The Pennsylvanian Cladid Crinoid Erisocrinus: Ontogeny and Systematics
Type of Degreethesis
Geology and Geography
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Cladid crinoids have among the highest disarticulation rates of all Paleozoic crinoids, so the study of morphology and systematics has been hindered by a lack of available specimens. An unusually large collection of the genus Erisocrinus from numerous museum collections has been studied in order to determine the growth of the type species of the genus, as well as the systematics. Included in the collections were specimens from Lagerstätten deposits, including a complete growth series of E. typus from the Barnsdall Formation and a large number of relatively pristine specimens from the LaSalle Limestone. A digital growth study using the complete growth series comprising eight crowns of E. typus collected from the Barnsdall Formation was performed using standard heads-up digitization methods in ArcGIS™. The sutures between all the plates of the crown were traced from high-resolution, two-dimensional photographs. Topological constraints that were put into effect prevented the digitized lines from overlapping and facilitated conversion into polygons. The perimeters, areas, and other measurements of these polygons, represented as individual plates, were automatically calculated by the software. A previous study of the ontogeny of this species concluded the growth of the cup to be isometric. However, results from this study concerning the relative rates at which plates changed size and shape show that E. typus grew anisometrically. The growth of the arm plates of the growth series appear to grow in three distinct stages, noted from the change in area in the arm plates of the growth series. The systematics study of the genus Erisocrinus took into account the 36 proposed species since its naming in 1865. The species previously synonymized or reassigned were reevaluated. Those species still named within the genus Erisocrinus were evaluated on the basis of a new diagnosis of Erisocrinus. Over two hundred specimens comprising a variety of proposed species were measured (radial height and width, basal height and width, stem diameter, cup height) so that a Principal Component Analysis and further analyses could be run. Of the 18 species still named within the genus, only eight of them are considered valid: E. typus Meek and Worthen 1865, E. propinquus Weller 1909, E. elevatus Moore and Plummer 1940, E. obovatus Moore and Plummer 1940, E. terminalis Strimple 1962, E. longwelli Lane and Webster 1966, E. mediator Strimple and Watkins 1962, and E. healdae Pabian and Strimple 1974. This study redefines the temporal span of the genus as being present primarily during the Pennsylvanian, with two of the eight species surviving into the Early Permian. As re-defined in this study, Erisocrinus was restricted to the mid-continental United States.