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Mie f Ae soe y ‘sig = Beal ee REE Sm ane ar PEs! pleted eee Syrittae Pte Ces AEP isl chalet armeabente > men ime! -s°e~ POF pee eee ome em Pi xe ienta papeiesel u 2 epee er Sete oe Vela gagerre et yes retains ss tyegee e+ forte a ‘ ‘ . 4 a gee ter aee mere ae ene ceeey ey a are ‘ poe te he , f 4 eSisegeet oe , ri . cgewe tbyare sPee0 08 : +a! Titan ‘ Seta i ica Hint cuieyee ey ve Pe a A ss ipo ' aUihtentet a4: ad a ap EES cach halted . - a ave ptcera ide ae sary ser . . ’ * : WER ine “ RN, siting aaace ee ad eens tin te ten sete rset Af ’ a a Las iy Py ieee reales 47 ; Powis Ue arco esr HARVARD UNIVERSITY e Library of the Museum of Comparative Zoology VOLUME 98, NUMBER 333 JUNE 5, 1990 Latest Ordovician to Earliest Silurian Solitary Rugose Corals of the East-Central United States by Robert J. McAuley and Robert J. Elias Paleontological Research Institution | 1259 Trumansburg Road Ithaca, New York, 14850 U.S.A. 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Numbers of Palaeontographica Americana are priced individually, and are invoiced separately on request. for additional information, write or call: Paleontological Research Institution 1259 Trumansburg Road Ithaca, NY 14850 USA (607) 273-6623 7 —— — DOLOMITIC LIMESTONE TATE SHALE CALCAREOUS SHALE DOLOMITIC SHALE CHERT OR SILICIFICATION ARGILLACEOUS 88° W CONGLOMERATIC LITTLE STURGEON WISCONSIN °*¥ KATELL FALLS@ HIGH CLIFF (ig OOLITIC CORAL-RICH INTERVAL FACIES CONTACT DEFINITE FACIES CONTACT APPROXIMATE crayton[ \ co. IOWA ILLINOIS Nip Ic OB OKLAHOMA eGuates @ST. 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At a few localities, it is in part argillaceous, phosphatic, and conglomeratic (Thompson and Sat- terfield, 1975, p. 87, fig. 10). The Noix overlies the Maquoketa Shale unconformably, and is overlain by the Bryant Knob Formation or Bowling Green Dolo- mite. It is a lateral equivalent of part of the Cyrene Formation, which is located to the west (Thompson and Satterfield, 1975, pp. 85, 103). Geographically, the distribution of the Bryant Knob Formation coincides approximately with that of the Noix (Thompson and Satterfield, 1975, p. 85). The type section of this formation and the Kissenger Lime- stone Member is Section 18 (Kissenger) (Thompson and Satterfield, 1975, p. 98). An unnamed member that occurs at the base of the Bryant Knob at the type section, and locally elsewhere, overlies the Noix with apparent conformity (Thompson and Satterfield, 1975, p. 103). This member consists of up to 2 m of dolomitic limestone and/or shale. The Kissenger Limestone Member, up to perhaps 5 m thick, is a light gray, me- dium- to coarse-grained, massively bedded, bioclastic calcarenite (Amsden, 1974, pl. 28, figs. 3, 4). It overlies the unnamed member with apparent conformity where both members are present, and overlies the Noix un- conformably where the unnamed member is absent. The Bryant Knob Formation is overlain unconform- ably by the Bowling Green Dolomite. The Cyrene Formation occurs immediately west of the Noix and Bryant Knob. The type section is about 4 km northeast of Edgewood, Missouri, and Section 13 (Bowling Green) is an excellent reference section (Thompson and Satterfield, 1975, pp. 82, 96). This unit is about 2 m thick. It is a brown to bluish gray, fine- to medium-grained, dolomitic limestone (Amsden, 1974, pl. 27, figs. la, b, 2a, b). The Cyrene overlies the Maquoketa Shale, and is overlain by the Bowling Green Dolomite. The latter contact is inconspicuous in the vicinity of Section 13 (Savage, 1914, p. 29; Row- ley, 1916, p. 317; Amsden, 1974, p. 11). The Bowling Green Dolomite is the upper unit of the Edgewood Group. The type section is located about 1 km east-northeast of Section 13 (Bowling Green), which is a principal reference section (Thompson and Satterfield, 1975, p. 99). This unit has an average thick- ness of 6 to 9 m in Pike County, Missouri (Krey, 1924, p. 27). It consists of buff, earthy, massive dolostone (Amsden, 1974, pl. 28, figs. 1, 2), locally with beds of chert nodules in the upper two-thirds. In the eastern part of this area, the Bowling Green unconformably overlies the Noix Limestone or Bryant Knob Forma- tion. To the west, it overlies the Cyrene Formation. The Bowling Green Dolomite is overlain unconform- ably by the Sexton Creek Limestone or younger strata. Biota The following workers identified fossils from strata in Missouri that probably represent the Noix Lime- stone, Bryant Knob Formation, and Cyrene Forma- tion: Rowley (1904 [see Ausich, 1987]; 1908, p. 23 [the odlitic limestone therein is probably the Noix and Bryant Knob, and the brown earthy limestone therein is probably the Cyrene]) and Savage (1913b, pp. 24, 25; 1917, pp. 82, 83 [the Noix odlite therein is the Noix and Bryant Knob, and the Edgewood limestone near Edgewood therein is probably the Cyrene]). Those authors recognized crinoids, brachiopods, gastropods, pelecypods, cephalopods, trilobites, stromatoporoids, tabulate corals, and solitary rugose corals. In addition, cystoid plates and tentaculitids, and conularids and cornulitids, were reported from strata that probably represent the Noix and Bryant Knob, respectively. Stromatoporoids from the coral-rich interval at the base of the Kissenger Limestone Member, Bryant Knob Formation, were described by Birkhead (1967 [the Cy- rene Member of the Edgewood Formation at loc. I therein]). In Illinois, Rubey (1952, p. 170) listed brach- iopods and solitary rugose corals from the Noix Lime- stone, and brachiopods, bryozoans, gastropods, pe- lecypods, trilobites, tentaculitids, cornulitids, tabulate corals, and solitary rugose corals from the Bowling Green Dolomite. Amsden documented brachiopods from the Noix Limestone, Bryant Knob Formation, and Cyrene For- mation of the Edgewood Group in Missouri and Illi- nois (Amsden, 1971b, pp. 21, 22 [included in the basal and younger Edgewood zones therein]; Amsden, 1974 [the Bryant Knob at loc. D and lower Bowling Green at loc. C therein are considered to be Cyrene herein]; Amsden, 1988, p. 24). In addition to brachiopods, Amsden (1974, fig. 9; 1986, figs. 29, 30) identified © pelmatozoans, bryozoans, gastropods, trilobites, ostra- codes, and corals from the Noix and Cyrene, as well as pelecypods and tentaculitids from the Noix. Con- odonts have been reported from the Noix Limestone, both members of the Bryant Knob Formation, and Cyrene Formation in Missouri by Thompson and Sat- terfield (1975, figs. 10-15) and McCracken and Barnes (1982). Graptolites occur in the unnamed member of the Bryant Knob (Thompson and Satterfield, 1975, pp. | 97, 98). In addition to fossils reported previously from the Bryant Knob Formation, we recognize the siphonous green alga Dimorphosiphon sp. in thin sections of sam- ples from the unnamed member at Section 18 (Kis- | | | | | | ORDOVICIAN-SILURIAN RUGOSE CORALS: MCAULEY AND ELIAS 15 senger), and the coral-rich interval at the base of the Kissenger Limestone Member at Sections 17 (Clarks- ville) and 16 (Clinton Spring) (Pl. 3, fig. 16). Algal coatings are rare on solitary rugosan coralla in the un- named member, but are relatively common in the cor- al-rich interval of the Kissenger. Microborings that were likely produced by algae occur in some coralla from the coral-rich interval, but are rare in those from overlying strata. Borings we identify as 7rypanites sp., which were probably formed by polychaete annelids, are rare in solitary coralla from the Kissenger Lime- stone. Elias (198 2a) described the holotype of Streptelasma subregulare (Savage, 1913b) from the Cyrene Forma- tion near Edgewood, Missouri. During the present study, S. subregulare was found elsewhere in the Cy- rene, as well as in both members of the Bryant Knob Formation. Streptelasma sp. A occurs in the Noix Limestone, and Streptelasma leemonense Elias, 1982a, and Grewingkia sp. A are present in the Kissenger Limestone. Dinophyllum sp., Dalmanophyllum sp., and Cyathactis? sp. are identified from the Bowling Green Dolomite, and Rhegmaphyllum sp., Dinophyllum sp., and Dalmanophyllum sp. are recognized in limestone patches of the Bowling Green. The distribution of these corals is shown in Text-figure 5. Correlation Early workers reported that the Bryant Knob For- mation (upper part of the “‘odlite”’ in their terminology) at exposures in eastern Pike County, Missouri, con- tained abundant solitary and colonial coralla, as did the upper portion of the Cyrene Formation (which in- cludes the Watson Limestone of Rowley) at western sections in the vicinity of Edgewood (Rowley, 1908, pp. 20, 21; Rowley, 1916, pp. 319, 320; Savage, 1913b, p. 22; Savage, 1917, p. 80). Savage (1913b, p. 25; 1917, p. 83) considered the “‘odlite” to correspond to the upper half or two-thirds of the Cyrene. Thompson and Satterfield (1975, pp. 85, 87, 103) interpreted the Noix Limestone (which underlies the Bryant Knob) as a fa- cies of the upper part of the Cyrene Formation, al- though conodonts used to correlate the Cyrene with the Maquoketa Shale also occur in one section of the Bryant Knob (Thompson and Satterfield, 1975, p. 96, figs. 10, 11). On the basis of brachiopods and lithologic similarity, Amsden (1974, pp. 9, 11, 14, 15) tentatively considered the upper half of the Cyrene of Thompson and Satterfield at Section 13 (Bowling Green), as well as the Watson Limestone of Rowley, to be Bryant Knob. However, subsequent collecting has reduced the dis- tinction between Noix—Cyrene and Bryant Knob brachiopod assemblages (Amsden, 1986, p. 29). We follow the interpretation that the Cyrene For- mation is a facies equivalent of the combined Noix Limestone and Bryant Knob Formation. The con- odont Noixodontus girardeauensis (Satterfield, 1971) is present in the upper middle portion of the Cyrene at Section 13 (Bowling Green) (see Thompson and Sat- terfield, 1975, fig. 11) and in the Noix. The Paltodus dyscritus conodont fauna first appears definitely in the upper portion of the Cyrene (Thompson and Satter- field, 1975, p. 101) and in the Bryant Knob. Strepte- lasma subregulare (Savage, 1913b) occurs in the Cy- rene Formation (just below and within the interval containing N. girardeauensis) and in the Bryant Knob Formation. Strata overlying the Maquoketa Shale in a quarry located 1.5 km north-northwest of Section 13 (Bowling Green) were assigned to the Bowling Green Dolomite by Amsden (1974, p. 8). The lower 3 m were tentatively correlated with the Noix Limestone on the basis of brachiopods (Amsden, 1974, pp. 15, 18). We suggest that these beds, now covered by water, correspond to the Cyrene Formation (see also Amsden, 1988, p. 36). Amsden (1974, pp. 16-18, figs. 12, 13) considered two hypotheses for the relationship between the Bryant Knob Formation and Bowling Green Dolomite. On the basis of lithostratigraphic data, he favored the in- terpretation that these units are laterally and vertically intergrading facies, rather than discrete units having separate depositional histories (see also Amsden, 1986, pp. 36, 37, fig. 34). Amsden inferred that basal strata of the Bowling Green at Section 17 (Clarksville) are equivalent to the unnamed member comprising the lower portion of the Bryant Knob at Section 18 (Kis- senger), and to the Kissenger Limestone Member of the Bryant Knob at Section 16 (Clinton Spring). At Section 18 (Kissenger), a thin interval containing abundant solitary rugosan coralla is present at the base of the Kissenger Limestone Member, immediately above the unnamed member of the Bryant Knob For- mation (see Text-fig. 5). This coral-rich interval also occurs in the Bryant Knob at Section 16 (Clinton Spring) (see Birkhead, 1967, fig. 7), immediately above a thin, shale-bounded carbonate bed. The latter bed did not yield conodonts, but was included in the Noix Lime- stone by Thompson and Satterfield (1975, fig. 12, sam- ple 9), presumably because it contained some odids. Elias (198 2a, p. 40, fig. 21) placed this bed in the Kis- senger because it contained the same solitary rugosan species as the overlying coral-rich interval, and con- sidered the odids to be reworked from the Noix. On the basis of lithology and stratigraphic position, we conclude that this bed represents the unnamed mem- ber of the Bryant Knob, and the coral-rich interval above it marks the base of the Kissenger. The coral- rich interval has been reported at a locality about 2 km southeast of Section 16 (Rowley, 1908, p. 20). We also found the interval containing abundant solitary 16 BULLETIN 333 coralla at the extreme southern end of Section 17 (Clarksville). There it occurs as a bed that is shale- bounded in places, overlies the Noix along an irregular contact, is overlain by the Bowling Green Dolomite along an undulatory surface, and pinches out north- ward along the exposure (Text-figs. 5, 6). This bed is assigned to the Kissenger. The coral-rich interval of the Kissenger Limestone contains solitary rugosan coralla that are abraded, have algal coatings, and have microborings of probable algal origin associated with micritized surfaces. We consider it to be a lag deposit that 1s likely isochronous. Strep- telasma subregulare (Savage, 1913b), the only solitary rugosan species in the interval, also occurs in the un- named member as well as other strata included in the Kissenger Limestone Member of the Bryant Knob For- mation. The solitary coral assemblage in the basal Bowling Green Dolomite at Section 17 (Clarksville), both above and lateral to the coral-rich bed, is entirely different. We infer that the unnamed member of the Bryant Knob is older than the Kissenger Limestone Member, which in turn is older than the Bowling Green Dolomite. This interpretation, based on the distribution of sol- itary Rugosa, is consistent with the nature of contacts between the various units in this sequence. Where the unnamed member of the Bryant Knob Formation is present (e.g., Section 18), it overlies the Noix Lime- stone with apparent conformity (Thompson and Sat- terfield, 1975, p. 103). Where the coral-rich interval at the base of the Kissenger Limestone Member of the Bryant Knob overlies the Noix, the contact is uncon- formable (Section 17). The contact between the Kis- senger Limestone and Bowling Green Dolomite is un- conformable at some sections (e.g., Sections 17, 18; see Thompson and Satterfield, 1975, pp. 98, 103). We conclude that both members of the Bryant Knob For- mation as well as the Bowling Green Dolomite are discrete units, as recognized by Thompson and Sat- terfield (1975, p. 103). Thompson and Satterfield (1975, pp. 89, 100, figs. 13, 14) reported an unnamed unit composed of soft, white limestone present locally at the base of the Bowl- ing Green Dolomite. It was described as thin “lenses” on the Bryant Knob Formation at Section 18 (Kissen- ger), and as two low “mounds” on the Noix Limestone at Section 17 (Clarksville). Thompson and Satterfield noted that the ““mound” at the north end of the latter section was associated with an irregularity along the upper surface of the Noix, and was enclosed by a thin shale seam. This unit was not exposed at Section 18 during the present study, but two ““mounds” were ob- served on weathered faces of the exposure at Section 17. One was at the north end, and the other was located toward the south. It is uncertain whether these are the same ““mounds” reported by Thompson and Satter- field. In 1983, slumping along parts of Section 17 ex- posed fresh surfaces. Two irregular patches composed of soft, white limestone that was indistinguishable from the ““mounds” were observed within the Bowling Green Dolomite about 3 m above the Noix. One was situated above, and separated from, the southern ““mound”’. The other, found farther north along part ofan inclined joint or fracture, contained chert nodules at the same stratigraphic positions as the surrounding dolostone. These observations suggest that areas of limestone at the base of, and within, the Bowling Green represent undolomitized portions of this unit, rather than a dis- crete stratigraphic interval. Dal/manophyllum sp. and Dinophyllum sp. were found in both lithologies. Al- though Rhegmaphyllum sp. is recognized only in the limestone and Cyathactis? sp. is known only from the dolostone, these apparent differences may be related to the overall rarity of solitary coralla in the Bowling Green, and the relatively poor preservation of fossils in the dolostone. Age Berry and Marshall (1971) identified graptolites of the Dicellograptus complanatus var. ornatus Zone from an exposure of the Maquoketa Formation in eastern Missouri. The age of these Late Ordovician strata was considered to be within the late Maysvillian to Rich- mondian interval. Before the 1970’s, workers followed Savage’s as- signment of Edgewood strata in northeastern Missouri to the Early Silurian Alexandrian Series (Savage, 19 13a, p. 352; Thompson and Satterfield, 1975, fig. 1). Liebe (1962, pp. 10, 35) was evidently first to recognize that conodonts in the Noix Limestone are Ordovician. Amorphognathus ordovicicus Branson and Mehl, 1933, Text-figure 6.—Contacts between Kissenger Limestone Member of the Bryant Knob Formation and underlying Noix Limestone (thumb), and overlying Bowling Green Dolomite (forefinger), ex- treme southern end of Section 17 (Clarksville), Pike County, Mis- souri (photographed in 1983). ORDOVICIAN-SILURIAN RUGOSE CORALS: MCAULEY AND ELIAS 17 and other taxa indicate a Late Ordovician age (Thomp- son and Satterfield, 1975, p. 87). McCracken and Barnes (1982, p. 1477) considered the Noix conodont fauna to be late Richmondian (see also McCracken and Lenz, 1987, p. 649). However, Barrick (1986, pp. 64, 66) suggested that Noixodontus girardeauensis (Satterfield, 1971) may be restricted to the Hirnantian Stage (7.e., late Gamachian). Thompson and Satterfield (1975, p. 93) reported the Late Ordovician Prioniodus ferrarius conodont fauna from strata at Section 14 (Higginbotham Farm) that were assigned to the Bryant Knob Formation by Ams- den (1974, p. 83, loc. A) and by us. Conodonts in the unnamed member at the base of the Bryant Knob in- clude representatives of Amorphognathus ordovicicus Branson and Mehl, 1933, Jcriodella? sp., and the Pal- todus dyscritus fauna (Thompson and Satterfield, 1975, p. 97, figs. 10, 14). These conodonts, as well as grap- tolites (Berry, written commun., 1971, cited in Thomp- son and Satterfield, 1975, pp. 97, 98), were considered to indicate an Early Silurian age (Thompson and Sat- terfield, 1975, pp. 72, 101). The overlying Kissenger Limestone Member of the Bryant Knob contains 4. ordovicicus as well as the Prioniodus ferrarius and Pal- todus dyscritus faunas, and was also assigned an Early Silurian age (Thompson and Satterfield, 1975, p. 101, figs. 10, 12, 14, 15). However, Nowlan (in Bolton and Nowlan, 1979, pp. 5, 21) suggested that mixed faunas including A. ordovicicus and P. dyscritus Rexroad, 1967, might be Late Ordovician, based on an occurrence in undoubtedly Ordovician strata in the District of Kee- watin. The Bryant Knob Formation was tentatively assigned to the Early Silurian (Early Llandovery) by Amsden (1971b, pp. 21, 22 [the younger Edgewood zone therein includes the Bryant Knob]; 1974, p. 14). He based this primarily on stratigraphic position and the absence of certain characteristic Noix brachiopods, but subsequent collecting demonstrated that some of the species previously thought to have been confined to the Bryant Knob are also present in Noix—Cyrene strata (Amsden, 1986, p. 29). Thompson and Satterfield (1975, p. 96, fig. 11) re- ported Amorphognathus ordovicicus Branson and Mehl, 1933, as well as the Prioniodus ferrarius and Paltodus dyscritus conodont faunas in the Cyrene Formation. They considered this unit to be Late Ordovician, but we equate it with the combined Noix Limestone and Bryant Knob Formation. The conodont Noixodontus girardeauensis (Satterfield, 1971) is present in the up- per middle portion of the Cyrene at Section 13 (Bowl- ing Green) (see Thompson and Satterfield, 1975, fig. 11) and in the Noix. The Paltodus dyscritus fauna first appears definitely in the upper portion of the Cyrene (Thompson and Satterfield, 1975, p. 101) and in the Bryant Knob. Brachiopods in the Cyrene Formation as recognized by us were tentatively assigned Late Or- dovician and Early Llandovery ages by Amsden (1974, pp. 14, 15 [locs. C and D therein]). Thompson and Satterfield (1975, pp. 96, 97, 101, 103) identified the Paltodus dyscritus fauna in the Bowling Green Dolomite (including the unnamed unit of those authors), and concluded that this formation is Early Silurian. They considered conodonts in eastern exposures to be younger than those in western sections on the basis of three specimens identified as Neospath- ognathodus celloni (Walliser, 1964). McCracken and Barnes (1982, p. 1475) suggested that the latter con- odonts represent Oulodus? cf. O.? nathani McCracken and Barnes, 1981, of which they found one specimen in the Bowling Green. They inferred that the fauna in this unit may represent the Early Llandovery Oulodus? nathani or Distomodus kentuckyensis conodont zones (McCracken and Barnes, 1982, pp. 1474, 1477). How- ever, the Ordovician-Silurian boundary defined as the base of the Parakidograptus acuminatus Graptolite Zone (Cocks, 1985) may be situated within the O.? nathani Zone (Lespérance, 1985, figs. 3, 4; McCracken and Nowlan, 1988, p. 77). The brachiopod Platymerella manniensis Foerste, 1909, was reported at the base of the Sexton Creek Limestone (Kankakee Formation), which locally over- lies the Bowling Green Dolomite, by Savage (1913b, p. 30; 1917, p. 88), Willman (1973, p. 16), and Willman and Atherton (1975, p. 97). This zone was placed in the Middle Llandovery by Berry and Boucot (1970, pl. 2). Specimens of Stricklandia triplesiana (Foerste, 1890), with an interior similar to Stricklandia lens ul- tima Williams, 1951, from the Sexton Creek in Illinois were considered to be Late Llandovery (C,_;) by Ams- den (1974, pp. 18, 24; 1986, p. 41). Age assignments we follow are shown in Text-figure 5, and discussed further on pp. 25, 26. Paleoenvironment Savage (1914, p. 30) concluded from the lithostrat- igraphic record that the sea in which the lower Edge- wood Group formed was deepest in the west, where the Cyrene Formation accumulated, and became pro- gressively shallower toward the eastern margin, where the Noix Limestone and Bryant Knob Formation were deposited. The shoreline gradually receded westward, reaching a position a few km west of Louisiana, Mis- souri. Deposition of the Bowling Green Dolomite was initiated by a slight uplift west of the basin, accom- panied by subsidence resulting in an eastward overlap on the Noix and Bryant Knob surface. Sedimentation was uninterrupted to the west, where the Bowling Green overlies the Cyrene. Thompson and Satterfield (1975, pp. 93, 103, fig. 16) interpreted the coarse, bioclastic limestone at Sec- 18 BULLETIN 333 tion 14 (Higginbotham Farm) as a bioherm situated beneath the Bryant Knob Formation. We consider these strata to be a coquina and include them in the Bryant Knob (see also Amsden, 1974, p. 83, loc. A). They suggested that this deposit may have been a source of nuclei for odids that formed to the east, and acted as a barrier that separated Noix and Cyrene deposition. Conodont assemblages indicate that the Noix Lime- stone formed in relatively shallow water, and the Bowl- ing Green Dolomite was deposited during a transgres- sion (McCracken and Barnes, 1982, p. 1477). On the basis of conodont data, Thompson and Satterfield (1975, p. 97) concluded that deposition of the Bowling Green began earlier at Section 13 (Bowling Green) in the west than at Section 17 (Clarksville) in the east. However, McCracken and Barnes (1982, pp. 1475, 1477) considered conodonts from the latter section to represent an earlier zone than that indicated by Thompson and Satterfield. Local structural move- ments possibly contributed to the complex facies re- lations and unconformities in the Edgewood sequence (Thompson and Satterfield, 1975, p. 103). Most solitary rugosan coralla in the Bryant Knob Formation are abraded, suggesting relatively high en- ergy levels and comparatively low sedimentation rates. The highest proportions of abraded specimens ob- served in this study are from the coral-rich interval at the base of the Kissenger Limestone Member. Algal coatings are relatively common on solitary coralla and bioclastic grains in that interval, and some of the coral- la have probable algal microborings associated with micritized surfaces. These features suggest transpor- tation and prolonged exposure before burial, and we interpret the coral-rich interval as a lag deposit. The directional orientation pattern for solitary coralla in this interval at Section 16 (Clinton Spring) indicates that they were rolled almost perpendicular to water flow or nearly parallel to wave crests, with currents from the northwest (Elias, McAuley, and Mattison, 1987, p. 810). The paleocurrent direction is parallel to depositional strike of the Bryant Knob and to the in- ferred shoreline, suggesting longshore currents and per- haps waves. NORTHEASTERN ILLINOIS Lithostratigraphy The lithostratigraphic terminology we use (Text-fig. 7) follows Willman (1973, pp. 6, 9, 12-17, fig. 6), who summarized the history of nomenclature (see also Will- man and Buschbach, 1975, p. 86, fig. O-27; Willman and Atherton, 1975, pp. 96, 97, fig. S-7). The type section of the Wilhelmi Formation, as well as the Schweizer Member, is Section 4 (Schweizer West), and the type section of the Birds Member is Sections 4 and 5 (Schweizer North) (Willman, 1973, pp. 12- 14). The Wilhelmi is up to 30 m thick where it fills or nearly fills channels eroded into the underlying Maquo- keta Group, but is thin or absent elsewhere in the area. The Schweizer Member, which is up to 24 m thick, is generally present only where the formation is relatively thick. The lower portion consists primarily of gray, dolomitic shale, whereas the upper part is very argil- laceous, silty, thinly bedded dolostone. This unit is overlain conformably by the Birds Member, which is up to 6 m thick. The Birds is a gray, slightly argilla- ceous, typically flaggy dolostone. At Section 3 (Garden Prairie), the basal bed above the Maquoketa and the overlying flaggy dolostone were identified as the Wil- helmi Formation by Willman (1973, p. 12) and are assigned to the Birds Member by us. The Wilhelmi Formation unconformably overlies strata of the Maquoketa Group, ranging from the up- permost unit, the Neda Formation, down to the top of the Fort Atkinson Limestone, which underlies the Brainard Shale. The Wilhelm is overlain conformably by slightly argillaceous to pure dolostone of the Elwood Formation, which contains numerous layers of chert (Willman, 1973, p. 14). Dolostone of the Kankakee Formation overlies the Elwood conformably. Biota Fossils from the Wilhelmi Formation at exposures along Des Plaines River about 1.6 km, 3.2 km, and 5.6 km to the south of Channahon were identified by Savage (1913b, pp. 26, 27; 1914, p. 31; 1917, pp. 84, 85 [the Channahon Limestone therein]). He recognized brachiopods, gastropods, pelecypods, cephalopods, tri- lobites, ostracodes, tabulate corals, and solitary rugose corals. Fisher (1925, pp. 26, 27 [the Edgewood therein]) reported brachiopods and trilobites from a section of platy dolostone, which possibly represents the Birds Member of the Wilhelmi, along Du Page River about 7 km north of Channahon. The section south of Bel- videre that was described by Savage (1926, p. 518 [the Edgewood limestone therein]) is very similar to Sec- tions 34 (Belvidere South) and 3 (Garden Prairie). He identified solitary rugose corals from unit 3 (his ter- minology), which we assign to the Birds Member. The following groups have been listed from the Wil- helmi Formation along Horse Creek about 2.4 km east of Essex: brachiopods, gastropods, pelecypods, algae?, tentaculitids, cornulitids, conularids, tabulate corals, and solitary rugose corals (Savage, 1913b, p. 29; Sav- age, 1917, p. 87; Athy, 1928, pp. 40, 41 [the Essex Limestone therein]). From the same unit along Horse Creek 0.4 km west of Custer Park, Athy (1928, p. 41) reported brachiopods, trilobites, algae?, and tentacu- litids. ORDOVICIAN-—SILURIAN RUGOSE CORALS: MCAULEY AND ELIAS 19 Ross (1962 [the Edgewood Formation therein]) de- Schweizer Member at Section 4 (Schweizer West). Sco- scribed graptolites from a silty dolostone bed situated lecodont fragments were reported in the Wilhelmi— 7.6 m (shown at 4.6 m in his fig. |) above the base of Elwood sequence of northeastern Illinois by Buschbach the Wilhelmi Formation, in the upper part of the (1964, p. 59 [the Edgewood Formation therein]). Liebe w w =x > <= a 5 co fe) aye a 28 6 1 E o 2 es 8 = c ! z eg§ ses ° Sez a6 § =a E a a=? 5 ® Joliet f] ° Dae = 40" 1 fe} SEE > = ° = - "ch nahon ! a 25a : sone WILL CO ; u zaa g Custer 3 ' x = ' Fe 1 3 rfulu o ! z ! 3 ! 3 c < w Zc =n ' =e oe ] ie i rs zo = a Ir J is 5 fg Soke 3 4 = = —— > H w 3 « if > a w ° o > oe . 8 = 3 z = e 3 ee : 22 w aa” a Exon? 2 Goe B “3 ra} os w 2a o 2 FS &SF oS= 1m z/= oe ase Zi ie 358 ae i ~ i= N 5 od a e C: wu < § 3 : ie ie iz w 3 : rs iz S) WS = = 2 ge ee : igs 3 — WwW z 3 f2 / 38 3 Oss — a > sx wo ® U2 Sj oS Is E 1 fe) = = Qg 25 ra < a v= == 2 p fe) = a5 2 7 |S = tc ° ' 2 w = . x 6 ! — = ae fu ! S a eos = < O2/\t 2 mn) A oS Fa z eile z SECTION 5 SECTION 6 o a = SECTION 3 SECTION 29 SECTION 4 (SCHWEIZER (PLAINES (GARDEN PRAIRIE) (SEARS PIT) (SCHWEIZER WEST) NORTH) WEST) Text-figure 7.—Stratigraphic sections (to scale) and locality map (E) in northeastern Illinois (see Text-fig. 1A). For legend, see Text-figure 1B (foldout inside front cover). For references and precise locations of sections, see Appendix. 20 BULLETIN 333 and Rexroad (1977, figs. 2, 4) listed conodonts from the Schweizer Member of the Wilhelmi Formation at Section 4, and from the Birds Member at Sections 4 and 5 (Schweizer North) and National Quarry on the south side of Joliet. Mikulic er a/. (1985, pp. 10, 32, 33) also reported conodonts from the Wilhelmi at Na- tional Quarry, and noted inarticulate brachiopods, tri- lobites, and trace fossils in the basal, shaly strata, and pelmatozoan debris, brachiopods, bryozoans, gastro- pods, cephalopods, and trilobites in the upper, dolo- mitic strata. Conodonts, brachiopods, gastropods, and trilobites are present in the Wilhelmi at Section 29 (Sears Pit) (Mikulic et al., 1985, p. 23). Elias (1982a) studied two collections of solitary ru- gosan coralla made by Savage from the unit he termed Channahon Limestone. These included specimens that Savage (1913b) identified and illustrated as two new species, Zaphrentis ambigua and Zaphrentis subregu- laris. Elias concluded that they represent one species, Streptelasma subregulare (Savage, 1913b). Savage (1913b, p. 26; 1917, p. 84) reported numerous solitary coralla in unit 2 (his terminology) of the Channahon Limestone at a section located 1.6 km southeast of Channahon. He noted that corresponding strata are exposed above Maquoketa shale 1.6 km farther south (Savage, 1914, p. 31; see also Savage, 1916, p. 306), suggesting a position near the base of the Wilhelmi Formation. Elias (1982a, p. 40) assumed that Savage’s specimens came from Wilhelmi beds that lie strati- graphically above the graptolite bed of Ross, based on statements by Ross (1962, p. 1385) and Willman (1973, pp. 12, 13). From collections made during the present study at Section 4 (Schweizer West), it is apparent that S. subregulare occurs in the Schweizer Member, below and above the position of the graptolite bed. Brachio- pods, bryozoans, gastropods, trilobites, cornulitids, co- lonial corals, and solitary corals were observed at the latter locality. During the present study, we examined additional solitary coralla obtained by Savage in the vicinity of Channahon. Streptelasma subregulare (Savage, 1913b) is the only taxon that is represented. The same species occurs in his collections made along Horse Creek and near Belvidere, and is common in a bed of the Birds Member, Wilhelmi Formation, exposed on the quarry floor at Section 34 (Belvidere South; interval 34-1). Streptelasma subregulare was also found in the Birds Member at Section 3 (Garden Prairie) and at the base of the undivided Wilhelmi at Section 29 (Sears Pit). Solitary coralla were not observed in the Birds Member at Section 5 (Schweizer North). Rhegmaphyllum sp., Dinophyllum sp., and Dalmanophyllum sp. appear at the base of the Elwood Formation at Section 5, and the latter two occur in the upper Elwood at Section 6 (Plaines West). In the Elwood at Section 29, Cyathac- tis? sp. is present. At Section 7 (Kankakee River), sol- itary coralla were not found in the 0.3-m-thick Wil- helmi Formation, or in the underlying Neda Formation of the Maquoketa Group. The Kankakee Formation overlies the Wilhelmi at that location. The distribution of solitary corals in northeastern Illinois is shown in Text-figure 7. In eastern Wisconsin at High Cliff Park (see Text- fig. 1A), Willman (1973, p. 13) noted that the lower 3 m of the Mayville Dolomite, which overlies the Maquoketa Shale, resemble the Wilhelmi Formation, and the overlying 6 m of Mayville are like the Elwood Formation. During the present study, solitary rugosan coralla were not observed in that basal 3-m-thick unit, or in immediately overlying strata, of the Mayville. Willman (1973, p. 13) stated that the Mayville Do- lomite at Katell Falls (see Text-fig. 1A) consists largely of Kankakee lithology, but that approximately the low- er | m overlying the Neda Formation is similar to the Wilhelmi. Solitary coralla were not found in the lower 1 m of the Mayville or in the Neda at that section during the present study. Katell Falls is the only locality at which the Neda is known to contain fossils (Savage and Ross, 1916, p. 191; Mikulic and Kluessendorf, 1983, p. 29), but solitary corals have not been reported. Beneath the Mayville in the vicinity of Little Sturgeon Bay (see Text-fig. 1A), E. O. Ulrich observed a coral- rich dolostone unit up to 2 m thick that apparently fills a channel at the top of the Maquoketa shale and is in places overlain by a thin interval of the Neda (Mikulic and Kluessendorf, 1983, p. 36, figs. 27, 28). Elias (1982a, pp. 29, 67, fig. 18, pl. 10, fig. 8) identified the solitary rugosan species Grewingkia canadensis (Bil- lings, 1862) in Ulrich’s collection from the coral-rich unit. Age The history of age assignments for Maquoketa strata in Illinois was summarized by Willman and Buschbach (1975, pp. 81, 83, 84, fig. O-27). It is inferred that the Maquoketa Group in northern Illinois is probably pri- marily Maysvillian and Richmondian, based mainly on conodont data (Kolata and Graese, 1983, pp. 5, 6; Mikulic et al., 1985, pp. 6, 8). Savage and Ross (1916, p. 191) and Savage (1916, p. 309) reported that fossils from an undisturbed zone in the Neda Formation of eastern Wisconsin show little evidence of wear, are characteristic of the Maquoketa shale, and indicate a Richmondian age. Mikulic (1979; see Mikulic and Kluessendorf, 1983, p. 3) found occurrences of Brain- ard-like deposits overlying the Neda, suggesting a re- lation to Maquoketa sedimentation. The history of age assignments for strata overlying the Maquoketa Group in northeastern Illinois was summarized by Willman (1973, pp. 3, 5, fig. 6) and ORDOVICIAN-SILURIAN RUGOSE CORALS: MCAULEY AND ELIAS 21 Willman and Atherton (1975, fig. S-7). Ross (1962) considered graptolites from a bed in the upper part of the Schweizer Member, Wilhelmi Formation, to rep- resent a subzone at the top of the Parakidograptus acuminatus Zone in Wales or the basal part of the Orthograptus vesiculosus Zone in Scotland. An Early Silurian (Early Llandovery) age was confirmed by Ber- ry (in Berry and Boucot, 1970, p. 145). Conodonts of the Schweizer have been assigned to the Panderodus simplex Zone, inferred to be Early Llandovery, by Liebe and Rexroad (1977, pp. 848, 849, fig. 5) and Rexroad and Droste (1982, pp. 10, 12, fig. 6). However, cor- relation and age assignments on the basis of these con- odonts are unreliable because diagnostic, biostrati- graphically important taxa are not present (Liebe and Rexroad, 1977, p. 848; Mikulic et al., 1985, p. 16). Conodonts in the Birds Member of the Wilhelmi For- mation and the overlying Elwood Formation were as- signed to the Ozarkodina hassi interval, which was included at the base of the Jcriodina irregularis Zone (= Paltodus dyscritus fauna of Thompson and Satter- field, 1975; = Distomodus kentuckyensis Zone of Coo- per, 1975). Mikulic et al. (1985, p. 10) noted that O. hassi (Pollock, Rexroad, and Nicoll, 1970) suggests a Middle to early Late Llandovery age, but the range of that taxon may extend down into the latest Ordovician (McCracken and Nowlan, 1988, p. 77). The brachiopod Platymerella manniensis Foerste, 1909, has been reported from the upper Elwood For- mation and basal Kankakee Formation by Savage (1913b, p. 31; 1917, p. 89 [the Sexton Creek Limestone therein]), Willman (1973, pp. 14, 15), and Willman and Atherton (1975, p. 97). This zone was placed in the Middle Llandovery by Berry and Boucot (1970, pl. 2). Age assignments we follow are shown in Text-figure 7, and discussed further on pp. 25, 26. Paleoenvironment The Wilhelmi Formation was deposited during a transgression that followed a period of erosion, as rec- ognized by Savage (1913b, pp. 34, 35; 1916, p. 314; 1917, p. 92 [the Edgewood therein]). Willman (1973, p. 12) noted that this unit occupies channels that had been cut into the Maquoketa Group. The irregular sur- face of the Maquoketa exhibits more than 30 m of relief in places (Mikulic et al., 1985, p. 9). The basal bed of the Wilhelmi is conglomeratic at Section 4 (Schweizer West), and contains clasts of Maquoketa shale at Section 3 (Garden Prairie). The Schweizer Member, which is very argillaceous and silty, occurs only in the deeper parts of major channels (Willman, 1973, p. 13). As the surface of the Maquoketa became covered, the amount of argillaceous material in the overlying deposits decreased. The Birds Member of the Wilhelmi is slightly argillaceous and the overlying Elwood Formation contains little or no clastic mate- rial. Liebe and Rexroad (1977, p. 844) reported that reworked Ordovician conodonts decrease in abun- dance upward in the Schweizer, and occur sporadically above it. The lithologies and bedding in the Wilhelmi For- mation suggest deposition in relatively low energy con- ditions. All solitary rugosan coralla from the Wilhelmi in the vicinity of Channahon and at Section 34 (Bel- videre South) are nonabraded, also suggesting a low energy environment and perhaps a high sedimentation rate. NORTHWESTERN ILLINOIS AND EASTERN IOWA Lithostratigraphy The lithostratigraphic terminology we use (Text-fig. 8) follows Willman (1973, pp. 26, 27, 29, 31-36, fig. 9), who summarized the history of nomenclature (see also Willman and Atherton, 1975, pp. 98, 99, fig. S-8). The type section of the Mosalem Formation is Sec- tion 8 (King) (Willman, 1973, p. 32). The Mosalem is up to 30 m thick where it fills channels eroded into the underlying Maquoketa Group, but thins almost to absence above paleotopographic highs (Brown and Whitlow, 1960, pp. 34, 36-39, figs. 9, 10; Whitlow and Brown, 1963, pp. 11, 13, fig. 62.2; Willman, 1973, pp. 31-33). Where the Mosalem is relatively thick, the lower part is composed of gray, dolomitic shale and very argillaceous dolostone. The clastic content de- creases upward. The upper portion of the unit consists of slightly argillaceous dolostone with a few bands of chert. Where the Mosalem is comparatively thin, only the upper, dolomitic portion is present. This formation unconformably overlies strata of the Maquoketa Group ranging from the Neda Formation, preserved on pa- leotopographic highs, down into the underlying Brain- ard Shale. Within channels, the base of the Mosalem is characterized by a thin, persistent conglomerate con- taining clasts derived from the Maquoketa. The Mosalem Formation is overlain with apparent conformity by massive, vuggy, pure dolostone of the Tete des Morts Formation in the northern part of this area, and by relatively pure, cherty dolostone of the Blanding Formation in the south. Biota Brachiopods and trilobites from the Mosalem For- mation in the vicinity of Section 26 (Bellevue) were reported by Savage (1906, p. 601 [the transition beds therein]). From a locality near Section 9 (Winston), he listed inarticulate and articulate brachiopods, and tri- lobites (Savage, 1914, p. 34 [the Winston Limestone therein]). Brachiopods, trilobites, and solitary rugose corals from near the base of the Mosalem in the vicinity 22 BULLETIN 333 of Section 30 (Thomson Northeast) were listed by Sav- age (1926, p. 527 [the lower part of the Edgewood Limestone therein]). Brown and Whitlow (1960, pp. 37-39) stated that brachiopods, bryozoans, and trilobites are the domi- nant fossils in the Mosalem Formation of Dubuque County, Iowa. They noted minute objects that are probably conodonts, especially in the basal 1.5 m where burrow mottling is common. At localities where the Mosalem is thin, possible algal stromatolites were re- ported at the base. In the lowermost bed of the for- mation at one section, small phosphatic fossils resem- bling the “depauperate fauna” at the base of the Maquoketa Group were observed. Comminuted fossil fragments from the Cornulites zone in the upper Brain- ard Shale were noted within the basal conglomerate of the Mosalem. Ross (1964) documented graptolites from a horizon situated about 3.4 m above the base of the Mosalem Formation as currently recognized at Section 26 (Belle- vue) (see Rose, 1967, p. 45, fig. 21). He reported typical Maquoketa fossils from the basal 0.6-m-thick con- glomeratic, silty, dolomitic calcarenite of the Mosalem at that locality (Ross, 1964, p. 1107), but it is not known if they were reworked. Whitlow and Brown (1963, p. 13) noted phosphatic fossil fragments in that bed. During the present study, three solitary rugosan cor- alla were collected from an interval situated 1.8 to 2.4 m above the base of the Mosalem Formation at Section 26 (Bellevue). Unfortunately, they are too poorly pre- served for identification, as are specimens at Section 30 (Thomson Northeast). At Section 32 (Thomson East), Streptelasma subregulare (Savage, 1913b) is rare 0.65 to 0.70 m above the base of the Mosalem exposure (interval 32-1la). Lateral to those coralla are local chan- nels that cut down from a position 0.8 m above the base of the section (interval 32-1b). They are filled with argillaceous dolostone, are shale-bounded, and contain uncommon specimens of S. subregu/are and abundant remains of fasciculate colonial rugose corals we iden- tify as Pycnostylus? sp. (Text-fig. 9). Streptelasma sub- regulare is common with tabulate corals in a bed exposed on the old quarry floor to the west (interval 32-1z). That bed appears to be at about the same strati- graphic position as intervals 32-la and 32-1b. Dal- manophyllum sp. occurs at heights of 2.2 and 3.75 m above the base of the exposure, Rhegmaphyllum sp. is present at 3.95 m, and Dinophyllum sp. was recovered between 3.95 and 4.45 m. Dalmanophyllum sp. and Cyathactis? sp. were found near the top of the Mosalem at Sections 10 (Lost Mound) and 9 (Winston), respec- tively. The distribution of these solitary corals is shown in Text-figure 8. One unidentifiable specimen was also found in the upper 2 m of the Mosalem at Section 11 (Schapville). Solitary coralla were not observed in this formation at Sections 8 (King) and 12 (Stockton). Sal- vadorea randi (Elias, 1981) was described from the Brainard Shale of the Maquoketa Group at Sterling, Illinois, and in Clayton County, Iowa, by Elias (1982a, pp. 61, 62, pl. 6, figs. 1-7; 1985, p. 45; see Text-fig. 1A). - : 2 ¢3 o|= = z u ,Ea ale 5 zi/2 ese Son | é3: -- fea oa =, Ir = Ir ir De ir Pe oe => ' H se ae tc te = az 2 » < 3 z 2 S SECTION 9 : a me (WINSTON) ra i ~ 2 wi? -s 2) oe 2 a al = | e 4428 5 5 5 a ad 1 we . ao-€| cole tan ete es jtcoz -~- »® - o Ir Ooms Se atc oes gees 725 z SEsxRe2s < 215 SECTION 32 zs ZERE — -o = 2 2/£ (THOMSON EAST) >E8$ 2 = > o; oa Qa o ee KE and ewe oye os = ° 5 z < SECTION 30 CCI ey eye SECTION 10 SECTION 26 (THOMSON NORTHEAST) (LOST MOUND) (BELLEVUE) Text-figure 8.—Stratigraphic sections (to scale) and locality map (F) in northwestern Illinois and eastern Iowa (see Text-fig. 1A). For legend, see Text-figure |B (foldout inside front cover). Gam. = Gamachian. For references and precise locations of sections, see Appendix. ORDOVICIAN-SILURIAN RUGOSE CORALS: MCAULEY AND ELIAS 23 Age The history of age assignments for Maquoketa strata in Illinois was summarized by Willman and Buschbach (1975, pp. 81, 83, 84, fig. O-27). Faunal and lithostrat- igraphic correlation of the upper Brainard Shale and uppermost Cornulites zone of the Brainard in lowa and Illinois with the Late Ordovician (latest Richmondian) “Elkhorn” strata in the Cincinnati Arch region was considered highly probable by Ladd (1929, pp. 369, 370) and Templeton and Willman (1963, pp. 132, 133). On the basis of conodonts, Glenister (1957, pp. 720, 721) assigned the Maquoketa Group of eastern Iowa to the Maysvillian through Richmondian. Specimens of the Maysvillian—Richmondian solitary rugosan species Salvadorea randi (Elias, 1981) in the Brainard Shale are considered to be Richmondian on the basis of their position above the Fort Atkinson Formation, which contains the Richmondian species Bighornia patella (Wilson, 1926) (see Elias, 1987). The history of age assignments for strata overlying the Maquoketa Group in northwestern II]linois and the adjacent area in Iowa was summarized by Willman (1973, pp. 29, 31, fig. 9) and Willman and Atherton (1975, fig. S-8). Ross (1964) noted that graptolites from a bed about 3.4 m above the base of the Mosalem Formation apparently represent the same Early Silu- rian (Early Llandovery) zone as those in the Schweizer Member of the Wilhelmi Formation in northeastern Illinois. The age of units overlying the Mosalem For- mation is imprecisely known, because important bio- stratigraphic marker zones are absent. Age assignments we follow are shown in Text-figure 8, and discussed further on pp. 25, 26. Paleoenvironment The Mosalem Formation was deposited during a transgression, as recognized by Savage (1914, p. 34 [the Winston Limestone therein]; 1926, p. 528 [the Edgewood Limestone therein]). Brown and Whitlow (1960, p. 36) concluded that the Mosalem was depos- ited in a shallow marine environment as a sea ad- vanced over the eroded Maquoketa surface. While sed- iment accumulated in low areas, topographic highs continued to be eroded, either above or below sea level. The detrital content of the Mosalem decreased as the summits became covered with marine deposits. Ero- sional relief on the Maquoketa is 41 m in Dubuque County, south of Dubuque, Iowa (Brown and Whitlow, 1960, p. 23; Whitlow and Brown, 1963, p. 11). Small, local channels in the upper Mosalem at Section 32 (Thomson East) may record a minor regressive event during the transgressive phase. SOLITARY RUGOSE CORAL ASSEMBLAGES Four solitary rugosan assemblages are recognized in the uppermost Ordovician—lowermost Silurian se- quence within the study region in the east-central United States: (1) Late Ordovician “‘epicontinental” assemblage (Richmondian); (2) Late Ordovician “‘con- tinental margin”’ assemblage (Gamachian); (3) Edge- wood assemblage (Gamachian-early Early Llandov- ery); and (4) Silurian assemblage (post-Edgewood Llandovery) (Text-fig. 10, Table 1). The lowest is a Late Ordovician “epicontinental” assemblage in the upper Maquoketa Group. Salvadorea randi (Elias, 1981), a Maysvillian—Richmondian species (Elias, 1985, p. 45), is present in the Orchard Creek and Brain- ard shales (Elias, 1982a, pp. 35, 36). Grewingkia can- adensis (Billings, 1862) occurs immediately below the Neda Formation in eastern Wisconsin (Elias, 1982a, p. 29). It is a Richmondian species (Elias, 1982a, p. 67). The upper, Silurian assemblage includes Dinophyl- lum sp., Dalmanophyllum sp., Cyathactis? sp., and Rhegmaphyllum sp. These genera are typical of the Early to Middle Silurian (Hill, 1981, pp. 159-161, 163, 308), and are present in the late Early to Late(?) Llan- dovery Brassfield Formation of the Cincinnati Arch region in Kentucky—Indiana—Ohio (Laub, 1979). One of the genera in the Silurian assemblage has recently been found in Late Ordovician deposits near the North American continental margin. At Pointe Laframboise on Anticosti Island, Québec, Rhegma- phyllum sp. appears in the basal 1 m of the Becscie Formation (Elias and Petryk, unpublished data; see Petryk, 1981la, fig. 11), in beds considered to be late Gamachian. The specimens occur about | m above the Ordovician-Silurian boundary of conodont work- ers (McCracken and Barnes, 1981), but strata equiv- Bet | “pec Y= 3 - Text-figure 9.—Small, local channel within Mosalem Formation at Section 32 (Thomson East), Carroll County, Illinois (assistant pointing to base and top of channel, photographed in 1986). 24 BULLETIN 333 alent in age to the basal Silurian Parakidograptus acuminatus Graptolite Zone may be much higher in the sequence (Lespérance, 1985, fig. 3; McCracken and Nowlan, 1988, p. 77). Coralla of Rhegmaphyllum sp. are associated with Grewingkia pulchella (Billings, 1865), a species that is also present in the Ellis Bay Formation (Gamachian) and upper member of the Vauréal Formation (Richmondian) on Anticosti Island (Elias, 1982a, p. 45). In the southeastern United States, Rhegmaphyllum sp. occurs in strata considered to be Richmondian (Elias and Stock, unpublished data). It has been found, together with Grewingkia sp. cf. G. pulchella (Billings, 1865), in the Sequatchie Formation at Birmingham, Alabama (see Drahovzal and Neath- ery, 1971, pp. 25, 237, stop 10), and in the Shellmound Formation at Pope Spring, Georgia (see Milici and Wedow, 1977, pp. 8, 9, 33, sec. 26b) (for age of strata, see also Colbath, 1986, p. 945). In the Beaverfoot For- mation of southeastern British Columbia, Rhegma- phyllum sp. and Streptelasma sp. are present in latest Ordovician or possibly earliest Silurian strata above the Bighornia—Thaerodonta Assemblage Zone, which is probably entirely Richmondian (Buttler, Elias, and Norford, 1988, pp. 59, 60). At the St. Clair Spring Section in eastern north-central Arkansas, Rhegma- phyllum sp. is the only solitary coral in the Cason oGlite, which is considered to be Gamachian (Hirnan- tian) in age (Amsden, 1986, pp. 20, 22, 26; Barrick, 1986, pp. 64, 66). This occurrence represents the Late Ordovician “continental margin” assemblage within the present study region. Our study is focused on the assemblage that is here termed the Edgewood. It is situated stratigraphically between the Late Ordovician “‘epicontinental” and the Silurian assemblages, and geographically lateral to the “continental margin” assemblage. Of the 709 speci- mens identified at the species level, percentages rep- resenting the various taxa are as follows: Streptelasma subregulare (Savage, 1913b), 83.1%; Streptelasma sp. cf. S. subregulare (Savage, 1913b), 0.1%; Streptelasma amsdeni, n. sp., 10.0%: Streptelasma leemonense Elias, 1982a, 4.7%; Streptelasma sp. cf. S. leemonense Elias, 1982a, 0.4%; Keelophyllum oklahomense, n. gen., n. sp., 0.9%: Streptelasma sp. A, 0.4%; Grewingkia sp. A, 0.3%: and Bodophyllum shorti Elias, 1982a, 0.1%. Streptelasma subregulare is the most widely distrib- uted species, followed by S. leemonense. AGE OF UNITS AND REGIONAL CORRELATION The stratigraphic position of the Edgewood solitary rugosan assemblage between Late Ordovician (Rich- mondian) and typical Early to Middle Silurian assem- blages suggests an age in the range of latest Ordovician to earliest Silurian. The species are comparable pri- Table 1.—Latest Ordovician to earliest Silurian solitary rugose corals in the study region, east-central United States. Silurian Assemblage Suborder Streptelasmatina Family Streptelasmatidae Subfamily Streptelasmatinae Rhegmaphyllum sp. Subfamily Dinophyllinae Dinophyllum sp. Subfamily Dalmanophyllinae Dalmanophyllum sp. Suborder Cyathophyllina Family Ptychophyllidae Cyathactis? sp. Edgewood Assemblage [Edgewood Province] Suborder Streptelasmatina Family Streptelasmatidae Subfamily Streptelasmatinae Streptelasma subregulare Streptelasma sp. cf. S. subregulare Streptelasma amsdeni Streptelasma leemonense Streptelasma sp. cf. S. leemonense Streptelasma sp. A Grewingkia sp. A Subfamily Dalmanophyllinae Bodophyllum shorti Suborder Monacanthina Family Lambelasmatidae Subfamily Coelostylinae Keelophyllum oklahomense Late Ordovician “Continental Margin”” Assemblage Suborder Streptelasmatina Family Streptelasmatidae Subfamily Streptelasmatinae Rhegmaphyllum sp. Late Ordovician “Epicontinental” Assemblage Suborder Streptelasmatina Family Streptelasmatidae Subfamily Streptelasmatinae Salvadorea randi [Red River-Stony Mountain Province] Grewingkia canadensis [Richmond Province] marily to taxa from various Late Ordovician (Rich- mondian—Gamachian; Ashgill, including Hirnantian) and Early Silurian (Llandovery) units in North Amer- ica and Baltoscandia, and a latest Ordovician (Hir- nantian) unit in China (see pp. 40-49, 53). However, the overall assemblage, in which 97.8 percent of spec- imens belong to Streptelasma subregulare (Savage, 1913b), Streptelasma amsdeni, n. sp., and Streptelas- ma leemonense Elias, 1982a, seems to be most similar to that in the Da/manitina Beds (Hirnantian) or pos- sibly earliest Llandovery beds of Ostergétland, Swe- den, and the Guanyingiao Beds (Dalmanitina Beds; Hirnantian) of Guizhou Province, China. Of the four species comprising the Swedish assemblage (see Neu- man, 1975, p. 336), three are similar to some speci- ORDOVICIAN-SILURIAN RUGOSE CORALS: MCAULEY AND ELIAS mens of S. subregulare (Streptelasma unicum Neuman, 1975, Helicelasma simplex Neuman, 1969, and Bor- elasma crassitangens Neuman, 1969), one resembles S. amsdeni (S. unicum), and one is comparable to S. leemonense (Streptelasma ostrogothicum Neuman, 1969). Many coralla from the Chinese assemblage, il- lustrated by He (1978, 1985), resemble specimens of S. subregulare and S. amsdeni (see pp. 42, 45). Amsden (1971b, pp. 21, 22; 1974, p. 26) considered the Keel-—Edgewood brachiopod assemblage in south- central Oklahoma, southern Illinois and southeastern Missouri, and northeastern Missouri and west-central Illinois to be characteristic of the latest Ordovician (late Ashgill) to earliest Silurian (Early Llandovery). He noted that brachiopods in the Keel Formation, Leemon Formation, and Noix Limestone most closely resemble the latest Ordovician Hirnantia fauna, but the species are different. It was suggested that this may be due, at least in part, to ecologic factors. Amsden (1986, pp. 18, 20, 22, 41, 42) concluded that brachio- pods in the Keel, Cason odlite, Leemon, Noix, and Cyrene formations indicate a latest Ordovician Hir- nantian (i.e., late Gamachian) age. The brachiopod as- semblage in the Bryant Knob Formation, which over- lies the Noix in northeastern Missouri, was initially thought to be different in some respects, and was ten- tatively assigned to the Early Llandovery almost en- tirely on the basis of associated conodonts (Amsden, 1971b, p. 22; Amsden, 1974, p. 26). However, sub- sequent collecting demonstrated that some of the species previously thought to have been confined to the Bryant Knob are also present in Noix—Cyrene strata (Amsden, 1986, p. 29). Brachiopods of the lower Edgewood assemblage were provisionally considered to be post-Hirnantian by Les- pérance (1974, p. 22). Lespérance and Sheehan (1976, pp. 719, 720) noted that these brachiopods could rep- resent a latest Ordovician endemic North American fauna, with species derived from the Hirnantia com- munity and other North European Province species. However, they suggested that it was most likely a Si- lurian fauna with a few holdovers from the Late Or- dovician North American Province. A Hirnantian age was accepted by Jaanusson (1979, p. 154), based on a trilobite and ‘other indications” from beds in Illinois. In view of the presence of Ordovician (possibly Hir- nantian) conodonts in the lower Keel Formation, Lee- mon Formation, Noix Limestone, and middle Cyrene Formation (Thompson and Satterfield, 1975; Mc- Cracken and Barnes, 1982; Barrick, 1986), the Hir- nantian aspect of the brachiopods and solitary corals, and the position of these beds above Late Ordovician (Richmondian) strata, we accept a latest Ordovician (Gamachian) age, as suggested and discussed by Elias (1982a, pp. 38, 39) (Text-fig. 10). If the Hirnantian is N nn equivalent to the upper part of the Gamachian in the North American succession (Cocks and Copper, 1981, p. 1033; Lespérance, 1985, p. 844, fig. 4; McCracken, 1987, p. 1454, fig. 2), assignment of Keel-Edgewood strata to the Hirnantian implies an unconformable re- lationship with the underlying Sylvan—Maquoketa beds, which are accepted as Richmondian. Although this contact is clearly unconformable in most areas, Ams- den (1980, p. 10; 1986, p. 6) indicated that deposition may have been continuous from the Sylvan to the Keel. Therefore, use of the term Gamachian is followed herein for this North American sequence. Streptelasma sub- regulare (Savage, 1913b), Streptelasma leemonense Elias, 1982a, Grewingkia sp. A, and other solitary cor- als in the Keel, Leemon, Noix, and middle Cyrene, are considered to have first appeared in the east-central United States during Gamachian time. Correlation of the Keel-—Edgewood with strata in north-central Arkansas is indicated by the following: Ordovician (possibly Hirnantian) conodonts in the Ca- son odlite and dolomitic shale (Craig, 1969; Craig, 1975b, pp. 77, 80, 85; Craig in Craig, Ethington, and Repetski, 1986, pp. 18, 19; Barrick, 1986); brachio- pods of Hirnantian aspect in the Cason odlite (Ams- den, 1986, pp. 20, 22, 26); and the presence of Strep- telasma leemonense Elias, 1982a, and Streptelasma sp. cf. S. subregulare (Savage, 1913b) in Cason dolomitic shale. The Cason o6lite, which contains Rhegmaphyl- lum sp., and dolomitic shale are considered Gama- chian in age (Text-fig. 10). In northeastern Illinois, Streptelasma subregulare (Savage, 1913b) is present in the Schweizer and Birds members of the Wilhelmi Formation. It occurs both below and above a bed in the upper Schweizer that contains Early Llandovery graptolites, which possibly represent the Parakidograptus acuminatus Zone (Ross, 1962, p. 1383: Berry in Berry and Boucot, 1970, p. 145). Therefore, we conclude that the range of S. sub- regulare extends into the Silurian. Neuman (1982, p. 34) noted that several solitary rugosan species in Nor- way range from the Ashgill into the Early Llandovery. It remains a possibility that lower Schweizer strata infilling the deepest channels in the eroded Maquoketa, below the position of the graptolite bed, may be Ga- machian in age, as suggested by Elias (1982a, p. 40, fig. 21) (Text-fig. 10). The lower part of the Wilhelmi Formation has been correlated with the lower portion of the Mosalem For- mation in northwestern Illinois and eastern Iowa on the basis of graptolites (Ross, 1964, p. 1107) and l- thology (Willman, 1973, pp. 13, 31, fig. 2). Graptolites near the base of the Mosalem apparently represent the same zone as those in the Wilhelmi, and are Early Llandovery in age (Ross, 1964). Streptelasma subregu- lare (Savage, 1913b) occurs at a higher stratigraphic 26 BULLETIN 333 position in the formation. We suggest that lower Mosa- lem strata infilling the deepest channels eroded into the Maquoketa, below the position of the graptolite bed, may be Gamachian in age (Text-fig. 10). In northeastern Illinois, corals of the Silurian assem- blage appear at the base of the Elwood Formation and occur with the brachiopod Platymerella manniensis Foerste, 1909, in the upper Elwood. The Platymerella zone includes the upper Elwood and base of the over- lying Kankakee Formation (Savage, 1913b, p. 30; Sav- age, 1917, p. 88; Willman, 1973, pp. 14, 15; Willman and Atherton, 1975, p. 97). It was considered to be Middle Llandovery in age by Berry and Boucot (1970, pl. 2). In northwestern Illinois, this solitary rugosan assemblage appears in the upper part of the Mosalem Formation, just above the stratigraphic position of lo- cal channels at Section 32 (Thomson East) that contain Streptelasma subregulare (Savage, 1913b) and Silurian fasciculate rugose corals identified as Pycnostylus? sp. (see Hill, 1981, p. 140). We consider the upper Mosa- lem to be younger than the upper part of the Wilhelmi Formation because S. swbregu/are occurs in the Birds Member of the Wilhelmi, whereas the Silurian assem- blage is present in the upper Mosalem. The Edgewood and Silurian solitary rugosan assemblages are not known to co-occur. Although Willman (1973, pp. 15-17, 35, 36) tentatively correlated the Elwood with the Blanding Formation of northwestern Illinois on the basis of li- thology, he noted that the Tete des Morts Formation as well as the Blanding could correlate with the lower Kankakee. From the coral evidence, we infer that the upper Mosalem must be equivalent to at least the lower part of the Elwood (Text-fig. 10). Johnson, Rong, and Yang (1985, fig. 5) placed the Mosalem, Tete des Morts, and lower Blanding in the Early Llandovery (Rhud- danian) on the basis of correlations involving sea-level curves. In northeastern Missouri, the Silurian coral assem- blage occurs in the Bowling Green Dolomite at an eastern exposure. The Platymerella manniensis zone, considered to be Middle Llandovery, is present at the base of the Sexton Creek Limestone, which overlies the Bowling Green (Savage, 1913b, p. 30; Savage, 1917, p. 88; Willman, 1973, p. 16; Willman and Atherton, 1975, p. 97). Therefore, we correlate the Bowling Green with the lower Elwood and upper Mosalem and con- sider it to be Early Llandovery in age (Text-fig. 10). Corals of the Edgewood assemblage in the Bryant Knob Formation, which overlies the Noix Limestone and underlies the Bowling Green Dolomite in north- eastern Missouri, could be Gamachian or Early Llan- dovery in age. Streptelasma subregulare (Savage, 1913b), Streptelasma leemonense Elias, 1982a, and Grewingkia sp. A are also known from the Keel For- mation (Gamachian), and S. subregulare and S. lee- monense occur in the Leemon Formation (Gamach- ian). However, the range of S. subregulare is known to extend into Early Llandovery strata of the Wilhelmi and Mosalem formations, in which the other species are not represented. The Bryant Knob is tentatively considered to be Early Llandovery on the basis of con- odonts (Thompson and Satterfield, 1975) and grap- tolites (Berry, written commun., 1971, cited in Thomp- son and Satterfield, 1975). However, this remains questionable because some of the conodont collections could be Ordovician (Nowlan in Bolton and Nowlan, 1979, pp. 5, 21) and the graptolites were not identified to the species level. The upper portion of the Cyrene Formation immediately to the west is equated with the Bryant Knob (Text-fig. 10). The evidence for this cor- relation, based on conodonts, brachiopods, and lith- ologic similarity, was discussed on p. 15. The lower part of the Sexton Creek Limestone in southeastern Missouri and southern Illinois could be as old as the Bowling Green Dolomite (Early Llan- dovery). Both units contain conodonts representing the Paltodus dyscritus fauna (Thompson and Satterfield, 1975, figs. 6, 7, 9, 11-15) and solitary corals of the Silurian assemblage. The Silurian solitary coral assem- blage is also represented in basal beds of the Cochrane Formation in south-central Oklahoma (Text-fig. 10). Conodonts in the lower Cochrane were thought to be Llandovery (position uncertain) by Barrick (1986, pp. 57, 67). Amsden (1986, p. 6) considered the brachio- pod Triplesia alata Ulrich and Cooper, 1936, which apparently ranges through most of the Cochrane (Ams- den, 1971la, p. 145), to indicate an early Late Llan- dovery C,_, age. However, he noted that it could be younger or older. The Edgewood solitary coral assemblage 1s latest Or- dovician (Gamachian) to earliest Silurian (early Early Llandovery; early Rhuddanian) in age, and therefore these Rugosa cannot be used to delineate the Ordo- vician-Silurian boundary in the east-central United States. In Illinois and Missouri, they occur in strata that were included within the Alexandrian Series by Savage (papers from 1908a through 1926, inclusive). Reeds (1911) extended the use of that term for cor- relative beds in Oklahoma. This series was proposed by Savage (1908a, pp. 433, 434; 1908b, pp. 110, 111) to include strata, thought to be earliest Silurian in age, situated between the Richmondian Stage of the Cin- cinnatian Series (Ordovician) and the Niagaran Series (Silurian). It has been recommended that Alexandrian be discontinued as a series term because of synonymies (Fisher, 1954, pp. 1982, 1984), and because outcrops in the type area are meager, the units are not especially fossiliferous, and unconformities are present within the sequence (Amsden, 1974, p. 5). ORDOVICIAN-SILURIAN RUGOSE CORALS: MCAULEY AND ELIAS 27 BIOGEOGRAPHY AND EVENTS Two biogeographic provinces are represented by sol- itary rugose corals of the Late Ordovician “epiconti- nental” assemblage in the upper Maquoketa Group. Salvadorea randi (Elias, 1981) in Illinois and Iowa is also known from middle Maysvillian strata in the Sel- kirk Member (see Elias, 1985, p. 45) and middle Rich- mondian beds in the Fort Garry Member of the Red River Formation in southern Manitoba (Elias, Now- lan, and Bolton, 1988, pl. 1, figs. 4-7). It belongs to the Red River—-Stony Mountain Solitary Rugose Coral Province (Elias, 1982a, p. 48; Text-fig. 11). The dis- tribution of this species indicates dispersion across the Transcontinental Arch between the Williston Basin and the area of Maquoketa deposition. Grewingkia cana- densis (Billings, 1862), which is present in the upper Maquoketa Group of eastern Wisconsin, is character- istic of the Richmond Solitary Rugose Coral Province (Elias, 1982a, pp. 49, 50; Text-fig. 11). Elias (1982a, p. 29) suggested that this occurrence, in an apparent channel-fill deposit at the top of the Maquoketa shale but beneath the Neda Formation, represents a west- ward shift in the geographic range of this species as- sociated with the regression during late Richmondian time. Solitary Rugosa of the Red River—Stony Mountain and Richmond provinces in east-central North Amer- ica became extinct during the terminal Richmondian regression of the epicontinental sea caused by a major glacio-eustatic sea-level drop (Elias, 1982a, pp. 48, 51). In northern [Illinois and eastern Iowa, the presence of 30-m-deep channels eroded into the upper, Richmon- dian portion of the Maquoketa Group and subsequent- ly filled with Gamachian(?) and Early Llandovery de- posits of the Wilhelmi and Mosalem formations suggests a period of post-Richmondian emergence, at least in the northern portion of the study region. To the east, the Cincinnati Arch region likely remained emergent until late Early Llandovery time (Grahn and Bergstrom, 1985, pp. 178, 179). On Anticosti Island, Québec, where deposition near the continental margin was essentially continuous from Richmondian time into the Silurian, the Richmondian—Gamachian boundary (placed at the contact of the Vauréal and Ellis Bay formations) appears to coincide with a major regression (Johnson, Cocks, and Copper, 1981, fig. 3; Petryk, 198 1b, fig. 1; Long and Copper, 1987, pp. 1829, 1830). Solitary Rugosa of the Edgewood assemblage rep- resent the Edgewood Solitary Rugose Coral Province (Elias, !982a, pp. 51, 52; Text-fig. 11). The inclusion of south-central Oklahoma in this province is con- firmed herein, and the boundary is extended to include western north-central Arkansas and northwestern II- linois. Units containing the Edgewood assemblage re- cord a succession of oscillatory transgressions that reached progressively farther north, according to Sav- age (1913a, p. 374; 1913b, pp. 34, 35; 1917, p. 92). Amsden (1986, pp. 2, 45) considered the Keel—Edge- wood o6litic deposits to represent a regressive sedi- mentary cycle reflecting eustatic lowering of sea level due to glaciation. Both interpretations may be in part correct. The Edgewood assemblage is Gamachian to earliest Llandovery in age. The major glacio-eustatic sea-level drop during Gamachian/Hirnantian time was followed by a rapid rise in the latest Gamachian/Hir- nantian (e.g., Brenchley and Newall, 1980, fig. 22; Johnson, Cocks, and Copper, 1981, fig. 3; Petryk, 198 1b, fig. 1; Woodcock and Smallwood, 1987, p. 393). The Keel—Edgewood odlites in Oklahoma and Mis- souri likely mark the regressive phase, but could have been deposited during minor transgressions if sea level fluctuated during that time interval (see Brenchley and Newall, 1980, pp. 29, 30, fig. 22). A number of oscil- lations are evidently recorded in the latest Richmon- dian to latest Gamachian regressive phase on Anticosti Island (Petryk, 1981b, fig. 1). Channel-fill sediments of the Wilhelmi and Mosalem formations in northern Illinois and eastern Iowa were likely deposited during the major latest Gamachian/Hirnantian-earliest Silu- rian transgression associated with deglaciation. The Edgewood solitary rugosan species were not de- rived from corals of the Late Ordovician “epiconti- nental”’ assemblage in this region. Their resemblance to some taxa previously restricted to the continental margin of North America suggests that they originated from such forms. Within the Edgewood Province, di- versity is highest in Oklahoma, intermediate in south- ern Illinois and Missouri, and lowest in northern IIli- nois (western north-central Arkansas is excluded because only two identifiable specimens are known) (Text-fig. 10). This apparent northward decrease in diversity corresponds to an environmental gradient from relatively open conditions near the continental margin to increasingly restricted conditions in the con- tinental interior. It is noteworthy that specimens of Rhegmaphyllum sp., but no representatives of the Edgewood solitary coral assemblage, are present in the Cason odlite at the St. Clair Spring Section in eastern north-central Arkansas. The associated conodonts and brachiopods comprise a typical Keel-—Edgewood as- semblage (Craig, 1969, pp. 1624, 1625; Craig, 1975b, p. 77; Craig in Craig, Ethington, and Repetski, 1986, p. 19; Amsden, 1986, pp. 20, 22; Barrick, 1986, p. 64). Streptelasma leemonense Elias, 1982a, and Streptelas- ma sp. cf. S. subregulare (Savage, 1913b), both Edge- wood solitary Rugosa, occur in the Cason about 100 km to the west at Section 33 (Buffalo River). Thus, it is possible to place the boundary of the Edgewood 28 BULLETIN 333 Solitary Rugose Coral Province and the area inhabited by the Late Ordovician “‘continental margin” solitary rugosan assemblage between these two sections (Text- fig. 11). Genera recognized in the Silurian assemblage were not derived from Edgewood taxa. They must have orig- inated elsewhere and been introduced to this region. Laub (1975, p. 280: 1979, p. 45) noted that rugosan species in the late Early to Late(?) Llandovery Brass- field Formation of the Cincinnati Arch region are not known in pre-Brassfield strata of North America, but some occur in the Baltic area, the Siberian platform, and possibly Venezuela. Recent work indicates that Rhegmaphyllum Wedekind, 1927, was confined to areas near the North American continental margin in the Richmondian and Gamachian (see Solitary Rugose Coral Assemblages, pp. 23, 24). During Early Llan- dovery time, the Silurian assemblage, including Di- nophyllum sp., Dalmanophyllum sp., Cyathactis? sp., and Rhegmaphyllum sp., succeeded the Edgewood as- “ RED RIVER- STONY MOUNTAIN PROVINCE RICHMOND PROVINCE EDGEWOOD PROVINCE “CONTINENTAL MARGIN” ASSEMBLAGE lext-figure | |.— Biogeography of North American Late Ordovician to earliest Silurian solitary Rugosa. Red River-Stony Mountain Province of Edenian to Gamachian age (Elias, 1981, pp. 2, 8, 10; Elias, 1982a, pp. 48, 49; Elias, 1983a, pp. 927-931; Elias, 1985, pp. 16-20; Elias, unpubl. data). Richmond Province of Richmondian age (Elias, 1982a, pp. 49-51). Edgewood Province of Gamachian to early Early Llandovery age (Elias, 1982a, pp. 51, 52: present study). Late Ordovician “continental margin” assemblage of Richmondian—Gamachian age (for discussion of taxa in eastern Québec and northern Maine, see Elias, 1982a, pp. 48, 49; present study). Solid and dashed lines show established and uncertain boundaries, respectively. ORDOVICIAN-SILURIAN RUGOSE CORALS: MCAULEY AND ELIAS 29 semblage. This occurred as water depth (and temper- ature) increased during the Early Llandovery transgres- sion related to deglaciation (see Johnson, Rong, and Yang, 1985, fig. 5, eastern Iowa). However, the ap- pearance of the Silurian solitary rugosan assemblage above local channels containing Edgewood corals with- in the Mosalem Formation at Section 32 (Thomson East), and the unconformity between the Bryant Knob Formation and Bowling Green Dolomite at some sec- tions in northeastern Missouri, suggest that Silurian solitary Rugosa were introduced after a minor regres- sive event. THE EDGEWOOD SOLITARY RUGOSE CORALS GROWTH A small minority of solitary rugose corals in the Edgewood Province were attached throughout ontog- eny to surfaces such as bryozoans, colonial corals, and possibly other solitary coralla. These epizoans belong to Streptelasma sp. A (Pl. 9, figs. 8, 9, Pl. 10, figs. 1- 5), Grewingkia sp. A (PI. 10, fig. 8), Bodophyllum shorti Elias, 1982a (Elias, 1982a, pl. 13, fig. 10), Keelophyl- lum oklahomense, n. gen., n. sp. (Pl. 12, figs. 4-6), and possibly Streptelasma leemonense Elias, 1982a (PI. 7, fig. 9). The vast majority of individuals, belonging to Streptelasma subregulare (Savage, 1913b) and Strep- telasma amsdeni, n. sp., behaved as unattached objects on the substrate. A few specimens representing the latter two species have right-angle bends, indicating that the polyp had the ability to redirect its growth axis after being fully overturned (Elias, 1984b, pp. 534, 535; Pl. 1, fig. 17, Pl. 6, fig. 11). These are the earliest North American species presently known that could do so. The ability to survive such events, which apparently resulted in the death of polyps belonging to other taxa, must have been advantageous. Although specimens of S. leemonense lack right-angle bends, this species was able to produce offsets that diverged from the parent at a high angle (Pl. 8, figs. 1-4). They could have per- formed a function analogous to redirection of the growth axis. Prominent constrictions of the coral at apparently regularly spaced intervals are known only from one specimen of Streptelasma amsdeni, n. sp. (PI. 6, fig. 11; identified as Streptelasma sp. in Elias, 1984b, p. 535). Seven consecutive constrictions are 6 to 7 mm apart (average, 6.8 mm). Apparent periodicity involv- ing tabulae was observed only in one specimen of Streptelasma subregulare (Savage, 1913b) (Pl. 3, figs. 2-4). Four thick tabulae, separated from one another by one to three thin tabulae, occur at intervals of 4 to 6 mm (average, 5.2 mm). Elias (1984b, pp. 535, 536) documented comparable examples of periodic growth involving constrictions and dilated tabulae at intervals of 3 to 13 mm in Late Ordovician (Richmondian— Gamachian) taxa from Anticosti Island, Québec. It is possible that these figures represent annual growth rates (see Risk, Pagani, and Elias, 1987, pp. 328, 329). Elias (1984b, p. 536) reported an unusually high growth rate for three coralla identified as Streptelasma sp. [referred herein to S. amsdeni], based on the relatively wide spacing of fine growth ridges. Following an examina- tion of additional material during the present study, we conclude that all growth increments may not be preserved on those specimens. ABRASION During life, these solitary Rugosa produced septal grooves and interseptal ridges covered by a very thin epitheca with fine growth ridges on the outer wall (Elias, 1982a, pl. 4, figs. 10, 11, 14, 15, 19; Pl. 1, fig. 17, Pl. AV ings Gh NOL IL Seti, Ay Ab Gy ies, I, 0), JUL, Web S) figs. 1, 2, Pl. 11, fig. 6). Nonweathered, well-preserved specimens lacking these features are considered to have been abraded prior to burial. The degree of abrasion was related to the duration and intensity of this process. The length of time a corallum was exposed on the substrate was determined by the sedimentation rate. The intensity of abrasion prior to burial of the speci- men was determined by the energy level of the envi- ronment. The proportion of nonabraded and abraded Edge- wood coralla in various stratigraphic units is shown in Table 2. All or the majority of specimens in the Keel Formation, Leemon Formation at Sections 31 (Thebes North) and 19 (New Wells), and Wilhelmi Formation are nonabraded. Lithologies of the laminated calcilu- tite unit of the Keel, the Leemon at Section 19, and the Wilhelmi suggest deposition in comparatively low energy conditions. However, the other Keel facies and the Leemon at Section 31 were apparently deposited in higher energy environments, and a higher propor- tion of abraded coralla might be expected. A relatively high sedimentation rate could have resulted in quick burial of these individuals, thus protecting them from abrasion. Most coralla in the Bryant Knob Formation, and in the Leemon Formation at Section 20 (Short Farm), are abraded. The lithologies of these units suggest that en- ergy levels were relatively high, but probably not sig- nificantly higher than those in which facies of the Keel Formation other than the laminated calcilutite unit were deposited. The high proportions of abraded spec- imens in these strata are likely a reflection of compar- atively low sedimentation rates. The coral-rich interval of the Kissenger Limestone Member, Bryant Knob Formation, contains the highest observed proportions of abraded specimens. 30 BULLETIN 333 ALGAL COATINGS Coatings having micritic, oncolitic, and Girvanella- like appearances in thin section were observed on Streptelasma subregulare (Savage, 1913b) from bio- herms in the Leemon Formation at Section 19 (New Wells), the unnamed member of the Bryant Knob For- mation at Section 18 (Kissenger), and the coral-rich interval at the base of the Kissenger Limestone Mem- ber, Bryant Knob Formation, at Sections 18 (PI. 2, fig. 6), 17 (Clarksville), and 16 (Clinton Spring) (PI. 3, figs. 14-18). Such coatings, presumed to be of algal origin, are rare except in the coral-rich interval of the Kissen- ger, where they are relatively common on solitary ru- gosan coralla and bioclastic grains. The coating usually completely surrounds the corallum, although in some cases it is thicker on one side. Coatings were observed Table 2.—Condition of corallum exterior, for specimens of Strep- telasma subregulare (Savage, 1913b) unless otherwise noted, deter- mined from an examination of the corallum and/or transverse thin section(s). Nonabraded if growth ridges, epitheca, and/or septal grooves and interseptal ridges are present; abraded if those features are absent on nonweathered, well-preserved material. unil section-interval nonabraded abraded Keel Fm. 23-2a and 23-3, and 23a-1 14 (93%)! 1 (7%) Keel Fm., Ideal Quarry Mbr. 21 16 (100%) 0 (0%) Keel Fm., Brevilamnulella beds 23-2 16 (100%) 0 (0%) Keel Fm., laminated calcilutite unit 24-2 68 (100%)? 0 (0%) Leemon Fm. 31 14 (70%) 6 (30%)* 20 12 (41%) 17 (59%) 19 (bioherms) 72 (87%) 11 (13%) Bryant Knob Fm., unnamed mbr. 18-1 4 (40%) 6 (60%) Bryant Knob Fm., Kissenger Lst. Mbr. 18-2 (coral-rich interval) and 18-3 4 (24%) 13 (76%) 17-0 (coral-rich interval) 7 (39%) 11 (61%) 16-1 (coral-rich interval) 3 (16%) 16 (84%) 15-1 5 (42%) 7 (58%)* 14-1 5 (45%) 6 (55%) Wilhelmi Fm., Schweizer Mbr. Channahon 25 (100%) 0 (0%) Wilhelmi Fm., Birds Mbr. 34 31 (100%) 0 (0%) includes three specimens of Streptelasma leemonense Elias, 1982a, and two of Streptelasma sp. cf. S. leemonense Elias, 1982a. all Streptelasma amsdeni, n. sp. includes one specimen of S. /eemonense. * includes four specimens of S. /eemonense. to extend into the calices of two individuals. In most specimens, the corallum surface under the coating is abraded. We conclude that coatings generally devel- oped during post-mortem transport, when coralla were rolled along the substrate. One silicified corallum of Streptelasma subregulare (Savage, 1913b) from the Brevilamnulella beds of the Keel Formation at Section 23 (Lawrence Quarry) has a lamellar incrustation that covers the calice. It prob- ably represents an alga or a stromatoporoid. A speci- men of S. subregulare from the Mosalem Formation at Section 32 (Thomson East) has an unidentifiable lamellar incrustation on one side. EPIZOANS Bryozoans rarely incrust algal coatings on Strepte- lasma subregulare (Savage, 1913b) in the coral-rich interval at the base of the Kissenger Limestone Mem- ber, Bryant Knob Formation, at Section 16 (Clinton Spring) (Pl. 3, fig. 15). They became associated with the host after the algal coating developed during trans- portation. In bioherms of the Leemon Formation at Section 19 (New Wells), these epizoans are common on the same species (Pl. 2, fig. 3). In one case, the bryozoan incrusts an algal coating on a corallum. The bryozoans observed on 21 coralla are situated on seven counter sides, 20 alar sides (there are two alar sides per corallum), and seven cardinal sides, suggesting a random distribution. An incrusting bryozoan is present on one specimen of S. subregulare from Section 32 (Thomson East). Epizoic bryozoans were found in a cluster of Streptelasma leemonense Elias, 1982a, from the Leemon Formation at Section 20 (Short Farm) (PI. 8, figs. 1, 4). An epizoan that is possibly a bryozoan is present on a specimen of Keelophyllum oklahomense, n. gen., n. sp., from the Keel Formation at Section 23 (Lawrence Quarry) (Pl. 12, fig. 6). BORINGS Vermiform borings were observed in transverse thin sections of three coralla assigned to Streptelasma sub- regulare (Savage, 1913b). We identify them as Trypa- nites sp., and they were probably produced by poly- chaete annelids (see Elias, 1986b, p. 33). In one specimen from a bioherm in the Leemon Formation at Section 19 (New Wells) (Pl. 1, fig. 18), the boring has a maximum observed diameter of 0.6 mm. It passes through the matrix as well as a thin algal coating on the corallum, indicating that the substrate was hard. The other two borings have maximum observed di- ameters of 0.25 mm. One is from the coral-rich interval at the base of the Kissenger Limestone Member of the Bryant Knob Formation at Section 18 (Kissenger), and the other is from the Kissenger at Section 15 (Calumet). This type of boring is rare in the Edgewood Province, ORDOVICIAN-SILURIAN RUGOSE CORALS: MCAULEY AND ELIAS 3] but occurs in solitary rugosan coralla from many strati- graphic units and localities in the Red River—Stony Mountain and Richmond provinces (Elias, 1986b, ta- ble 1). Microborings about 5 um in diameter were observed in the outer wall of some specimens of Streptelasma subregulare (Savage, 1913b) from bioherms in the Lee- mon Formation at Section 19 (New Wells) (Pl. 2, fig. 4), and possibly in one or two from the Leemon at Section 20 (Short Farm). These small borings could be algal or fungal (see Bromley, 1970, pp. 54, 55; Golubic, Perkins, and Lukas, 1975, p. 243). They occur beneath epizoic bryozoans in two coralla and beneath an algal coating in one, indicating that the borers became as- sociated with these hosts relatively early. Similar bor- ings have been reported in Richmondian solitary ru- gosan coralla from the Richmond and Red River-Stony Mountain provinces (e.g., Elias, 1982a, pl. 9, fig. 20; Elias, 1982b, fig. 4h). A second type of microborings, with branches hav- ing highly variable diameters of up to 5O um, was probably produced by algae (Pl. 4, figs. 1, 2). Such borings were observed in a few coralla of Streptelasma subregulare (Savage, 1913b) from the Leemon For- mation at Section 20 (Short Farm), and from the Kis- senger Limestone Member of the Bryant Knob For- mation at Sections 18 (Kissenger), 17 (Clarksville), and 16 (Clinton Spring). All but one of the Kissenger spec- imens are from the basal coral-rich interval, and most are from Section 16. The exterior surface of the cor- allum was micritized in almost all cases, and the bor- ings occur beneath algal coatings in some individuals and within the calice in two. Such borings and asso- ciated micritization have not been observed in Rich- mondian solitary rugosan coralla. ORIENTATION Virtually all of the solitary rugosan coralla found during this study were lying sideways within the strata, in what would have been stable depositional orienta- tions after being overturned and possibly transported. Several clusters representing colonies and/or pseudo- colonies of Streptelasma leemonense Elias, 1982a, in- clude corallites oriented with calices facing upward, as they would have during life. These may have been preserved in growth position. The greater size and weight of the clusters likely made them more stable than individual coralla. Such specimens were found at Sections 23 (Lawrence Quarry), 20 (Short Farm), and 15 (Calumet). Data sets large enough to permit statistical analysis of directional orientations were obtained for solitary rugosan coralla in the upper half of the Keel Formation, including the Brevilamnulella beds, at Section 23 (Law- rence Quarry), and the coral-rich interval at the base of the Kissenger Limestone Member, Bryant Knob Formation, at Section 16 (Clinton Spring). These dis- tributions were interpreted by Elias, McAuley, and Mattison (1987, p. 810), and the results are summa- rized below. Coralla at Section 23 are considered to be randomly oriented (Text-fig. 12A). Although the energy level was probably high enough to transport these objects, the directions of fluid motion may have been sufficiently variable that a preferred orientation pattern did not result. It seems unlikely that an initial pattern was subsequently obscured by the activity of burrowers, because there is minimal evidence for bioturbation. Coralla at Section 16 are preferentially oriented, and the distribution is bimodal (Text-fig. 12B). Peaks in the northeast and southwest are opposite one another but skewed slightly toward the southeast. These indi- viduals were rolled almost perpendicular to water flow or nearly parallel to wave crests, with the apical end facing either way but directed slightly into currents from the northwest. The current direction is parallel to depositional strike of the Bryant Knob and the in- ferred shoreline, suggesting that coralla were oriented by longshore currents and perhaps waves. From an examination of loose slabs, it is apparent that cylindrical coralla of Streptelasma amsdeni, 0. sp., in the laminated calcilutite unit of the Keel Formation at Section 24 (Coal Creek) are generally aligned parallel to one another (PI. 6, figs. 9, 10). This 1s considered to be a result of hydraulic action. Unfortunately, too few specimens were found in situ to conduct a paleo- current analysis. N A uy B \ > a aa AEN “we . 7 € pew N= 100 N: 69 4 Se] |1em Text-figure 12.—Rose diagrams showing directional orientations of solitary rugosan coralla (NV = sample size). Orientation convention is illustrated using the north arrow and a typical specimen (to scale). A, Mostly Streptelasma subregulare (Savage, 1913b), intervals 23-2 (Brevilamnulella beds), 23-2a, 23-3, Keel Formation, Section 23 (Lawrence Quarry). B, Streptelasma subregulare, interval 16-1 (cor- al-rich interval), Kissenger Limestone Member, Bryant Knob For- mation, Section 16 (Clinton Spring); thick line represents deposi- tional strike of Bryant Knob Formation. B93 BULLETIN 333 PALEOECOLOGY Solitary rugose corals of the Edgewood Province are absent or rare in units representing the highest energy environments. They are absent in the cross-bedded odlite at the base of the Leemon Formation at Section 20 (Short Farm). These fossils are generally absent or rare in relatively pure oGlitic portions of the Keel For- mation, such as below and above the laminated cal- cilutite unit at Section 24 (Coal Creek), in which Strep- telasma amsdeni, n. sp., is abundant. Streptelasma sp. A, the only taxon known from the odlite comprising the Noix Limestone, is rare at Section 15 (Calumet). This small, epizoic species may have been protected by its position on a larger bryozoan colony. Edgewood solitary Rugosa are also absent or rare in argillaceous units considered to represent the lowest energy envi- ronments. They are absent in the Leemon Formation above the basal bioherms at Section 19 (New Wells), in which Streptelasma subregulare (Savage, 1913b) is abundant. These corals are absent or rare in Wilhelm1 and Mosalem strata, except near the base of the Wil- helmi at a few localities and on one bedding surface in the Mosalem at a single locality. Streptelasma sub- regulare is the only Edgewood species known from the Wilhelmi and Mosalem formations. Edgewood solitary rugosan species generally occur in relatively pure bioclastic calcarenite beds, such as those in the Keel Formation at Sections 21 (Rock Crossing) and 23 (Lawrence Quarry), the upper Lee- mon Formation at Sections 31 (Thebes North) and 20 (Short Farm), and the Bryant Knob Formation. This suggests that they favored clear water, moderate en- ergy, normal marine conditions. Streptelasma subregu- lare (Savage, 1913b), by far the most common and widely distributed species (see Text-figs. 2-5, 7, 8), was evidently able to inhabit a wider range of environments than the other taxa. Streptelasma leemonense Elias, 1982a, and Grewingkia sp. A are the second and third most widely distributed species, respectively. Deposition of the Kissenger Limestone Member at Sections 18 (Kissenger), 17 (Clarksville), and 16 (Clin- ton Spring) occurred nearer to shore and possibly in shallower water than at Sections 15 (Calumet) and 14 (Higginbotham Farm) (Text-fig. 5). This is inferred from the southeast-northwest depositional strike of the Bryant Knob Formation, and the apparent position of a shoreline immediately to the northeast. Streptelasma subregulare (Savage, 1913b) is the only solitary coral at Sections 18, 17, and 16, but it occurs together with Streptelasma leemonense Elias, 1982a, at Sections 15 and 14, and also with Grewingkia sp. A at Section 14. Perhaps the distribution of Edgewood taxa was deter- mined by proximity to shore and/or water depth. Streptelasma subregulare is the only species present in northern Illinois (Text-figs. 7, 8), where deposition ev- idently occurred in the most restricted conditions, far- thest from the continental margin. Evidence indicating that the distribution of solitary rugosan species in the Richmond and Red River-Stony Mountain provinces was related to water depth and/or the degree of envi- ronmental restriction was presented by Elias (1982a, pp. 13, 26; 1983b, pp. 2, 3: 1985, pp. 14-16) and Elias, Zeilstra, and Bayer (1988, p. 33). INTRASPECIFIC VARIATION AND EVOLUTION Streptelasma subregulare (Savage, 1913b) is a highly variable species. The continuous spectrum of values for measurements of numerous characteristics indi- cates that there are no morphologic discontinuities among the 589 specimens assigned to the species. Those characteristics exhibiting anomalous trends at certain sections or in particular stratigraphic intervals (Table 3) fall within or overlap the range of values for coralla from elsewhere. Such features could reflect genetic dif- ferences among populations and/or environmental dif- ferences that are not understood at present, given the limited amount of detailed paleontologic and sedi- mentologic work that has been done on these units. Morphologic characteristics of coralla from the lam- inated calcilutite unit of the Keel Formation at Section 24 (Coal Creek), and a specimen from the underlying Keel odlite, generally lie within the range of variability of Streptelasma subregulare (Savage, 1913b), but most are not typical of that species (Table 3). The most striking features are the long, cylindrical growth form and thin septa. We consider these coralla to represent a distinct taxon, Streptelasma amsdeni, n. sp., because of the consistent differences. It is inferred that S. ams- deni was derived from S. subregulare by geographic speciation. This is the only evolutionary event recog- nized within the Edgewood Province. SYSTEMATIC PALEONTOLOGY INTRODUCTION Our morphologic terminology follows Hill (1981) and Elias (1981, p. 3). Most of the biometric and other data used to prepare Text-figures 13-23 and Tables 4, 6, and 7 were presented by McAuley (1985, appendixes 1-4). The synonymies include only forms considered identical to the one under discussion. The suprageneric classification followed herein is that of Hill (1981) for Streptelasmatina Wedekind, 1927, and Cyathophyllina Nicholson in Nicholson and Ly- dekker, 1889, and Neuman (1984) for Monacanthina Neuman, 1984. Generic treatments are included where our study increases understanding of an established genus, or requires erection of a new one. ORDOVICIAN-SILURIAN RUGOSE CORALS: MCAULEY AND ELIAS 33 Table 3.—Morphologic characteristics of Streptelasma subregulare (Savage, 1913b) and the closely related species Streptelasma amsdeni, n. sp., for sections with relatively large sample sizes, based on an inspection of corallum lengths and data in Tables 4-6 and Text-figure 17; unusually high frequency (H), typical frequency (T), unusually low frequency (L), insufficient data (—; fewer than 10 data points for comparison, except for “‘closely spaced tabulae,”’ where all points were used). sections 18 (in- 18 (in- tervals tervals characteristics 21 23 31 20 19 l and 2) 3 and 4) 17 16 15 14 34 32 24 S ams- Streptelasma subregulare deni long coralla T i H H H ap H T T al i iT a H trochoid coralla = H 10 H a i Ay L L H Ib i H L cylindrical coralla T ab ar 40 T 1 1 ap in 4 T T H noncurved coralla — Te T L Ww 1s T uw oe H 18 1p at i greatly curved coralla — T H T wp T I, al it L i Ae Ai L numerous septa L a H H L ie it 50 i at TP 1 T L long major septa WT T T T iW a H a it 1 T a aT 1 thin major septa ap ap H H H IL, IV Ww it ak L T H H long cardinal septum H 1 H T aw a Iv IL re iC, 16 ap ih Ti thick cardinal septum H 1 T als T Ww Wr a ap a 1 J a dhs wide cardinal fossula T T ap i H H H ar 1 1h i a AB a closely spaced tabulae T = T T WD at iT H T 1p dy Wh T L Coates (1984) summarized recent advances in taxo- ABBREVIATIONS OF REPOSITORIES nomic methods applied to living and fossil scleracti- UCGM: University of Cincinnati Geological Mu- nian coral species. The recognition and documentation Conn Cincnmate OF US of intraspecific variability based on large numbers of UL University of Illinois at Urbana—Cham- coralla is critical. In modern studies of Early Paleozoic paign, Urbana, IL, U.S.A solitary rugose corals, the ontogeny of individuals as USNM: National Museum of Natural History, well as intraspecific variability are of fundamental im- Smithsonian Institution, Washington, DC portance (see Neuman, 1974, 1977). USA A total of 731 specimens are identified herein. Ex- ternal features of the corallum, ontogeny and internal ABBREVIATIONS OF COLLECTIONS structures, and microstructure are documented as cane Te Arado completely as possible for 709 coralla representing the ey ae peas ; i C coll.: W. W. Craig. Edgewood assemblage. Biometric and other data are Fl coll.: R. J. Elias (1978) presented and analyzed in graphic and tabular form F2 eae R. . Elias (1988). where feasible. The named species that we recognize EM Ae R. i Bie ed R. J. McAuley (1983) are considered valid because they are separated from EMM SAT : R. J. Elias. B. W : MATER v A others by morphologic gaps. A continuous spectrum mi a d R F rene (1982) ? of features among coralla included in a species indi- EMZ coll. RJ Elias RJ ae A is ; cates that there are no discontinuities. The range of ii An d R G ees (1 38 ae characteristics developed during ontogeny of individ- EZ coll.: RJ Elias eh are Feileten (1986) uals aids in the confirmation of intraspecific variabil- Senile zi T. E. Saree a , ity. In some cases, assignment of specimens to a named species is uncertain because their ontogeny or range of variability is incompletely known. We use “‘sp. cf.”’ to qualify such identifications. A letter designation (sp. A) is given if there are insufficient data to assign a group of conspecific coralla to an existing species or a new one. Two specimens representing the Late Ordovician “continental margin”’ assemblage and 20 from the Si- lurian assemblage are documented briefly. The generic name is followed by “sp.” alone, because we are not certain that all the included coralla are conspecific. Suborder STREPTELASMATINA Wedekind, 1927 Family STREPTELASMATIDAE Nicholson in Nicholson and Lydekker, 1889 Subfamily STREPTELASMATINAE Nicholson in Nicholson and Lydekker, 1889 Genus STREPTELASMA Hall, 1847 Streptelasma Hall, 1847, p. 17 (as Streptoplasma), and page facing p. 338; Neuman, 1969, pp. 8-10; McLean, 1974, pp. 38-41; Laub, 1979, pp. 59-61; Elias, 1982a, p. 52. Type species. — Designated by Roemer (1861, p. 19): 34 BULLETIN 333 Streptelasma corniculum Hall, 1847; lower Trenton Limestone (upper Middle Ordovician), Middleville, New York. Diagnosis. —Solitary or with few offsets. Septa non- dilated to completely dilated in early stage, dilation usually decreases during ontogeny. Major septa straight or wavy, generally extend to axis in early stage and become shorter during intermediate to late stages. Ax- ial structure, if present, typically simple, composed of septal lobes and rarely lamellae in intermediate and/ or late stages. Cardinal septum and fossula inconspic- uous to prominent. Discussion.—Neuman’s (1969, pp. 8-11) study of the lectotype of Streptelasma corniculum Hall, 1847, formed a basis for the generally accepted concept of Streptelasma Hall, 1847. This genus was considered to include streptelasmatids with major septa that are long, thin to moderately thick, and usually joined to form a simple axial structure during early to inter- mediate stages. These septa become shorter and thin- ner in late stages, when an axial structure is seldom present. However, because intraspecific variability of the type species is unknown, Streptelasma remains poorly understood. Furthermore, most species pres- ently included in the genus are based on small collec- tions. McLean (1974, pp. 38-41) stated that Streptelasma Hall, 1847, Dinophyllum Lindstrom, 1882, and Por- firieviella \vanovskiy, 1963, may prove to be syno- nymous. He also noted that many species resembling Streptelasma in late stages are not well enough known in earlier stages to be included in the genus with cer- tainty. Elias (1982a, p. 52) indicated that further study may demonstrate the synonymy of Streptelasma, He- licelasma Neuman, 1969, Borelasma Neuman, 1969, and Grewingkia Dybowski, 1873. He showed that in Streptelasma divaricans (Nicholson, 1875) there is complete gradation from coralla with open axial re- gions to those with axial structures similar to Gre- wingkia (Elias, 1982a, pp. 52, 55, 56, pl. 1, figs. 1-19). Neuman (1986, p. 352) considered the North Ameri- can fauna to show an extreme range of variation not representative of most other areas, including Balto- scandia. Streptelasma subregulare (Savage, 1913b), described in this study on the basis of a large collection, is im- portant in establishing the range of variability in Strep- telasma Hall, 1847. In this highly variable species there is continuous gradation from coralla that closely re- semble Streptelasma corniculum Hall, 1847, to those that are similar to Helicelasma Neuman, 1969, Bor- elasma Neuman, 1969, and Ullernelasma Neuman, 1975. Within some individuals, different ontogenetic stages would be assigned to different genera if they were considered independently. We therefore broaden the diagnosis of Streptelasma to include all representatives of S. subregulare. Neuman (1986, p. 352) stated that the only fossular structures characteristic of Streptelasma Hall, 1847, are a pseudofossula (lacking tabular depression) or a septofossula (shortened cardinal septum). The cardinal fossula in Streptelasma subregulare (Savage, 1913b) is highly variable, and the tabulae are markedly de- pressed in some specimens (including the lectotype; Elias, 1982a, pl. 4, fig. 7). Streptelasma subregulare (Savage, 1913b) Plate 1, figures 1-19; Plate 2, figures 1-12; Plate 3, figures 1-18; Plate 4, figures 1-13; Plate 5, figures 1-10 Zaphrentis subregularis Savage, 1913b, p. 62, pl. 3, fig. 5, pl. 7, fig. 1; Savage, 1917, p. 113, pl. 5, fig. 5, pl. 9, fig. 1. Zaphrentis ambigua Savage, 1913b, pp. 109, 110, pl. 7, fig. 2; Savage, 1917, p. 149, pl. 9, fig. 2. Streptelasma sp. Elias, 1982a, pp. 56, 57, pl. 4, figs. 4-6. Streptelasma subregulare (Savage, 1913b). Elias, 1982a, pp. 57, 58, pl. 4, figs. 7-22. Holotype. —By original designation: UI X-851 (Sav- age, 1913b, pl. 3, fig. 5; Savage, 1917, pl. 5, fig. 5; Elias, 198 2a, pl. 4, figs. 7, 8), S coll., Cyrene Formation, near Edgewood, Pike County, Missouri. Additional material. —USNM 422831, 422832, in- terval 21-1, EMM coll., USNM 422833422836, in- terval 21-la, EM coll., USNM 422837-422842, inter- val 21-1b, EM coll., USNM 422843-422852, interval 21-lc, EM coll., Ideal Quarry Member, Keel Forma- tion, Section 21 (Rock Crossing), Carter County, Okla- homa; USNM 422853-422856, interval 25-1, EMM coll., Keel Formation, Section 25 (Hunton), Coal County, Oklahoma; USNM 422857, interval 23-1, EMM coll., USNM 422858422909, interval 23-2 (Brevilamnulella beds), EMM coll., USNM 422910- 422912, interval 23-2a, EM coll., USNM 422913, 422914, interval 23-3, EMM coll., USNM 422915- 422932, interval 23-3, EM coll., USNM 422933- 422935, interval 23a-1, EM coll., Keel Formation, Sec- tion 23 (Lawrence Quarry), Pontotoc County, Okla- homa; UI X-6683, X-6684, S coll. [labelled “‘Edge- wood’, ‘“‘near Thebes’’], USNM 422936-422952, interval 31-1, EMZ coll., USNM 422953422965, in- terval 31-la, EMZ coll., USNM 422966, interval 31- 1b, EMZ coll., Leemon Formation, Section 31 (Thebes North), Alexander County, Illinois; USNM 365919, 422967-422978, interval 20-1, EMM coll., USNM 422979422981, interval 20-1, EM coll., USNM 422982, 422983, interval 20-2, EMM coll., USNM 422984422992, interval 20-3, EM coll., USNM 422993, 422994, interval 20-4, EM coll., USNM 422995-423004, interval 20-5, EM coll., UCGM ORDOVICIAN-SILURIAN RUGOSE CORALS: MCAULEY AND ELIAS 35 45616, same interval as 20-3, 20-4, 20-5, El coll., Leemon Formation, Section 20 (Short Farm), Cape Girardeau County, Missourt; UCGM 45618-45634, E1 coll., USNM 365918, 423005—423018, interval 19- 1, EMM coll., USNM 423019-423041, interval 19-2, EMM coll., USNM 423042-423083, interval 19-3, EMM coll., Leemon Formation, Section 19 (New Wells), Cape Girardeau County, Missouri; USNM 423084423103, interval 18-1, EMM coll., USNM 423104—423108, interval 18-1, EM coll., unnamed member, Bryant Knob Formation, Section 18 (Kis- senger), Pike County, Missouri; USNM 423109- 423126, interval 18-2, EMM coll., USNM 423127- 423137, interval 18-3, EMM coll., USNM 423138- 423145, interval 18-3, EM coll., USNM 423146, in- terval 18-4, EM coll., Kissenger Limestone Member, Bryant Knob Formation, Section 18 (Kissenger), Pike County, Missouri; USNM 423147-423183, interval 17-0, EM coll., Kissenger Limestone Member, Bryant Knob Formation, Section 17 (Clarksville), Pike Coun- ty, Missouri; UCGM 45643, USNM 423184, interval 16-0, El coll., unnamed member, Bryant Knob For- mation, Section 16 (Clinton Spring), Pike County, Mis- souri; UCGM 45644, 45645, USNM 423185-423187, interval 16-1, El coll., USNM 423188423234, inter- val 16-1, EMM coll., Kissenger Limestone Member, Bryant Knob Formation, Section 16 (Clinton Spring), Pike County, Missouri; USNM 423235-423241, in- terval 15-1, EMM coll., USNM 423242-423252, in- terval 15-1, EM coll., Kissenger Limestone Member, Bryant Knob Formation, Section 15 (Calumet), Pike County, Missouri; USNM 423253-423280, interval 14-1, EM coll., Kissenger Limestone Member, Bryant Knob Formation, Section 14 (Higginbotham Farm), Pike County, Missouri; USNM 423281-423285, in- terval 13-1, EMM coll., Cyrene Formation, Section 13 (Bowling Green), Pike County, Missouri; UI C-864 [labelled ‘*Zaphrentis’’], S coll., Cyrene Formation [la- belled ‘“‘Edgewood’’], Edgewood, Pike County, Mis- souri; UI C-2258 [labelled “*Zaphrentis cf. subregular- is”), S coll., Wilhelmi Formation [labelled “‘Essex’’], Horse Creek, about 2.4 km east of Essex, Kankakee County, Illinois; UI C-1581 [five specimens labelled “Zaphrentis subregularis”), S coll., Wilhelmi Forma- tion [labelled ““Channahon’’], southeast of Channahon, Will County, Illinois; UI C-1560 [seven specimens la- belled “Zaphrentis channahonensis’’|, UI C-1563 [one specimen, a slab with one specimen, and a slab with three specimens, labelled “‘“Zaphrentis ambigua’’], S coll., Wilhelmi Formation [labelled ““Edgewood”’ and “Channahon’’, respectively], Channahon, Will Coun- ty, Illinois; UI C-1547 [slab with two specimens la- belled “‘Zaphrentis channahonensis’’|, UI C-1561 [10 specimens labelled ‘‘Zaphrentis subregularis’|, UI X-947 [type specimens of Zaphrentis ambigua, one specimen and a slab with five specimens, one figured by Savage, 1913b, pl. 7, fig. 2, and Savage, 1917, pl. 9, fig. 2], S coll., Wilhelmi Formation [labelled ““Edge- wood”’’], near Channahon, Will County, Illinois; UI X-926 [two specimens labelled “Zaphrentis subregu- laris”, one figured by Savage, 1913b, pl. 7, fig. 1, and Savage, 1917, pl. 9, fig. 1], S coll., Wilhelmi Formation [labelled ““Edgewood”’, ““Channahon’’], Will County, Illinois; USNM 423286-423291, interval 4-1, EMM coll., Schweizer Member, Wilhelmi Formation, Sec- tion 4 (Schweizer West), Will County, Illinois; USNM 423292, interval 29-1, EMZ coll., Wilhelmi Forma- tion, Section 29 (Sears Pit), De Kalb County, Illinois: USNM 423293-423296, interval 3-2, EMM coll., USNM 423297-423303, interval 3-3, EM coll., Birds Member, Wilhelmi Formation, Section 3 (Garden Prairie), McHenry County, Illinois; UI C-1619 [six specimens labelled “*Zaphrentis subregularis’’|, S coll., Birds Member, Wilhelmi Formation [labelled “Chan- nahon’’], near Belvidere, Boone County, Illinois; USNM 431136-431166, interval 34-1, E2 coll., Birds Member, Wilhelmi Formation, Section 34 (Belvidere South), Boone County, Illinois; USNM 423304, inter- val 32-0a, EZ coll., USNM 431167-431173, interval 32-1z, E2 coll., USNM 423305, 423306, interval 32-1a, EZ coll., USNM 431174-431177, interval 32-1b, E2 coll., Mosalem Formation, Section 32 (Thomson East), Carroll County, Illinois. Occurrences. —Uppermost Ordovician (Gamach- ian): Keel Formation including Ideal Quarry Member, south-central Oklahoma; Leemon Formation, south- eastern Missouri and southern I]linois; middle Cyrene Formation, northeastern Missouri. Uppermost Or- dovician (?) (Gamachian(?)) to lowermost Silurian (lower Lower Llandovery): Schweizer Member, Wil- helmi Formation, northeastern Illinois. Lowermost Si- lurian (?) (lower Lower Llandovery (?)): Bryant Knob Formation including unnamed member and Kissenger Limestone Member, northeastern Missouri. Lower- most Silurian (lower Lower Llandovery): Birds Mem- ber, Wilhelmi Formation, northeastern Illinois; mid- dle Mosalem Formation, northwestern Illinois. Diagnosis. —Solitary, usually ceratoid. In early stages, major septa generally moderately to greatly dilated and extend to axis, where distal ends join. During inter- mediate to late stages, septal length and dilation de- crease, axial region opens. Cardinal septum commonly conspicuous in late stages. Cardinal fossula usually in- conspicuous, less commonly very broad or distinc- tively shaped. Minor septa typically extend a short distance beyond relatively narrow stereozone. Tabulae generally moderately to widely spaced, commonly de- pressed in cardinal fossula. 36 BULLETIN 333 Description of coralla.—The largest specimen is 102 mm long and 44 mm in diameter at the calice rim (USNM 423146). Growth form is trochoid (PI. 4, fig. 10) to ceratoid (PI. 5, fig. 2) to rarely subcylindrical, and coralla vary from straight to moderately curved (Pl. 4, fig. 4) to gradually curved through 90 degrees (Table 4). Among curved coralla, the cardinal side is typically convex, but there are irregular exceptions (PI. 4, fig. 4). Several coralla have slight to moderate bends in two different directions. A few individuals have sin- gle right-angle bends (Pl. 1, fig. 17), but none have more than one such bend. Corallum shape in trans- verse section is generally circular, but is irregular in a few cases (PI. 2, fig. 6). Well-preserved specimens show prominent septal grooves and interseptal ridges, as well as growth lines. Coarse rugae are present on some in- dividuals. Rejuvenations, if present on a corallum, are few in number and not pronounced. On the convex cardinal side at the apex, two specimens have small grooves (USNM 422910, 422935) and one has a small expansion with a flat surface (USNM 422941). These are interpreted as attachment structures. Depth of the calice is commonly about 50 percent of the corallum length, but varies from 30 percent to perhaps as much as 90 percent in several individuals that have unusually short major septa throughout ontogeny (USNM 423006, 423009, 423025, 423034, 423075). Ontogeny and internal structures. —The relationship between number of septa and corallum diameter is shown in Text-figure 13 and Table 5. Major septa gen- erally extend to or near the axis, where their distal ends meet in small groups, during early ontogenetic stages (EP ly figs. 1) 135 Pls 2. figs TOS RIS Saiiesaeeel Omer 16, Pl. 4, fig. 5, Pl. 5, figs. 3, 7, 9). Septa are shorter in some specimens, but only rarely extend less than half the corallum radius. The septa usually decrease in length gradually during intermediate stages (PI. 1, figs. 2, 4, 6-11, 14, 15, 18, Pl. 2, figs. 1, 5, 11, Pl. 3, figs. 8, 9, 12, 17, Pl. 4, figs. 6, 8, 11, 12, Pl. 5, figs. 4, 5, 8) and late stages (PI. 1, figs. 3, 5, 12, 16, 19, Pl. 2, figs. 2,356; 125,Plh 35 figs: 135 18 Pll 4) fess Ss 7 Osloeele 5, figs. 6, 10) (Text-fig. 14, Table 5). However, they remain long in some coralla, and rapidly decrease in length in others. Septa are relatively straight to wavy in transverse sections. A few septal lobes are present in some individuals (Pl. 1, fig. 15, Pl. 2, fig. 12), but Table 4.—Growth form and curvature of specimens of Streptelasma subregulare (Savage, 1913b), Streptelasma amsdeni, n. sp., Streptelasma leemonense Elias, 1982a (and two specimens of Streptelasma sp. cf. S. leemonense Elias, 1982a, from Section 23), and Keelophyllum okla- homense, n. gen., n. sp. growth form curvature moderately section trochoid ceratoid cylindrical non-curved!' curved? greatly curved® Streptelasma subregulare 21 4 (80%) 1 (20%) 0 (0%) 2 (40%) 3 (60%) 0 (0%) 25 3 (100%) 0 (0%) 0 (0%) 2 (67%) 1 (33%) 0 (0%) 23 44 (80%) 11 (20%) 0 (0%) 26 (47%) 26 (47%) 3 (5%) 31 10 (56%) 8 (44%) 0 (0%) 8 (47%) 6 (35%) 3 (18%) 20 19 (76%) 6 (24%) 0 (0%) 6 (26%) 15 (65%) 2 (9%) 19 26 (29%) 60 (67%) 3 (3%) 42 (48%) 41 (47%) 5 (6%) 18 (intervals 1 and 2) 4 (36%) 7 (64%) 0 (0%) 3 (27%) 7 (64%) 1 (9%) 18 (intervals 3 and 4) 6 (S0%) 6 (50%) 0 (0%) 5 (42%) 7 (58%) 0 (0%) 17 6 (18%) 26 (79%) 1 (3%) 13 (43%) 14 (47%) 3 (10%) 16 8 (20%) 33 (80%) 0 (0%) 20 (50%) 17 (43%) 3 (8%) 15 9 (75%) 3 (25%) 0 (0%) 7 (58%) 5 (42%) 0 (0%) 14 4 (18%) 18 (82%) 0 (0%) 3 (17%) 14 (78%) 1 (6%) 13, Cyrene, and Edgewood 4 (100%) 0 (0%) 0 (0%) 3 (75%) 1 (25%) 0 (0%) 4 and Channahon 15 (56%) 12 (44%) 0 (0%) 6 (22%) 20 (74%) 1 (4%) 34 17 (59%) 12 (41%) 0 (0%) 3 (14%) 17 (81%) 1 (5%) 32 10 (100%) 0 (0%) 0 (0%) 5 (50%) 4 (40%) 1 (10%) Streptelasma amsdeni 24 0 (0%) 25 (53%) 22 (47%) 22 (48%) 23 (50%) 1 (2%) Streptelasma leemonense (and S. sp. cf. S. leemonense) 23, Gale, 20, 15, and 14 9 (43%) 10 (48%) 2 (10)% 8 (38%) 13 (62%) 0 (0%) Keelophyllum oklahomense 25 and 23 1 (20%) 4 (80%) 0 (0%) 3 (60%) 1 (20%) 1 (20%) 0-10° curvature of growth axis. 1 1—70° curvature of growth axis. 71—90° curvature of growth axis. ORDOVICIAN-SILURIAN RUGOSE CORALS: MCAULEY AND ELIAS 37 paliform septal lamellae are rare (Pl. 1, fig. 10). The degree of septal dilation is typically moderate to great in early stages and decreases during ontogeny, but the range among specimens is from nondilated to completely dilated in all stages. The actual thickness of septa generally increases in early to intermediate stages, and then decreases (Text-fig. 15, Table 5). How- ever, in some coralla the septa decrease in thickness throughout ontogeny, and in others their thickness re- mains about the same. The cardinal septum is most commonly inconspic- uous, being the same length and thickness as other major septa (Table 6). However, this septum is distinct if it is relatively short or long, and/or thin or thick. Its prominence varies among coralla, and from stage to stage within some individuals. In many specimens, the cardinal septum becomes relatively short and thin in late stages near the base of the calice. In early to in- termediate stages of a few individuals, the cardinal and counter septa are longer than other major septa (PI. 4, fig. 12). The cardinal fossula is usually inconspicuous, having the same width and shape as other pairs of interseptal spaces. However, it becomes very broad in some coralla (Text-fig. 16, Table 5). Five basic inter- gradational shapes are recognized in transverse sec- tions, as follows: (1) width decreases from periphery toward corallum axis; (2) width constant; (3) width increases toward axis; (4) biconvex with maximum width midway between periphery and axial end: and (5) hourglass-shaped with constriction midway be- tween periphery and axial end (Table 7). Shapes (3) to (5) are especially distinctive in many cases. Minor septa extend a short distance beyond the ste- reozone in ontogenetic stages where major septa are nondilated to slightly dilated. In a few individuals, the minor septa immediately adjacent to the counter sep- D2 50 ‘ ee ee =u) ra) Oo . oe oO 9S oO . oO Oo Coliezis, Oo o} o) (a) = o a” - i fe} 40 Gh +s ae . . o fe) fo) i} ° ® oOo . O2 O2 < Sama aes Th ane 0) = 0 oO ee 3° 2975 25 © OF O 2 OF2 O Gv2e 392 1e) i¢p) = = = 2° 644 5 joa ae < Laveeeaies c Ee: . 9 30 Digit es ei . 239 569338 . 2 © 5 +34 Bean 56 eet fo) 2e Ge 2 39 39238 x re Oo Se es oa Byer ee fea) 20 a roo O2 a 5 : = wate ee : =) : z Leap Sete Os 45 opel o Sections 21 and 19: 112 transverse sections from 57 coralla o Sections 31 and 20: 64 transverse sections from 42 coralla - All other sections: 295 transverse sections from 190 coralla (0) 5 10 15 20 25 30 DIAMETER (mm) Text-figure 13.—Relationship between number of major septa and corallum diameter in Streptelasma subregulare (Savage, 1913b). Data for “All other sections” are from Sections 25, 23, 13-18, 4, 3, 34, and 32. Numbers beside data points indicate frequencies greater than one. Arbitrary line was used to derive proportions shown in part of Table 5. oy) oo Table 5.— Frequency of data points above (A), or on and below (B), arbitrary lines drawn in Text-figures 13-16 and 18-21, for characteristics BULLETIN 333 of major septa and cardinal fossula in Streptelasma subregulare (Savage, 1913b) and Streptelasma amsdeni, n. sp. number of septa length of septa thickness of septa width of fossula section A B A B A B A B Streptelasma subregulare 21 3 (12%) 22 (88%) 1 (4%) 24 (96%) 12 (48%) 13 (52%) 0 (0%) 25 (100%) 25 0 (0%) 3 (100%) 0 (0%) 2 (100%) 1 (50%) 1 (50%) 0 (0%) 1 (100%) 23 8 (29%) 35 (81%) 13 (25%) 40 (75%) 23 (45%) 28 (55%) 1 (2%) 40 (98%) 31 17 (68%) 8 (32%) 0 (0%) 25 (100%) 4 (16%) 21 (84%) 3 (14%) 19 (86%) 20 30 (77%) 9 (23%) 8 (20%) 33 (80%) 9 (22%) 32 (78%) 1 (3%) 34 (97%) 19 8 (9%) 79 (91%) 5 (5%) 89 (95%) 11 (12%) 81 (88%) 31 (38%) 51 (62%) 18 (interval 1) 5 (36%) 9 (64%) 1 (7%) 13 (93%) 10 (71%) 4 (29%) 9 (69%) 4 (31%) 18 (interval 2) 6 (38%) 10 (63%) 1 (6%) 15 (94%) 10 (63%) 6 (38%) 5 (36%) 9 (64%) 18 (intervals 3 and 4) 9 (32%) 19 (68%) 20 (69%) 9 (31%) 20 (69%) 9 (31%) 7 (26%) 20 (74%) 17 29 (59%) 20 (41%) 6 (11%) 47 (89%) 27 (51%) 26 (49%) 2 (4%) 52 (96%) 16 12 (33%) 24 (67%) 9 (20%) 36 (80%) 27 (61%) 17 (39%) 2 (5%) 38 (95%) 15 3 (23%) 10 (77%) 3 (23%) 10 (77%) 6 (46%) 7 (54%) 0 (0%) 13 (100%) 14 7 (29%) 17 (71%) 5 (27%) 25 (83%) 21 (72%) 8 (28%) 2 (8%) 23 (92%) 13 3 (43%) 4 (57%) 0 (0%) 7 (100%) 2 (29%) 5 (71%) 0 (0%) 6 (100%) 4 5 (45%) 6 (55%) 1 (14%) 6 (86%) 3 (33%) 6 (67%) 1 (11%) 8 (89%) 3 3 (43%) 4 (57%) 1 (11%) 8 (89%) 4 (44%) 5 (56%) 0 (0%) 5 (100%) 34 6 (23%) 20 (77%) 0 (0%) 30 (100%) 11 (38%) 18 (62%) 1 (7%) 14 (93%) 32 8 (44%) 10 (56%) 1 (6%) 16 (94%) 1 (6%) 16 (94%) 0 (0%) 14 (100%) Streptelasma amsdeni 24 2 (4%) 52 (96%) 4 (8%) 49 (92%) 0 (0%) 52 (100%) 8 (17%) 38 (83%) tum are anomalous in being longer than the other mi- nor septa, and in some of these are almost as long as the major septa (Elias, 1982a, pl. 4, fig. 13; Pl. 2, figs. 2, 3, 12). Thickness of the stereozone ranges from 5 to 15 percent of the corallum radius. Specimens with thick septa tend to have thick stereozones. Tabulae are commonly complete and thin (PI. 2, figs. 7-9: Pl. 3, figs. 2-6, 14, 15). They are usually convex upward in the septal region, and less commonly flat or rarely concave upward. In the axial region, they are concave or flat. Tabulae are generally moderately to widely spaced, but in some cases are very closely spaced (Text-fig. 17). They are usually depressed in the car- dinal fossula (Elias, 1982a, pl. 4, fig. 7; Pl. 2, figs. 8, 9). Microstructure. —In transverse thin sections, the ma- jor septa are fibrous in all ontogenetic stages. However, fibers are difficult to discern if septa are thin. The fibers originate at the median line within the septum, and curve outward in the direction of the corallum axis so that their convex sides face axially. Where the major septa are greatly to completely dilated, minor septa appear as triangular wedges between them (PI. 5, fig. 1). Where major septa are nondilated to slightly dilat- ed, U-shaped lamellae with concave sides facing the corallum axis form the stereozone between adjacent major and minor septa (PI. 3, fig. 1). A contorted suture extends through the lamellae in a medial position be- tween adjacent septa. The epitheca is composed of short fibers oriented approximately perpendicular to the outer surface of the corallum. In longitudinal thin sections, septal fibers are inclined from the periphery of the corallum toward the axis at an angle of about 40 degrees. Discussion. —Savage (1913a) mentioned ‘‘Zaphren- tis subregularis” and “*Zaphrentis ambigua” in his text and tables, but descriptions and illustrations of these species were published later (Savage, 1913b, 1917). Elias (1982a, p. 57) established that Z. ambigua Sav- age, 1913b, is a synonym of Z. subregularis Savage, 1913b, and assigned the species to Streptelasma Hall, 1847. *Zaphrentis channahonensis, n. sp.”’ was listed in a table by Savage (1912, pp. 98, 99), but he did not provide a description or illustrations, and the species was not mentioned in subsequent literature. Therefore, this is a nomen nudem. Specimens labelled “*Zaphren- tis channahonensis” by Savage were examined in our study. They are from the same unit and location as others that he identified as Zaphrentis subregularis and Zaphrentis ambigua. Thin sections of one reveal that it is Streptelasma subregulare (Savage, 1913b) (Pl. 5, figs. 3-5). A small, incomplete corallum from the Leemon For- mation at Section 20 (Short Farm) was identified by Elias (1982a, pp. 56, 57) as Streptelasma sp. Compar- ison with the numerous specimens subsequently ob- tained at that locality indicates that it is Streptelasma subregulare (Savage, 1913b). The continuous spectrum of values for the numerous ORDOVICIAN-SILURIAN RUGOSE CORALS: MCAULEY AND ELIAS 39 = o ° © ° @ 9 N Section 18 (intervals 3 and 4): 35 transverse sections from 14 coralla LENGTH OF MAJOR SEPTA = CORALLUM RADIUS ° @ —— a 0.9 ZZ ZO ° a LENGTH OF MAJOR SEPTA = CORALLUM RADIUS ° > 0.3 0.2 All other sections: | 291 transverse sections from 123 coralla te) 5 10 25 30 15 20 DIAMETER (mm) Text-figure 14.—Relationship between length of major septa and corallum diameter in Streptelasma subregulare (Savage, 1913b), for specimens yielding more than one transverse thin section. The length of a typical septum was measured and divided by the corallum radius, yielding a ratio less than or equal to 1.0. Data for “All other sections” are from Sections 21, 23, 31, 20, 19, 18 (intervals 1 and 2), 13-17, 4, 3, 34, and 32. Arbitrary line (dotted) was used to derive proportions shown in part of Table 5. =) Text-figure 15.—Relationship between thickness of major septa and corallum diameter in Streptelasma subregulare (Savage, 1913b), for specimens yielding more than one transverse thin section. Septal thickness was measured in transverse thin sections halfway between the axial and peripheral ends of a typical septum on the counter side (usually the counter septum). Data for “All other sections” are from Sections 21, 23, 15-17, 13, 4, 3, and 34. Arbitrary line (dotted) was used to derive proportions shown in part of Table 5. 0.8 0.6 ° a SEPTAL THICKNESS (mm) °o °o e a 0.2 ° > SEPTAL THICKNESS (mm) ° @ 0.2 Sections 18 and 14 50 traneverse sections from 20 coralla 0.8 0.7 ° a ° a SEPTAL THICKNESS (mm) ° ° e > 0.2 Sections 31, 20, 19 and 32: 136 transverse sections from 57 coralla All other sections: 156 transverse sections from 67 coralla Ps en A LEY \e 5 10 256 30 15 20 DIAMETER (mm) 40 BULLETIN 333 Table 6.—Length and thickness of cardinal septum, compared with other typical major septa, in transverse thin sections of Streptelasma subregulare (Savage, 1913b) and Streptelasma amsdeni, n. sp. length of cardinal septum thickness of cardinal septum section shorter same longer thinner same thicker Streptelasma subregulare 21 1 (4%) 8 (35%) 14 (61%) 7 (29%) 10 (42%) 7 (29%) 23 5 (12%) 20 (48%) 17 (40%) 16 (33%) 26 (54%) 6 (13%) 31 4 (16%) 3 (12%) 18 (72%) 15 (60%) 9 (36%) 1 (4%) 20 4 (10%) 16 (41%) 19 (49%) 9 (23%) 28 (72%) 2 (5%) 19 19 (22%) 34 (40%) 32 (38%) 23 (27%) 57 (68%) 4 (5%) 18 (intervals | and 2) 5 (20%) 11 (44%) 9 (36%) 10 (37%) 15 (56%) 2 (7%) 18 (intervals 3 and 4) 5 (17%) 19 (66%) 5 (17%) 6 (21%) 20 (69%) 3 (10%) 17 8 (16%) 31 (63%) 10 (20%) 18 (37%) 25 (51%) 6 (12%) 16 5 (15%) 27 (79%) 2 (6%) 7 (21%) 24 (71%) 3 (9%) 15 1 (10%) 8 (80%) 1 (10%) 2 (20%) 7 (70%) 1 (10%) 14 3 (12%) 19 (73%) 4 (15%) 6 (23%) 17 (65%) 3 (12%) 13 2 (40%) 2 (40%) 1 (20%) 2 (40%) 2 (40%) 1 (20%) 4 1 (11%) 3 (33%) 5 (56%) 3 (33%) 5 (56%) 1 (11%) 3 0 (0%) 6 (100%) 0 (0%) 1 (17%) 3 (50%) 2 (33%) 34 5 (25%) 8 (40%) 7 (35%) 12 (60%) 7 (35%) 1 (5%) 32 4 (25%) 4 (25%) 8 (S0%) 7 (44%) 9 (56%) 0 (0%) Streptelasma amsdeni 24 0 (0%) 26 (53%) 23 (47%) 0 (0%) 48 (98%) 1 (2%) Table 7.—Frequency of each type of shape for the cardinal fossula in transverse thin sections of Streptelasma subregulare (Savage, 1913b) and Streptelasma amsdeni, n. sp. In cases where the shape on either side of a fossula is different, each side was counted as one-half. Fossula shapes (1-5) are described on p. 37. fossula shape section (1) (2) (3) (4) (5) Streptelasma subregulare 21 1.0 (4%) 11.5 (46%) 3.5 (14%) 7.0 (28%) 2.0 (8%) 23 6.0 (15%) 15.0 (37%) 8.5 (21%) 8.5 (21%) 3.0 (7%) 31 2.5 (12%) 6.5 (31%) 1.5 (7%) 7.0 (33%) 3.5 (17%) 20 0.5 (1%) 10.0 (28%) 6.0 (17%) 12.5 (35%) 7.0 (19%) 19 17.0 (21%) 17.5 (21%) 15.5 (19%) 18.5 (23%) 13.5 (16%) 18 (intervals | and 2) 2.5 (9%) 7.5 (28%) 6.0 (22%) 3.5 (13%) 7.5 (28%) 18 (intervals 3 and 4) 1.5 (6%) 12.0 (44%) 7.0 (26%) 1.0 (4%) 5.5 (20%) 17 16.0 (31%) 20.5 (39%) 7.5 (14%) 5.0 (10%) 3.0 (6%) 16 24.0 (56%) 7.5 (17%) 8.5 (20%) 1.0 (2%) 2.0 (5%) 15 0.0 (0%) 4.0 (31%) 4.0 (31%) 1.0 (8%) 4.0 (31%) 14 6.0 (24%) 4.0 (16%) 6.5 (26%) 3.0 (12%) 5.5 (22%) 13 0.0 (0%) 0.5 (8%) 3.0 (50%) 1.0 (17%) 1.5 (8%) 4 0.0 (0%) 1.0 (11%) 2.5 (28%) 2.0 (22%) 3.5 (39%) 3 1.0 (20%) 1.0 (20%) 3.0 (60%) 0.0 (0%) 0.0 (0%) 34 0.5 (3%) 7.0 (46%) 1.0 (7%) 4.0 (27%) 2.5 (17%) 32 0.5 (4%) 7.0 (54%) 0.5 (4%) 1.0 (7%) 4.0 (31%) Streptelasma amsdeni 24 19.0 (42%) 4.5 (10%) 1.0 (2%) 7.5 (17%) 13.0 (29%) features measured and compared in our study indicates that there are no morphologic discontinuities among the 589 specimens described above. Those character- istics exhibiting anomalous trends at certain localities or in particular stratigraphic intervals (Table 3) fall within or overlap the range of values for coralla from elsewhere. We conclude that Streptelasma subregulare (Savage, 1913b) is a highly variable species. An im- portant confirmation of this intraspecific variability is the great range of characteristics developed during the ontogeny of some individuals. Solitary coralla from the Keel Formation at Section 24 (Coal Creek) generally lie within the range of vari- ability for this taxon, but most of the individual char- acteristics are atypical (Table 3). They are considered to represent a different species, Streptelasma amsdeni, n. sp. Some specimens of Streptelasma subregulare (Sav- ORDOVICIAN-SILURIAN RUGOSE CORALS: MCAULEY AND ELIAS age, 1913b) with moderately dilated septa closely re- semble the lectotype of Streptelasma corniculum Hall, 1847, from the Trenton Limestone (upper Middle Or- dovician) of New York (Neuman, 1969, pp. 10, 11, figs. 5, 6). The number of septa, width of the cardinal fossula, and spacing of tabulae in the latter corallum are about average for S. subregulare. However, the major septa are slightly longer than usual, and the de- gree of septal dilation is greater on the cardinal side than on the counter side. Further comparison is not possible at this time because the range of variability in S. corniculum is unknown. Some representatives of Streptelasma subregulare (Savage, 1913b) with thin, wavy septa are similar to the following taxa: Streptelasma affine (Billings, 1865) 41 from the Vauréal Formation (Richmondian) and Ellis Bay Formation (Gamachian) on Anticosti Island, Qué- bec (Bolton, 1981, pl. 3, figs. 3-8; Elias, 1982a, pp. 59, 60, pl. 5, figs. 4-18); Streptelasma primum (Wedekind, 1927) in Stage 5a (Ashgill) of Norway, the Boda Lime- stone (Ashgill; Hirnantian) of Sweden, and the Pirgu Stage (Ashgill) of the Estonian S.S.R. (Neuman, 1969, pp. 11-17, figs. 7a—c, e-h, 8-10; see also Neuman, 1975, pp. 357, 358); Streptelasma cf. primum (Wede- kind, 1927) from boulders (Upper Ordovician; sub- Hirnantian, possibly Pirguan) in Sweden (Neuman, 1986, pp. 353, 355, 358, figs. 5-8); and Streptelasma unicum Neuman, 1975, from the Da/manitina Beds (Ashgill; Hirnantian) or possibly the lowermost Llan- dovery of Sweden (Neuman, 1975, pp. 353, 356-358, fo) fo) 5 ° o Sections 19 and 18: 136 transverse sections from 70 coralla 3 eS o Sn é fo) — Streptelasma subreguiare ———+ 26 20 . Strepte/asma amsdeni NUMBER OF TABULAE PER CENTIMETER OF CORALLUM LENGTH a . 214) S15 20 19" 136 20716) 164 ta o4enc2 eee SECTION Text-figure |7.— Number of tabulae (counted along corallum axis) per cm of corallum length in longitudinal thin sections of Strepte- lasma subregulare (Savage, 1913b) and Streptelasma amsdeni, n. sp. Numbers beside data points indicate frequencies greater than one. early stages, a feature that is rare in S. subregulare. The cardinal septum and fossula are inconspicuous in late stages. Borelasma spp. a and b were reported from Stages 6b and 6c (Llandovery; Rhuddanian—Idwian) and Stage 6c (Idwian), respectively, of Norway (Neu- man, 1982, pl. 1, figs. 14-19). They are similar to B. crassitangens, but the cardinal septum becomes short and a fossula is developed in late stages. Coralla of Streptelasma subregulare (Savage, 1913b) with a few distantly spaced tabulae, and thick septa that are greatly to completely dilated in early stages, resemble Ullernelasma svartoeyensis Neuman, 1975, from Stage 5b (Ashgill; Hirnantian) and Stage 6a (low- ermost Llandovery; Rhuddanian) of Norway (Neu- man, 1975, pp. 348, 350-353, figs. 10-13; Neuman, 1982, p. 34). In U. svartoeyensis, the cardinal septum is distinct and becomes short in late stages, and the cardinal fossula is conspicuous. Those features are less common in similar individuals of S. subregulare. Neu- man noted that a few incomplete tabulae seem to be present in U. svartoeyensis. Tabulae in S. subregulare are complete, but their presence is not always revealed in transverse sections of coralla in which they are widely spaced and approximately horizontal. Solitary Rugosa from the Guanyingiao Beds (Dal- manitina Beds) (Ashgill; Hirnantian) in the northern Guizhou Province of China have been documented by He (1978, 1985). The following illustrated specimens resemble some coralla of Streptelasma subregulare (Savage, 1913b), but the available information is in- sufficient for a thorough comparison of these taxa: Streptelasma insolitum He, 1978, pl. 1, figs. la—d, 2a, b. Streptelasma cf. distinctum Wilson, 1926, of He, 1978, pl. 1, fig. 3a-d. Streptelasma sp. of He, 1978, pl. 3, fig. la-e. Brachyelasma lindstroemophylloides He, 1978, pl. 2, figs. la-g, 3a— c; He, 1985, pl. 1, fig. 4a—c. Brachyelasma irregularis He, 1978, pl. 2, figs. 2a—d, 4a-c. Brachyelasma primum (Wedekind, 1927) of He, 1978, pl. 3, figs. 2a-c, 3a, b. Brachyelasma yentzekouense He, 1985, pl. 1, figs. la-g, 2a—c, 3a-e, 5a, b. Borelasma cornicum He, 1978, pl. 5, fig. la-f. Borelasma sinensis He, 1978, pl. 5, figs. 2a—e, 3a-c, 4a-c, Sa, b. Crassilasma polytabulatum He, 1985, pl. 5, figs. la, b, 7a, b; see also He, 1978, pl. 7, figs. la-g, 2a-c. Streptelasma species cf. S. subregulare (Savage, 1913b) Plate 5, figures 11, 12 Material. —USNM 423307, interval 33-1, C coll. Occurrence. —Uppermost Ordovician (Gamachian): Cason shale, Section 33 (Buffalo River), Searcy Coun- ty, western north-central Arkansas. Description. —This small specimen is incomplete and the outer wall is not preserved. The major septa are nondilated and withdrawn from the axis, leaving an ORDOVICIAN-SILURIAN RUGOSE CORALS: MCAULEY AND ELIAS 43 open axial region possibly with several septal lobes. The cardinal septum appears to be shorter than the other major septa. Discussion. —The septal arrangement of this speci- men appears to lie within the range of variability doc- umented herein for Streptelasma subregulare (Savage, 1913b). However, because it is incomplete, poorly pre- served, and the only corallum of this kind known from north-central Arkansas, it cannot be positively iden- tified. We therefore refer to it as Streptelasma sp. cf. S. subregulare (Savage, 1913b). Streptelasma amsdeni, new species Plate 5, figures 13-22; Plate 6, figures 1-11 Derivation of name.—The species name honors Thomas W. Amsden of the Oklahoma Geological Sur- vey, who collected some of the specimens described herein. Holotype. —USNM 423308, interval 24-2, EMM coll. Paratypes.—USNM 423309, interval 24-1, EMM coll., USNM 365920, 423310-423312, interval 24-2, A coll., USNM 423313423317, interval 24-2, EMM coll. Additional material. —USNM 423318423320 [three specimens on slab], 423321—423329, interval 24-2, A coll., USNM 423330423377, interval 24-2, EMM coll. Occurrence. —Uppermost Ordovician (Gamachian): lower o6litic and middle laminated calcilutite units (intervals 24-1 and 24-2, respectively) of the Keel For- mation, Section 24 (Coal Creek), Pontotoc County, south-central Oklahoma. ee e 54 transverse sections from 26 coralla z 10) 5 15 20 10 DIAMETER (mm) Text-figure 18.— Relationship between number of major septa and corallum diameter in Streptelasma amsdeni, n. sp. Numbers beside data points indicate frequencies greater than one. Arbitrary line [in same position as that for Streptelasma subregulare (Savage, 1913b) in Text-fig. 13] was used to derive proportions shown in part of Table 5. Diagnosis. —Solitary, slender ceratoid to cylindrical. Major septa thin, nondilated to slightly dilated and extend to axis in early stages, gradually withdraw from axis during ontogeny. Cardinal septum indistinct to relatively long and conspicuous. Cardinal fossula usu- ally inconspicuous, less commonly distinctively shaped. Minor septa typically extend a short distance beyond very narrow stereozone. Tabulae widely spaced. Description of coralla.—The greatest observed length and diameter are 155 mm (USNM 423319, incomplete at both ends; PI. 6, fig. 10) and 23 mm (USNM 423377, incomplete calice), respectively. In early ontogenetic stages, the majority of coralla are slenderly ceratoid in form, with moderate or slight curvature. The cardinal side is not always convex (Pl. 6, fig. 1). The typical 0.9 0.8 0.7 0.6 0.5 0.4 LENGTH OF MAJOR SEPTA += CORALLUM RADIUS 0.3 153 transverse sections from 26 coralla ie) 5 10 15 20 DIAMETER (mm) Text-figure 19.—Relationship between length of major septa and corallum diameter in Streptelasma amsdeni, n. sp. The length of a typical septum was measured and divided by the corallum radius, yielding a ratio less than or equal to 1.0. Lines join data points from individual specimens. Numbers beside data points indicate fre- quencies greater than one. Arbitrary line [dotted, in same position as that for Streptelasma subregulare (Savage, 1913b) in Text-fig. 14] was used to derive proportions shown in part of Table 5. 44 BULLETIN 333 adult form is long and cylindrical, but some remain slenderly ceratoid (Table 4). Specimens are generally relatively straight, but have slight to moderate bends as well as several constrictions and rejuvenations (PI. 6, fig. 10). A few have right-angle bends. Periodicity of growth has been observed only in the straight por- tion of one individual, where six successive constric- tions are spaced 6 to 7 mm apart (average, 6.8 mm; Pl. 6, fig. 11). Septal grooves and interseptal ridges, as well as growth lines, are preserved on all specimens. Attachment structures are not present. Depth of the calice is estimated to be about 20 percent of the cor- allum length in small individuals, and is probably less than 10 percent in adults. Ontogeny and internal structures. —The relationship between number of septa and corallum diameter is shown in Text-figure 18. In early ontogenetic stages (Pl. 5, figs. 17-20, Pl. 6, fig. 5), major septa extend to or almost to the axis, where they meet. The septa with- draw from the axis, leaving an open axial region, during intermediate stages (Pl. 5, figs. 14-16, 21, Pl. 6, figs. 6, 7). They shorten to half the corallum radius or less in late stages (Pl. 5, fig. 22, Pl. 6, fig. 8; Text-fig. 19). Major septa are generally straight to slightly curved in early stages, and become wavy by late stages in many individuals. They are thin throughout ontogeny (Text- fig. 20: Pl. 6, fig. 9), but in some cases are slightly dilated in early stages. Compared with other major septa, the cardinal sep- tum is the same length or longer, and almost always the same thickness, during all stages (Table 6). The ~_- E ~ 0.3 no ” e e 2 YO 24 O \ = . = =I Oal4 20 ©5040 © 04 0) 5 10 15 20 DIAMETER (mm) Text-figure 20.—Relationship between thickness of major septa and corallum diameter in Streptelasma amsdeni, n. sp. Septal thick- ness was measured in transverse thin sections halfway between the axial and peripheral ends of a typical septum on the counter side (usually the counter septum). Changes in septal thickness within individual specimens are shown by lines joining data points. Num- bers beside data points indicate frequencies greater than one. Arbitrary line [dotted, in same position as that for Streptelasma subregulare (Savage, 1913b) in Text-fig. 15] was used to derive proportions shown in part of Table 5. cardinal fossula is about the same width as other pairs of interseptal chambers in most specimens, but is somewhat wider in intermediate and late stages of a few (Text-fig. 21). Shapes of the fossula are as described for Streptelasma subregulare (Savage, 1913b) (Table 7). Minor septa generally project a short distance be- yond the very narrow stereozone. Throughout ontogeny, tabulae are usually complete, very thin, and relatively widely spaced (PI. 5, fig. 13, Pl. 6, figs. 2-4; Text-fig. 17). They are most commonly convex upward in the septal region, but some are flat. In the axial region, they are flat to concave upward. Microstructure.—The microstructure in transverse and longitudinal thin sections is the same as described for specimens of Streptelasma subregulare (Savage, 1913b) having nondilated to slightly dilated septa. Discussion. — There is relatively little variation among the 71 coralla described above. Morphologic charac- teristics generally lie within the range of variability in Streptelasma subregulare (Savage, 1913b), but are not typical of that species (Table 3). The most striking features are the long, cylindrical growth form and thin septa. We consider these coralla to represent a distinct taxon, Streptelasma amsdeni, n. sp., because of these consistent differences. In previous paleobiologic stud- ies, this species was referred to as Streptelasma sp. (Elias, 1984a, tables 1, 3-5; Elias, 1984b, pp. 535, 536). Streptelasma amsdeni, n. sp., resembles Streptelas- ma primum (Wedekind, 1927), Streptelasma cf. pri- mum (Wedekind, 1927) of Neuman, 1986, and Strep- telasma unicum Neuman, 1975, which were discussed 2e e 46 transverse sections from 25 coralla WIDTH OF CARDINAL FOSSULA (mm) 0 5 10 15 20 DIAMETER (mm) Text-figure 21.—Relationship between width of cardinal fossula and corallum diameter in Streptelasma amsdeni, n. sp. Width of cardinal fossula was measured between median lines of septa im- mediately adjacent to the fossula, midway between axial and pe- ripheral ends of septa bounding the fossula. Numbers beside data points indicate frequencies greater than one. Arbitrary line [in same position as that for Streptelasma subregulare (Savage, 1913b) in Text-fig. 16] was used to derive proportions shown in part of Table 5. ORDOVICIAN-SILURIAN RUGOSE CORALS: MCAULEY AND ELIAS 45 under Streptelasma subregulare (Savage, 1913b). How- ever, the new species is distinct in having more widely spaced tabulae. The cardinal septum does not become short, as in S. unicum. The following illustrated specimens from the Gua- nyingiao Beds (Dalmanitina Beds) (Ashgill; Hirnan- tian) in the northern Guizhou Province of China have a septal arrangement, tabulae, and thin stereozone that resemble Streptelasma amsdeni, n. sp., but the septa are thicker than in the latter species: Brachyelasma? simplotabulatum He, 1978, pl. 2, figs. Sa, b, 6. Paramplexoides cylindricus He, 1978, pl. 9, figs. la-g, 5, 6, pl. 10, figs. la—c, 2a, b, 3a, b, 4a-c, 5, 6. Paramplexoides attenuatum He, 1978, pl. 9, figs. 7a—-c, 8a, 9. Streptelasma leemonense Elias, 1982a Plate 7, figures 1-14; Plate 8, figures 1-4; Plate 9, figures 1, 2 Streptelasma leemonense Elias, 1982a, p. 56, pl. 4, figs. 1-3. Holotype. —UCGM 45614 (Elias, 1982a, pl. 4, figs. 1, 2), same interval as 20-3, 20-4, 20-5, El coll., Lee- mon Formation, Section 20 (Short Farm), Cape Gi- rardeau County, Missouri. Paratype.—UCGM 45615 (Elias, 1982a, pl. 4, fig. 3), same interval as 20-3, 20-4, 20-5, El coll., Leemon Formation, Section 20 (Short Farm), Cape Girardeau County, Missouri. Additional material. —USNM 423378, 423379, in- terval 23-2 (Brevilamnulella beds), EMM coll., USNM 423380, interval 23-2a, EM coll., USNM 423381- 423389, interval 23-3, EMM coll., USNM 423390, 423391, interval 23-3, EM coll., USNM 423392, 423393, interval 23a-1, EM coll., Keel Formation, Sec- tion 23 (Lawrence Quarry), Pontotoc County, Okla- homa; USNM 423394, interval 33-1, C coll., Cason shale, Section 33 (Buffalo River), Searcy County, Ar- kansas; USNM 423395, interval 31-1, EMZ coll., USNM 423396, interval 31-1b, EMZ coll., Leemon Formation, Section 31 (Thebes North), Alexander County, Illinois; UI C-1448a, S coll., Leemon For- mation [labelled ““Edgewood”’], Gale Section, Alex- ander County, Illinois; USNM 423397, 423398, in- terval 20-1, EMM coll., USNM 423399, 423400, interval 20-1, EM coll., USNM 423401, interval 20- 3, EM coll., USNM 423402, interval 20-4, EM coll., Leemon Formation, Section 20 (Short Farm), Cape Girardeau County, Missouri; USNM 423403, 423404, interval 15-1, EM coll., Kissenger Limestone Member, Bryant Knob Formation, Section 15 (Calumet), Pike County, Missouri; USNM 423405-423407, interval 14-1, EM coll., Kissenger Limestone Member, Bryant Knob Formation, Section 14 (Higginbotham Farm), Pike County, Missouri. Occurrences. —Uppermost Ordovician (Gamach- ian): Keel Formation, south-central Oklahoma; Cason shale, western north-central Arkansas; Leemon For- mation, southern Illinois and southeastern Missouri. Lowermost Silurian (?) (lower Lower Llandovery (?)): Kissenger Limestone Member, Bryant Knob Forma- tion, northeastern Missouri. Diagnosis. —Solitary or with peripheral offsets, gen- erally ceratoid. Major septa typically thin, extend to or almost to axis throughout ontogeny. Cardinal sep- tum indistinct to relatively long, cardinal fossula usu- ally inconspicuous. Minor septa at least half the length of major septa, commonly contraclined to contratin- gent, extend beyond narrow to very broad stereozone. Tabulae moderately to closely spaced. Description of coralla.—The greatest observed length and diameter are 55 mm (USNM 423387, incomplete at both ends) and 20 mm (USNM 423401, at base of calice), respectively. Of 12 individuals, six are ceratoid, three are trochoid, and three are cylindrical (PI. 7, fig. 1). Two are straight and ten are slightly to moderately curved; the cardinal side is not always convex (PI. 7, fig. 1). Corallum shape in transverse section Is generally circular, but is irregular in some cases (PI. 8, fig. 4). The presence of septal grooves and interseptal ridges has been verified in transverse thin sections of speci- mens enclosed in matrix. The measured depth of the calice is approximately 10 percent of the corallum length, but calice rims in those individuals are poorly preserved and may be incomplete. Clusters of up to about 20 individuals were collected at Sections 23, 20, and 15 (USNM 423391, 423401, 423403). Smaller clusters were also found, consisting of at least six coralla at Section 14 and five at Section 20 (USNM 423405, 423400). Enclosure in matrix or poor preservation makes it difficult to establish wheth- er they represent true colonies or pseudocolonies. Evi- dence from one large cluster and several other coralla indicates that budding did occur (USNM 423378, 423382, 423401; Pl. 8, figs. 1-4). Definite peripheral offsets are located between the older and younger walls at constrictions, and are initially oriented approxi- mately parallel to the protocorallite. Some diverge out- ward in later stages. Offsets are found on alar, counter, and cardinal quadrants of the parents with about equal frequency. One specimen has at least seven at the same height, and they are distributed over about 75 percent of the circumference (USNM 423382). Some offsets become quite large. In one cluster, several of the big individuals with numerous offsets began as buds them- selves (Pl. 8, figs. 1-4). Other specimens could represent budding or a gre- garious, epizoic habit (Pl. 7, fig. 9). In two clusters, several relatively small coralla located on only one side of a larger individual are oriented approximately per- pendicular to it (USNM 423400, 423403). They may 46 BULLETIN 333 have used an exposed side as a substrate for attach- ment. In other cases, small coralla occur on the exterior of a large individual that lacks constrictions at those sites (e.g, USNM 423384). They are usually oriented at high angles to the growth axis of the host or pro- tocorallite. Peripheral offsets and/or epizoic individ- uals are present in approximately 25 percent of the specimens studied herein. Ontogeny and internal structures. —The relationship between number of septa and corallum diameter is shown in Text-figure 22. Major septa extend to or al- most to the axis, where they commonly meet in small groups, in early ontogenetic stages (Pl. 7, figs. 11, 14, Pl. 8, figs. 1, 2), intermediate stages (Pl. 7, figs. 3, 5- 7,9, 12, Pl. 8, fig. 3), and late stages (Pl. 7, figs. 4, 10, 13, Pl. 8, fig. 4). A counterclockwise axial whorl is present in some specimens. The septa are usually slightly curved to wavy. They are generally thin, but can be slightly to (rarely) greatly dilated in early stages. The cardinal septum is as long as or longer, and in some cases slightly thicker, than other major septa. The cardinal fossula is usually inconspicuous, having the same width and shape as other pairs of interseptal chambers, but in some coralla it is slightly wider or biconvex in transverse section. The length of minor septa is generally about 50 percent of the corallum radius, but varies from 30 to 70 percent. Minor septa are commonly contraclined to contratingent. 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