Life Sciences Contributions Royal Ontario Museum 1 37 Palaeoecology of a Well-Preserved Crinoid Colony from the Silurian Rochester Shale in Ontario Carlton E. Brett James D. Eckert ROM ROYAL ONTARIO MUSEUM LIFE SCIENCES PUBLICATIONS INSTRUCTIONS TO AUTHORS Authors are to prepare their manuscripts carefully according to the following instructions. Failure to do so will result in the manuscript’s being returned to the author for revision. All manuscripts are considered on the understanding that if accepted they will not be offered for publication elsewhere. 1. GENERAL Papers for publication are accepted from ROM staff members, Research Associates, or from researchers reporting on work done with ROM collections. In exceptional cases,monographic works on the flora and/or fauna of Ontario will be considered for publication by authors not affiliated with the ROM. 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ECKERT . : : ° Crinoid Colony from the Silurian Rochester Shale in Ontario Y ROM ROYAL ONTARIO MUSEUM PUBLICATIONS IN LIFE SCIENCES The Royal Ontario Museum publishes three series in the Life Sciences. LIFE SCIENCES CONTRIBUTIONS, a numbered series of original scientific publications including monographic works. LIFE SCIENCES OCCASIONAL PAPERS, a numbered series of original scientific publications, primarily short and usually of taxonomic significance. LIFE SCIENCES MISCELLANEOUS PUBLICATIONS, an unnumbered series of publications of varied subject matter and format. All manuscripts considered for publication are subject to the scrutiny and editorial policies of the Life Sciences Editorial Board, and to review by persons outside the Museum staff who are authorities in the particular field involved. LIFE SCIENCES EDITORIAL BOARD Senior Editor: C. MCGOWAN Editor: P.H. von BITTER Editor: R. WINTERBOTTOM CARLTON E. BRETT is Professor in the Department of Geological Sciences, University of Rochester, Rochester, New York 14627 (Tel.: 716 275-2408). JAMES D. ECKERT is a former Graduate Student in the Department of Geology, University of Toronto. Canadian Cataloguing in Publication Data Brett, Carlton Elliot. Palaeoecology of a well-preserved crinoid colony from the Silurian Rochester shale in Ontario (Life sciences contributions, ISSN 0384-8159 ; no. 131) Bibliography: p. ISBN 0-88854-281-X 1. Crinoidea, Fossil. 2. Paleontology—Silurian. 3. Paleontology—Ontario—Sixteen Mile Creek (Niagara). I. Eckert, James D., 1952-—_ II. Royal Ontario Museum. III. Title. IV. Series. QE782.B73 563.91 '0971351 C82-094054-2 Publication date: 19 January 1982 ISBN 0-88854-281-X ISSN 0384-8159 © The Royal Ontario Museum, 1982 100 Queen’s Park, Toronto, Canada M5S 2C6 PRINTED AND BOUND IN CANADA AT THE ALGER PRESS Palaeoecology of a Well-Preserved Crinoid Colony from the Silurian Rochester Shale in Ontario Abstract An exceptionally well-preserved assemblage of crinoids and other fossils is described from the Rochester Shale (Upper Silurian, Wenlockian) of Ontario. These crinoids have retained complete crowns, columns, and holdfasts and are preserved essentially in life position, indicating rapid destruction and anastrophic burial of a level, mud-bottom community. The crinoid colony was _ numerically dominated by the minute inadunate Homocrinus parvus Hall, which occurs in densities of 1500 specimens/m*; these crinoids possess stems more than 100 mm long. Specimens of Eucalyptocrinites caelatus (Hall) from this assemblage exhibit shorter columns (60 to 100 mm) and dendritic radices. Columns of this species were utilized as attachment substrates by other organisms such as favositids, brachiopods, and other crinoids. Complete juvenile specimens of Dimerocrinites spp. occur attached to the Eucalyptocrinites columns by coiled distal stem termini. This assemblage represents a local dense aggregation of organisms on an otherwise sparsely populated sea floor. Soft, muddy bottoms and high rates of sedimentation may have inhibited settlement of the sea floor by most low-level suspension-feeding organisms. However, once ‘“‘pioneer’’ individuals of crinoids became established they permitted colonization by various other organisms. Conditions favourable for the preservation of complete crinoids occur most frequently in a limited range of depositional environments, that is, low energy, mud-bottom areas below normal wave base but within reach of storm waves. Consequently many of the best- preserved crinoid assemblages are probably derived from similar biotopes and represent rather unusual mud-bottom, rheophobic associations. Introduction Echinoderms are notoriously susceptible to post-mortem disarticulation (Ruhrmann, 1971; Meyer, 1971; Liddell, 1975, and in preparation; Kier, 1977). The occurrence of large numbers of completely articulated fossil crinoids is extremely rare and is a reliable indication of catastrophic annihilation and burial of a benthic community (Rosenkranz, 1971; Lane, 1973). However, examples of such exceptionally preserved crinoid assemblages (crinoid Konservat-lagerstdtten, sensu Seilacher, 1970) occur sporadically in the geologic record and they provide significant insights into the morphology and life habits of ancient pelmatozoans and associated organisms (Lane, 1963, 1972, 1973; Rosenkranz, 1971; Hagdorn, 1978). Since these assemblages often approximate in situ remains of marine communities, they are also of palaeoecologic significance. Detailed analyses of crinoid lagerstdtten can serve as models for the interpretation of associated, and far more numerous, assemblages of disarticulated crinoid remains. This paper focuses on one such lagerstdtten occurrence. During the spring of 1976 an assemblage of exceptionally well-preserved crinoids was discovered near the base of the Rochester Shale (Upper Silurian, Wenlockian) near Sixteen Mile Creek, Jordan, Ontario. Excavation at this site permitted collection of slabs, now housed at the Royal Ontario Museum, which show complete skeletal structure of the crinoids Eucalyptocrinites caelatus (Hall) and Dimerocrinites spp. In addition, numerous well-preserved individuals of Homocrinus parvus Hall, and other fossils are preserved in a closely packed aggregation. The Sixteen Mile Creek crinoid occurrence appears to be nearly as good an example of an in situ fossil assemblage as is ever seen in the geologic record. Apart from the non-preservable soft-bodied organisms, this assemblage is inferred to be a rather accurate representation of one type of Silurian level, mud-bottom community. Materials and Methods Well-preserved crinoids and a few associated fossils occur in a single horizon 1.4 m above the base of the Rochester Shale in a west-facing bank of Sixteen Mile Creek, near Jordan, Lincoln Co., Ontario (NTS Niagara 30M /3W; Fig. 1). The horizon was initially recognized when four complete crowns of Eucalyptocrinites were discovered, partially weathered-out and protruding from the surface of the outcrop. Subsequent excavation of approximately 3.7 m° of this horizon revealed two small clusters of complete Eucalyptocrinites and other fossils, each occupying about 1000 cm? and separated by about 2.3 m. These crinoid-rich patches were surrounded by spaces of apparently unfossiliferous shale. Most of the data pertaining to the Homocrinus-Eucalyptocrinites horizon are derived from a detailed examination of a large slab measuring 52 cm by 24 cm, which is in the Royal Ontario Museum (ROM 35832; Fig. 2). This slab was collected intact by channelling of the surrounding shale with chisels. The remainder of this colony and the second, smaller Eucalyptocrinites patch, were collected as isolated specimens or obtained on small slabs of shale. Further excavation at the Sixteen Mile Creek site would be extremely difficult owing = n = = ® ay ® © Zz NEW YORK [3] Silurian outcrop ~~ Escarpment ees ShOreline 40 miles 60 kilometres Fig. 1 Locality map for Sixteen Mile Creek crinoid occurrence; inset shows details of the Sixteen Mile Creek area (arrow indicates collecting site). Map reference NTS (National Topographic Series) Niagara 30M /3W. to the steepness of the bank and the thickness of overburden which must be removed to clear the crinoid horizon. Neither of the two crinoid patches appeared to continue back into the unexcavated areas beneath the bank, although some Homocrinus Specimens may have been overlooked in the field. The surfaces of the shale slabs were carefully prepared using an air-abrasive unit and dolomite powder. A thin layer of shale matrix coating the surface of the slabs was removed, revealing abundant Homocrinus and complete stem and root systems of many of the Eucalyptocrinites specimens. The area of the large slab was obtained from a traced outline, and abundances of various fossil taxa within the aggregation were determined. Stratigraphic Setting Outcrops of Rochester Shale along Sixteen Mile Creek are discontinuous and a complete measured section cannot be made at this locality. However, the lower third of the Rochester Shale in this outcrop differs markedly from the same interval in exposures both east and west of the Jordan area (see Bolton, 1957) in possessing a very high proportion of homogeneous, sparsely fossiliferous shale. Bryozoan-rich limestones and fossiliferous shales are restricted to the basal 0.5 m of the shale; two thin calcisiltite bands occur, interbedded with barren shale, the highest of which is about 0.6 m above the Irondequoit. The main crinoid horizon occurs within an interval of otherwise sparsely fossiliferous, dark grey, silty shale. Three additional well-preserved crinoid specimens were obtained from higher levels during the excavation of the main crinoid-bearing horizon. Two crowns of Asaphocrinus ornatus (Hall), one with a complete stem and holdfast attached to a brachiopod, were collected on a small slab of shale containing brachiopods and bryozoans about 2.16 m above the base of the Rochester Shale. A specimen of Dendrocrinus longidactylus Hall with a calyx and a nearly complete stem was also collected on a slab of silty shale at intermediate level, 1.68 m above the Irondequoit Limestone. Aside from these isolated small occurrences the rock both overlying and underlying the Eucalyptocrinites horizon proved to be barren. Although the top of the bank exposure is covered, there is at least 3 m of similar lithology overlying the highest Asaphocrinus crinoid-bearing bed. The entire Rochester Formation is continuously exposed in a small stream gully about 1 km east of Sixteen Mile Creek. Here the lower Rochester resembles that at Sixteen Mile Creek; fossiliferous limestones are again restricted to the base of the formation and the entire remainder of the lower Rochester is both unfossiliferous and lacking in interbeds of any sort. This is the only exposure east of Grimsby, Ontario, at which the ‘‘Bryozoa Beds’’ (Grabau, 1901)—thin biomicrites marking the top of the lower Rochester member—are completely missing. A high proportion of barren shale in these two outcrops is anomalous with respect to the majority of exposures along the Niagara Escarpment in Ontario, which contain abundant fossiliferous horizons up to 3 m above the base and, again, at about 6 to 8m. However, a large proportion of unfossiliferous shale and the virtual disappearance of bryozoan-rich beds is characteristic of outcrops only a few kilometres south of the main escarpment in the Fonthill Reentrant and in the southern end of Niagara Gorge (Fig. 1). Thus, the barren lower Rochester around Sixteen Mile Creek can be interpreted as a local northward extension of the “‘barren’’ mudstone facies which apparently existed immediately south of a “‘Bryozoan Belt’’ that normally coincides with the present escarpment edge (Brett, 1978c). The apparent northward deflection of the ‘‘Bryozoan Belt’’ in the Jordan, Ontario, area may be associated with a local thickening of the Rochester Shale in the subsurface south of this area around Port Colborne, Ontario. Both phenomena may be attributed to higher rates of fine-grained terrigenous sediment relative to surrounding areas, owing to a local depression of the sea floor or to a persistent offshore current. The shale at Jordan appears somewhat siltier than that of most Rochester Shale outcrops. The shale overlying the main crinoid horizon shows distinct laminae of silt- and clay-sized particles. Discrete calcisiltite bands are lacking in higher portions of the lower member of the Rochester Shale; however, the shales are highly calcareous, suggesting an input of carbonate sediment. No fossiliferous horizon corresponding to the Eucalyptocrinites bed could be located at an equivalent position in nearby outcrops; thus, this was probably a localized occurrence. Fig. 2 Overall view of large slab of Rochester Formation containing 10 well-preserved specimens of Eucalyptocrinites caelatus (Hall); note general alignment of specimens; arrow on slab near bottom of page indicates north direction. ROM 35832, x 0.35. N Faunal Composition and Autecology of Fossils Examination of a large surface area of the crinoid-bearing horizon yielded a total fauna of 10 species of fossils, numerically dominated by the crinoid Homocrinus parvus Hall. Fossil density data for the large slab assemblage are summarized in Nablewis Table 1 Data on Eucalyptocrinites-Homocrinus slab assemblage, Sixteen Mile Creek, Jordan, Ontario Area of slab 1139 cm? Area above Homocrinus bedding plane 439 cm? Area of Homocrinus bedding plane 700 cm? Fauna Taxa Number Comments Coelenterata Favosites parasiticus (Hall) ] encrusting Eucalyptocrinites column Bryozoa Fenestella elegans (Hall) a complete frondose zoaria bryozoan indet. 7 ramose, 2 species Brachiopoda Stegerhynchus neglectum (Hall) 1] all articulated Trilobita Dalmanites limulurus (Green) 2 complete, ventral sides up Crinoidea Dimerocrinites lilliformis (Hall) l 10-armed species Dimerocrinites(?) sp. 5) 20-armed; 1 specimen lost, 1 buried in slab Eucalyptocrinites caelatus (Hall) 11 mainly complete Homocrinus parvus (Hall) 130 Tabulata Two specimens of the tabulate coral Favosites parasiticus (Hall) were observed. Both occurred as small (diameter about 10 mm), symmetrical colonies attached to the columns of two Eucalyptocrinites specimens (Figs. 3, 4A). Corals evidently grew on the upright stems of living crinoids since they are symmetrically developed around the column. There was no observable reaction on the part of the crinoids (e.g., secretion of secondary stereom). Favosites parasiticus has previously been observed on columns of Eucalyptocrinites crassus (Hall) from the Waldron Shale of Indiana (Hall, 1881) and on stems of Caryocrinites from the Rochester Shale of New York (Hall, 1852). The persistent association of this tabulate coral with well-preserved (i.e., live at time of coral attachment) pelmatozoans suggests substrate selectivity by this tabulate species. Bryozoa Several well-preserved frondose zoaria of Fenestella sp. were observed on the prepared surface of the large slab. All were lying flat and were strongly compressed, but they are not fragmented. One possesses a single, small cemented crinoid holdfast, perhaps belonging to Homocrinus. In addition, two small ramose trepostomatous bryozoans occur on the slab. Fenestrate bryozoans were also abundant on the Asaphocrinus-bearing slab collected higher in the section. Bryozoans are probably somewhat commoner than recorded as they are lying flat on the slab beneath Homocrinus and other crinoids. Portions of the surface were not completely cleared, to prevent damage to the Homocrinus specimens. Brachiopoda Brachiopods are represented on the large slab by a single species of rhynchonellid tentatively identified as Stegerhynchus neglectum (Hall). The 11 specimens are completely articulated, tightly closed, and similar in size. Most are preserved lying on either the pedicle or brachial valve but the beaks in most cases are closely appressed to other objects. These brachiopods are closely associated with the stems and root systems of Eucalyptocrinites (Fig. 3). However, in no case were the roots observed to be centred on rhynchonellid brachiopods; rather the Stegerhynchus appear to have been pedically attached to exposed portions of the crinoid stems or roots. Similar beak-down orientations (interpreted as life positions) have been observed in other rhynchonellid brachiopods (Richards, 1972). Stegerhynchus specimens were also obtained from a layer a few centimetres above the crinoid bed and thus were not exclusively associated with this crinoid patch. Stegerhynchus as well as Leptaena was found in the higher Asaphocrinus-bearing slab. Trilobita Two complete exoskeletons of Dalmanites limulurus (Green) occur in inverted orientations on the surface of the large slab (Fig. 4C). Both are approximately 25 mm long and are complete except for the hypostomes. Aside from these, no other trilobites were present. Crinoidea The slab assemblage is most unusual for lower Rochester fossil occurrences in having such a high density of crinoids. A total of 148 specimens belonging to four species was counted in an area of 700 cm’ on the large slab; this corresponds to a density of 2114 specimens/m*. The preservation of all specimens is equally good, suggesting that all were living together at the time of burial. Density in this patch far exceeds the maximum ‘“‘high density crinoid garden’’ reported by Brower (1973). However, the biomass of crinoids in this community was probably not as high as that of other reported occurrences (e.g., Lane, 1973; Brower, 1973) since most of the observed crinoids were relatively minute specimens of Homocrinus parvus Hall (Fig. 4D). At least 130 calyces of Homocrinus were counted on the prepared portion of the slab; much of the surface is covered with the delicate stems and arms of these crinoids. Tangled masses of stems occur in the vicinity of Eucalyptocrinites radices and some Homocrinus stems may be coiled on the stems of the larger crinoids (Fig. 4c). The high density of specimens appears to continue to the broken edges of the slab and so it is reasonable to assume that Homocrinus covered areas of the sea bottom beyond the immediate vicinity of clusters of Eucalyptocrinites. Specimens were also observed on smaller Eucalyptocrinites-bearing slabs from the second patch. Unfortunately, Homocrinus specimens were virtually invisible on unprepared surfaces owing to a thin coating of clay matrix, thus the total extent of Homocrinus patches on the bedding plane is unknown. The broken edge of the slab reveals that the crinoids occurred through an interval of 1 to 2 cm of shale so that the total original density may have been 50 to 100 per cent higher than that recorded above. Specimens of Homocrinus are invariably preserved with arms and long sections of the stem intact. The longest section of stem is in excess of 100 mm long and is incomplete, lacking the calyx. Considering the extreme abundance of the Homocrinus specimens it 1s paradoxical that only a single small holdfast was obtained. Minute attachment discs, thought to belong to Homocrinus , are common in the Lockport Homocrinus beds. Possibly the holdfasts of these crinoids occur attached to small objects at a level slightly below the cleaned surface of the slab. Complete specimens of the camerate crinoid Eucalyptocrinites caelatus (Hall) are the most conspicuous element of the Sixteen Mile Creek crinoid assemblages (Figs. 3, 4B). Ten specimens are present on the 700 cm’ area of the large slab; an additional four crowns were obtained from the periphery of this area, yielding a total density of 14 individuals in an area of about 100 cm*. The second cluster produced 10 Eucalyptocrinites specimens. These are interpreted as in situ crinoids and suggest intraspecific aggregations of Eucalyptocrinites , as in many other echinoderms. Fig. 3. Detail of ROM 35832 A-F. Specimens of Eucalyptocrinites caelatus and Dimerocrinites spp. Larger Eucalyptocrinites specimen (A) exhibits elongate anal chimney, complete column, and radix holdfast. A specimen of Favosites parasiticus Hall and at least three dimerocrinitid crinoids are attached to the column of this individual. Note that two species of dimerocrinitids are present; the lowest crown (E) is a 10-armed form, Dimerocrinites cf. D. lilliformis (Hall), the other two (C, D) belong to an undescribed 20-armed form, Dimerocrinites(?) sp. The smaller Eucalyptocrinites specimen shows imperfect preservation, note partial collapse of column. Also present on this portion of the slab are three rhynchonellid brachiopods, Stegerhynchus sp., numerous threadlike columns and crowns of Homocrinus parvus Hall, and the column and root of a third Eucalyptocrinites individual (F), X 1.25. Preservation of the Eucalyptocrinites is exceptional; this is undoubtedly the best-preserved assemblage of these crinoids ever collected. All specimens consist of completely uncrushed to slightly compressed crowns, with long anal chimneys preserved intact (Fig. 4B); the arms are tightly folded into their tegmenal compartments. Of the 24 specimens, 13 were buried with the column intact; the remaining specimens probably also possessed stems but these have been lost, owing to weathering of the shale bank or to damage during collection. At least six specimens preserve the entire column, which terminates in a large branching root system (radix). The lengths of complete columns vary from 58 mm to 99 mm, and appear to be approximately correlated with crown size. Columns are uniform in diameter throughout their length, in contrast to the tapering column figured for a specimen of Eucalyptocrinites crassus (Hall) from the Waldron Shale at Waldron, Indiana (Macurda, 1968). They are strongly heteromorphic, with deep gaps at the internodes. Distally, nodal columnals become somewhat irregular and near the root they bear small knoblike pseudocirri which appear to grade into the stout non-aligned radicular cirri at the base of the column. Root systems are well developed in these specimens and are up to 80 mm in diameter, composed of stiff, branched radicles. Roots did not penetrate vertically into the sediment for more than 10 or 20 mm; rather the rootlets spread out laterally, forming a fairly broad support base for the crown and column. The initial attachment objects within these roots, if any, are unknown since the Specimens were only partially extracted from the matrix. Portions of the rootlets may have been temporarily exposed at the sediment-water interface, as they appear to have been utilized as attachment substrates for rhynchonellid brachiopods. Several Eucalyptocrinites specimens exhibit an abrupt curvature of the proximal column, which directs the crown at an angle to the column (Fig. 4B). This does not appear to have been a permanent bend, as the involved columnals are not cuneiform; rather it indicates a moderate flexibility of the proxistele. Probably this is an adaptation for turning the crown in a downcurrent direction for rheophillic feeding. Apart from the proximal quarter, the columns were quite rigid. That the columns were held vertically during life is strongly suggested by the symmetry of the root systems, by their restriction to the distal terminus of the stem, and by the symmetrical growth of epizoic favositids on the columns. The 24 specimens exhibit a rather broad range of crown size (measured from the inflected edge of the radial plates to the base of the anal chimney) from 29.7 mm to 41.1 mm (Table 2). However, this represents only a small fraction of the total size range observed for Eucalyptocrinites caelatus specimens in other beds of the Rochester Shale (11 to more than 75 mm). Furthermore, the size-frequency distribution of the 24 specimens appears to be continuous and unimodal rather than bimodal. A single generation of Eucalyptocrinites individuals is probably represented in each of the two local patches. Therefore the observed size range is thought to represent individual variation. Four of the smallest specimens (29.7—30.7 mm) occurred in a single clump on the large slab; stem lengths are approximately equal. Aside from the size variation, crowns exhibit some variation with respect to ornamentation. All are pustulose to some degree, but the pustules are smaller and less closely spaced in some specimens than in others. Completely smooth specimens of E. caelatus have been observed in some other beds of the Rochester Shale. Several of the Eucalyptocrinites columns bear distal coils of other crinoid stems lying between the nodal columnals. Position of the coils varies; one specimen has a 10

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