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Kansas LIBRARY OCCASIONAL PAPERS 2 4 1992 of the MUSEUM OF NATURAL HISTORY The University of Kansas Lawrence, Kansas number 148, pages 1-11 17 june 1992 Comments on North American Fossil Zapodidae (Rodentia: Mammalia) with Reference to Megasminthus, Plesiosminthus, and Schaubeumys Morton Green1 ABSTRACT Megasminthus tiheni Klingener from the Bijou Hills, South Dakota, is redescribed. Evidence is presented that the alleged character differences between Plesiosminthus and Schaubeumys are not valid and that the latter name should not be used. Parasminthus may be present in the North American Miocene. Key Words: Zapodidae. Miocene, Megasminthus, Plesiosminthus. Schaubeumys. The purposes of this paper are to amplify knowledge of Megasminthus tiheni Klingener based on material from the Bijou Hills local fauna (Green, 1 985 ). South Dakota, and to discuss the validity of the genera Plesiosminthus and Schaubeumys and their occurrence in North America and Europe. The possibility of a migration of Parasminthus between Eurasia and North America in the Miocene also is considered. Green (1977:1008) referred specimens from SDSM Loc. V731, Bijou Hills local fauna, to Megasminthus tiheni: this site later was designated as the F.O. Quarry (Green, 1985:141). Herein, I deal with a few additional speci- mens from the F.O. Quarry, a larger number of specimens from the nearby F.U. Quarry, and specimens of M. tiheni from the Egelhoff Locality, Nebraska. The availability of these specimens allows a more accurate assessment of variations in dental pattern than was possible in 1977. •Division of Vertebrate Paleontology, Museum of Natural History, The University of Kansas, Lawrence, Kansas 66045-2454, USA. © Museum of Natural History. The University of Kansas. Lawrence. ISSN 0091-7958 2 UNIV. KANSAS MUS. NAT. HIST. OCC. PAP. No. 148 Wilson ( 1 960) reviewed the question of the generic validity of Schaubeumys and opted to assign it subgeneric status within the genus Plesiosminthus. Green (1972) placed Schaubeumys in the synonymy of Plesiosminthus and Engesser (1979) subsequently reestablished Schaubeumys as a valid genus. Although a number of authors have accepted Engesser 's taxonomic arrange- ment, I remain unconvinced of the validity of their rationale and address this issue herein. MATERIALS AND METHODS In an earlier zapodid paper. Green ( 1977) used the dental terminology recommended by Wood and Wilson ( 1 936). Their paper not only established names for the major cusps in rodents, but also for the various enamel ridges that connect the cusps; this terminology generally has been accepted, espe- cially by North American workers. The terminological changes suggested by Lindsay ( 1988) are not followed here because I think that substitution of the term "mure" for ectoloph and ectolophid adds confusion. Measurements in millimeters were made using an Olympus binocular microscope with a Daedal two-axis linear motion stage having rotary motion positioning. Accurate measurements to 0.01 mm were possible. Each tooth was oriented as closely as possible in the same plane. Chi-square tests of independence were obtained using MINITAB. Specimens of Plesiosminthus grangeri referred to in this paper are those from Green (1977). Abbreviations: SDSM, Museum of Geology, South Dakota School of Mines and Technology; BBQII, Black Bear Quarry II V-673; F:AM, Frick Collection, American Museum of Natural History; Fm., formation; Loc, locality; SD, standard deviation; CV, coefficient of variation; AP, anteroposterior length; mx, maxilla; Tr. transverse width. SYSTEMATIC PALEONTOLOGY Order Rodentia Bowdich, 1821 Family Zapodidae Coues, 1875 Subfamily Zapodinae Trouessart, 1880 Genus Megasminthus Klingener, 1966 Emended diagnosis. — Cusps and cuspids bulbous; valley between protoloph and mesoloph completely open or nearly so; ectolophid weak or incomplete between protoconid and mesoconid. NORTH AMERICAN FOSSIL ZAPODIDAE 3 Megasminthus tiheni Klingener, 1966 Referred specimens.— Loc.V731.F.O. Quarry: SDSM 10475: 11 M1, 15 M2, 12 M3. SDSM 10476: 2 maxillary fragments, one with P4, one with M'. SDSM 10464: 7 P4, 10 M1, 9 M2, 5 M3. Loc. V73 1 . F.U. Quarry: SDSM 1046 1 : 65 M,; SDSM 10462: 117 M,; SDSM 10463: 112M3;SDSM 10465: 10 P4; SDSM 10458: 101 M1; SDSM 10459: 170 M2; SDSM 10460: 141 M\ Loc. V733, Valentine Fm., SDSM 8010: left and right P4-M3. Egelhoff local fauna, Keya Paha Co., Nebraska (see Holman, 1973): F:AM 101677: jaw w/M,-M,; 101675: jaw w/MrM3; 101678: jaw w/M,-M3; 101681: jaw w/M,-M,: 101684: jaw w/MrM3; 101673: jaw w/ M,-M3; 101687: jaw w/I. MrM3; 101685: jaw w/I, M,-M3; 101679: jaw/M,-M3; 101682: jaw w/M,-M3; 101672: jaw w/I,MrM3; 101680: jaw w/I, M,-M3; 101686: jaw w/I, M,-M,: 101688: jaw w/MrM?: 101691 : mx w/L P4-M\ R P4-M2; 101694: mx w/ L P4-M2, R P4-M2: 101695: mx w/ L P4-M\ R P4-M3: 101693: mx /L M'-M\ RM'-M3; 101699: mx w/L P4-M\RM'-M3; 101698: mx w/LP4-M2,RM'; 101692: mx w/LP4-M2; 101697: mx w/ L M2, R M2; 101696: mx w/L P4-M\ RP4-M3; 101689: mx w/LP4-M2.RM'-M2: 101690: mx w/LP4-M\RP4-M\ Description. — The following characters of the first lower molar (Mt) of Megasminthus tiheni have been tabulated: ( 1 ) anteroconid joined to ectolophid or not, (2) anteroconid median or lingual. (3) mesostylid crest present or absent, (4) ectostylid present or absent, (5) ectostylid crest present or absent, (6) ectolophid connected to Metalophulid II or not, and (7) hypolophulid weak or absent. Observed variation in these characters of the first lower molar of Megasminthus tiheni from the Bijou Hills, South Dakota, and the Egelhoff Quarry, Nebraska, are tabulated in Table 1 . The different values for the total number of specimens observed for various characters result from some specimens lacking a character because of breakage or wear. The same is true if these numbers are matched against the total number of teeth studied (see Fig. 1 ). There are noticeable differences in percentages for Characters 2 and 4-6. Chi-square results for probability of differences as large as those observed indicate that there are no differences in proportion of the characters in the two populations for Characters 2, 4, and 5, but the results for Character 6, are ambivalent. I attribute this to the small sample of selected specimens from Egelhoff. The same is true for Character 3 of M' shown in Table 2. Character 2, anteroconid median or lingual, is included because it has been used by other authors. I find this trait highly subjective and believe that it should not be used at all. Characters for the first upper molar seem more reliable than those for the lower teeth. These are: (1) anterior valley, open or closed, (2) endoloph between mesocone4iypocone, present or absent, (3) endoloph between mesocone-protocone, present or absent, (4) mesoloph, present or absent, UNIV. KANSAS MUS. NAT. HIST. OCC. PAP. No. 14S c u cu X — D- P- O &-|a o ^" cu >_ . . cu X! Dh o o cu TU- tU ° ri ID L) c C o cu o i- X U X) 03 T3 X "S 2 o 2 c C aj 03 Jr c CU CO cu — D. CO £2 "- ca is IS U g s o E 3 X Q. _o "o3 aj o Cu •c: s cu "§ 1 s: fa K X oo . - o c U .2 c -a x Q. 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SO + r) + + SO sO O — — ■ O CO OS i/-] go r- co o o o d co d in d A o Os O d d >r, p i — i . — 1 ^^ r--' — <■ 0.21 >0.5 m p CO Os r- GO CO GO CO -1- Os ^Ct- rl l/"i SO iri o o d v m tj- O p c-] d ri m d d A iri SO Os O 00 sd d OS O rn !»H rr, in rM SO r-^ CM ro co d in d A in Os in in so sO CU CfH 03 T1 tC 3 -1h o trr 3 O X "aj 00 z^ ISO X 3Q W U NORTH AMERICAN FOSSIL ZAPODIDAE 1^ 8 10 Fig. 1. Megasminthus tiheni, SDSM 10461. SEM stereopairs. 1. Anteroconid connected to ectolophid; left M, . 2. Anteroconid not connected to ectolophid; right M,. 3. Anteroconid median; left M,. 4. Anteroconid lingual; left M,. 5. Ectolophid connected to Metalophulid II; left M,. 6. Ectolophid connection to Metalophulid II absent; left M,. 7. Ectostylid present; left M,. 8. Ectostylid absent; right M,. 9. Ectostylid crest present; right M . 10. Ectostylid crest absent; left M,. (5 ) mesostyle, present or absent, and (6) Protolophule II, present or absent. The Chi-square value for Character 3 indicates that there might be a real difference in the two populations, but as with the lower teeth, this may reflect the small sample size of selected specimens. 6 UNIV. KANSAS MUS. NAT. HIST. OCC. PAP. No. 148 Remarks. — The presence or absence of enamel-ridge connections in lower and upper teeth historically has been used as a criterion to distinguish species and genera. This has been possible because usually there have been few specimens from a given locality for a particular genus. In the case of SDSM Loc. V731, F.U. Quarry, South Bijou Hill, the number of specimens (i.e., individual teeth) is enormous in comparison to previous discoveries. Thus, I have been able to observe many characters and to tabulate percentages of presence or absence. It becomes increasingly clear that differences ob- served in only a few specimens from different localities are not necessarily taxonomic criteria for establishing new species because they may reflect individual or populational differences rather than interspecific variation. When this has been done, are we really dealing in "quarry or locality species"? I urge extreme caution in naming new species when only a few specimens are available, especially in the Muroidea. The statistical analysis confirms the observational evidence of the vari- ability of enamel crests in the molars of Me gasminthus tiheni. In addition, I refute Korth's (1980) allegation that SDSM 8010 from the Valentine Forma- tion (M. tiheni fide Green, 1977: 1 0 1 2) should be referred io Me gasminthus sp. Me gasminthus tiheni lacks a complete endoloph. Korth ( 1 980) also stated that the right M1 of SDSM 8010 has an endoloph that extends from the mesocone to the anterocone; this is true only in the left and right M2. He considered SDSM 8010 too large to be referred to M. tiheni; however, the results presented herein indicate that SDSM 8010 fits neatly into the size range of specimens of M. tiheni from the Egelhoff Quarry. Here again, it is a matter of having a sufficient number of specimens from a given locality in order to determine the range of variability of the species or population (Tables 3—4). Table 3. Measurements in millimeters of M of Megasminthus tiheni. Stat = statistic; M = mean; SD = standard deviation; R = range; CV = coefficient of variation. Egelhoff Bijou Hills (n=n) (ii = 19) Stat AP Tr(a) Tr(p) AP Tr(a) Tr(p) M 1.90 1.12 1.41 1.87 1.10 1.40 SD 0.10 0.07 0.07 0.10 0.08 0.08 R 1.74-2.06 0.89-1.22 1.26-1.51 1.07-2.11 0.91-1.29 1.25-1.64 CV 5.25 6.40 4.99 5.4 7.27 5.71 NORTH AMERICAN FOSSIL ZAPODIDAE 7 Subfamily Sicistinae Allen, 1901 Plesiosminthus Viret, 1926 Schaubeumys Wood, 1935 Plesiosminthus grangeri (Wood) Schaubeumys grangeri Wood. 1935 Referred specimens.— Loc: V673 BBQII: SDSM 7951: 119 M,; SDSM 7954: 97 M1. Early Hemingfordian. Discussion. — Green (1977) referred Schaubeumys Wood, 1935 to Plesiosminthus Viret, 1 926. Later. Engesser ( 1 979:35 ) separated Schaubeumys from Plesiosminthus despite acknowledging that he "found no morphological differences, but only quantitative ones" between European and North Ameri- can species of zapodids. It should be borne in mind that a primary objective in Engesser 's paper was to show that there was no migration from the Old World to the New World in the early Miocene. These differences are not many and seem to me to be of specific characters rather than generic. For example, the primary difference concerns the ectolophid of Mj. which according to Engesser, "almost always joins the protoconid" in European forms, whereas in American species, it "extends straight anteriorly towards the anteroconid and contacts the ridge connecting protoconid and metaconid." (This ridge is called Metalophulid II by Green, 1977:Fig. IB). Engesser also pointed out that the type of S. grangeri resembles the European species, rather than the North American and noted that in 80% of isolated M 's from Black Bear Quarry II, the ectolophid does not reach the Metalophulid II (see Green, 1977:Fig. 2E). In individuals in which the ectolophid continues anteriorly, it connects to the protoconid 60% of the time and connects with the center of the Metalophulid 1140% of the time (see Green. 1977:Fig. 2F); these variations are clearly marked in Green's (1977) figure and labeled in the legend. Korth ( 1 980: 1 987 ) followed Engesser in the use of Schaubeumys for North American species and. in particular ( 1980). emphasized the following Table 4. Measurements in millimeters of M' of Megasminthus tiheni. Eg- elhoff Bijou Hills (n = 20) (" = 101) Statistic AP Tr AP Tr Mean 1.91 1.86 1.88 1.85 SD 0.07 0.11 0.09 0.11 Range 1.81-2.04 1.70-2.09 1.61-2.08 1.58-2.08 CV 3.44 5.69 4.63 5.83 8 UNIV. KANSAS MUS. NAT. HIST. OCC. PAP. No. 148 as important differences between the two genera. Plesiosminthus , M { ectolophid always to protoconid; Schaubeumys, M} ectolophid joined to metalophid or protoconid (Metalophulid II fide Green, 1977) via anteroposterior lophid (ectolophid fide Green, 1977). Plesiosminthus, M] mesoconid weak; Schaubeumys, M strong. Plesiosminthus, M1 protoloph connects toendoloph nearer the protocone; Schaubeumys, M1 protoloph connects to endoloph nearer the mesoloph. Plesiosminthus, M1 protocone and hypocone com- pressed transversely (= AP greater); Schaubeumys, M ' protocone and hypocone compressed anteroposteriorly (Tr greater); Plesiosminthus, M1 metacone anteroposterior^ compressed with a straight metaloph; Schaubeumys, M1 metacone rounded with a curved metaloph. In order to test Korth 's ( 1 980; 1 987 ) assertions regarding the anteroposterior and transverse compressions of Schaubeumys versus Plesiosminthus in M', measurements of specimens from Black Bear Quarry II (from Green's 1977 study) were made of the protocone, hypocone, and metacone of 82 individual M1. Because of the curvature of the cusps, they were measured at their bases and not at the crowns which enlarge with wear (Fig. 2 ). The results are shown in Table 5. It can be seen that transverse or anteroposterior compression of the protocone and hypocone is not a reliable character to use for generic separation. Moreover, if these specimens are supposed to belong to Schaubeumys, 28% of them should be separated as Plesiosminthus on the basis of protocone measurements and 23% on the basis of hypocone measure- ments. In addition, some of the differences are only 0.01 mm — 19 for the protocone and six for the hypocone. Differences of 0.02 mm are are four for the protocone and 1 4 for the hypocone. As for the metacone, all 82 specimens Fig. 2. Diagram of zapodid left M' showing limits of individual cusps for AP and Tr measurements. Lingual side to the left. NORTH AMERICAN FOSSIL ZAPODIDAE 9 from BBQII are compressed anteroposteriorly (Green, 1977:Fig. 3). This is precisely the opposite of Korth's statement (1980:314) (see Table 6). Curiously, Korth (1980) referred one North American species, Plesiosminthus clivosus Galbreath to what he claims is an Old World genus {Schaubeumys). In doing so, he placed P. geringensis L. D. Martin (1974) in the synonymy off. clivosus. Whether or not this is correct is moot. However, according to Korth's analysis, the type of P. geringensis, a lower jaw, should be referred to Schaubeumys whereas the upper teeth have the characters of Plesiosminthus. If this were true, we would have to refer the lower teeth to Schaubeumys and the upper teeth to Plesiosminthus. This seems unlikely. Clearly, Korth (1980) and Engesser (1979) disagree. Korth believed both genera to be present in North America, whereas Engesser reserved Plesiosminthus for Old World species only. Barnosky ( 1986) followed Korth using Schaubeumys rather than Plesiosminthus on the basis of size. Skwara (1988) referred small zapodid teeth from the Topham local fauna (Saskatchewan) to Schaubeumys clivosus (contra Korth. 1979: 1980). This is a result of her belief that the European and North American forms are not congeneric. Skwara (1988) stated that circularity of arguments regarding intercontinental migration is eliminated by allocating the species to different genera. In essence, she derives North American forms of Zapodidae from a North American ancestor and the European Zapodidae from an Old World ancestor ascribing the similarities to parallelism: Skwara, as Engesser, does not subscribe to Miocene migration. Plesiosminthus well may have migrated from Asia to North America in the late Oligocene or early Miocene. There is no real justification for retaining Schaubeumys as a valid genus. In my opinion, based on the variety of patterns evident in the more than 200 lower molars and 200 upper molars examined, the differences allegedly separating the two genera are at best no more than characters of specific value. One other factor may alter current thinking regarding the geographic distribution of sicistine zapodids. Rasmussen (1977) pointed out that a characteristic of Plesiosminthus and Schaubeumys is grooved upper incisors. Table 5. Measurements of protocone versus hypocone of specimens of Plesiosminthus grangeri (SDSM 7954). Protocone Hypocone (/! = = 82) (n = 82) Statistic AP Tr AP Tr Mean 0.45 0.47 0.47 0.50 SD 0.04 0.05 0.04 0.05 Range 0.38-0.53 0.39-0.55 0.35-0.57 0.36-0.61 CV 7.77 10.85 8.72 10.20 AP > Tr = 19 = 23% AP > Tr = 0 = 0% Tr >AP = 58 =70.7% Tr >AP = 82 =100% AP = Tr = 5 = 6% — 10 UNIV. KANSAS MUS. NAT. HIST. OCC. PAP. No. 148 Table 6. Plesiosminthus grangeri, SDSM 7954. Percentages of cusp measure- ments in millimeters of 82 M1. Protocone Hypocone Metacone AP >Tr = 23 = 28% Tr >AP= 52 = 63% AP =Tr = 7 = 8.5% In the collection from the Cabbage Patch Locality, there are two sicistine anterior moieties with smooth upper incisors, a characteristic of the Asian genus Parasminthus. There are two implications here. First, identification of sicistine genera based on molars alone can be risky. Second, if these Cabbage Patch (=Gering/Monroe Creek) specimens are Parasminthus, migration from Asia to North America did take place in the late Oligocene or early Miocene (contra Engesser, 1979 ). Grooved upper incisors are known from the Cabbage Patch Locality thereby indicating the presence of two genera — Parasminthus and Plesiosminthus. A comment should be made regarding the temporal distribution of speci- mens that have been referred to Plesiosminthus grangeri. This species ranges from the Arikareean to the Barstovian (see Korth. 1980:Table 4). This is an exceptionally long time for the existence of one species; it may be the longest specific range given for any mammal. Given the variation seen in teeth both as to enamel patterns and size and lacking skulls, our ability to discriminate species of sicistines on teeth alone may not be great. The range of individual variation seen in BBQII specimens confirms my interpretation that Schaubeumys Wood should remain in the synonymy of Plesiosminthus Viret and that the alleged differences separating them into Old World and New World genera are either individual variations or, at best, specific differences. Acknowledgments: Thanks are given to PR. Bjork and J.E. Martin for lending the South Dakota specimens; to the American Museum of Natural History for the loan of the Nebraska specimens and to the late Morris F. Skinner who arranged the loan. R.W. Wilson, L.D. Martin, N. Slade, and H.-P Schultze critically reviewed the manuscript. H.-P. Schultze continuously supported my efforts and constantly urged me on. Thanks also to reviewers whose comments helped me clarify my thesis. Apologia: Figure 1 A in Green ( 1977) is of a right M1 :, not a left. NORTH AMERICAN FOSSIL ZAPODIDAE 1 1 LITERATURE CITED B arnosky, A.D. 1 986. Arikareean, Hemingfordian and Barstovian mammals from the Miocene Colter Formation, Jackson Hole. Teton County, Wyoming. Bull . Carnegie Mus. Nat. Hist. 26:1-69. Engesser, B. 1979. Relationships of some insectivores and rodents from the Miocene of North America and Europe. Bull. Carnegie Mus. Nat. Hist. 14:1-68. Green, M. 1977. Neogene Zapodidae (Mammalia: Rodentia) from South Dakota. J. Paleontol. 51(5):996-1015. Green, M. 1985. Micromammals from the Miocene Bijou Hills local fauna, South Dakota. Dakoterra 2(2): 141-1 54. Holman, J. A. 1973. Reptiles of the Egelhoff local fauna (Upper Miocene) of Nebraska. Contr. Mus. Paleontol. Univ. Michigan 24(12): 125-134. Korth, W.W. 1980. Cricetid and zapodid rodents from the Valentine Formation of Knox County, Nebraska. Ann. Carnegie Mus. 49(19):307-322. Korth, W.W. 1987. New Rodents (Mammalia) from the late Barstovian (Miocene) Valentine Formation. Nebraska. J. Paleontol. 61(5): 1058-1067. Lindsay, E.H. 1988. Cricetid rodents from Siwalik deposits near Chingi Village. Part I: Megacricetodontinae, Miocricetodontinae and Dendromurinae. Palaeovert. Montpellier 18(2):95-154. Martin, L.D. 1974. New rodents from the lower Miocene Gering Formation of western Nebraska. Univ. Kansas Mus. Nat. Hist. Occ. Pap. 32:1-12. Rasmussen, D. 1977. Geology and Mammalian Paleontology of the Oligocene- Miocene Cabbage Patch Formation, Central-western Montana. Doctoral Disser- tation. Lawrence: Univ. of Kansas. Skwara.T. 1988. Mammals of the Topham local fauna: Early Miocene (Hemingfordian), Cypress Hills Formation, Saskatchewan. Saskatchewan Parks. Recreation and Culture. Mus. Nat. Hist. Regina. Nat. Hist. Contr. 9:1-169. Wilson, R.W. 1960. Early Miocene rodents and insectivores from northeastern Colorado. Univ. Kansas Paleontol. Contr. 24(7): 1-92. 3 2044 093 361 715