EXAMINING THE ONTOGENY OF THE PENNSYLVANIAN CLADID CRINOID ERISOCRINUS

Crinoids were major constituents of the Late Carboniferous (Pennsylvanian), but their soft tissue and rapid disarticulation rate translates to few well-preserved specimens, limiting the study of their growth. This is amplified for cladids, who had among the highest disarticulation rates of all Paleozoic crinoids, due to the relatively loose suturing of the plates of their calyx. However, Erisocrinus typus has exceptionally well-preserved specimens from the Barnsdall Formation in Oklahoma. The Barnsdall Formation is predominantly composed of fine- to medium-grained sandstone, overlain by shale. Severe compaction of the crinoid skeletons allowed for exceptional preservation of the plates, although the specimens were laterally compressed.

A growth study of eight crowns of Erisocrinus typus, reposited at the University of Nebraska State Museum, was conducted using high-resolution photographs. The fossils encapsulate a full growth series of this species. The photographs were imported into ImageJ; area, length, and width were recorded for all preserved basal, radial, and brachial plates. Cup height geometries were measured both in ImageJ and with calipers, as the 2D photographs may have distorted the measurements of these; measurements of cup width is not possible due to severe compaction. Results show that the plates of the cup grew in a slightly anisometric pattern. Additionally, we found that as E. typus progressed through its life stages, the skeleton underwent various changes. For the primibrachial plates, the growth rate is more rapid during the juvenile phase, and slows down during the later stages of growth. The arm plates started as uniserial wedge shapes (i.e. cuneiform) as juveniles, but shifted and became biserial early in ontogeny. The statistical analyses performed in this study showed that there is a trend of allometric growth throughout. E. typus broadly shared a similar growth trajectory to other cladids, however, some differences are noted.