PONDERING OVER PIECES: SOLVING THE PUZZLE OF EDRIOASTEROIDS AND OPHIUROIDS FROM THE GLEN DEAN FM

The Mississippian is well known for rich echinoderm faunas dominated by stemmed echinoderms, that have robust, well-preserved and easily recognized calyces and thecae. Other, taphonomically more fragile echinoderms are often overlooked because they are preserved dominantly as isolated ossicles. Unless these ossicles are particularly familiar, abundant or distinctive they may go unnoticed except if they are being searched for particularly. The upper Mississippian (Chesterian) Hopkinsville, KY locality of the Glen Dean Fm. has an echinoderm fauna characterized by large numbers of blastoids and a diversity of crinoids, typical of Chesterian faunas. However, the fauna also contains skeletal elements from edrioasteroids, echinoids, asteroids, and ophiuroids.

The dominant edrioasteroid in the fauna is a large species of the discocystinoid Hypsiclavus based on the presence of organized peduncular plates, interambulacral plates with vertical sutures, and floor plates with intrathecal extensions. A second edrioasteroid, likely Lepidodiscus is known from a single floor plate with imbricated sutures. Ophiuroids are much more diverse with at least five taxa identified from both vertebrae and lateral arm plates. Stem lineage taxa include at least two different furcasterid species represented by J-shaped lateral arm plates with large conspicuous lateral spine articulations, and high, narrow fused vertebrae with a rectangular lateral view and triangular cross-section. Large, massive, rounded fused vertebrae with well-defined zygapophyses and epanapophyses indicate an Onychaster-like taxon. High lateral arm plates with a strongly convex distal edge carrying large spine articulations and conspicuous pointed protrusions are assignable to an eospondylid. Remarkably, the material also includes a modern type ophiuroid represented by vertebrae and lateral arm plates. Ophiuroid identification, especially stem lineage taxa, is challenging because most Paleozoic species were described from fully articulated skeletons providing little or no detail for lateral arm plates and vertebrae. Nevertheless, investigating isolated ossicles from Paleozoic sieving residues is worthwhile because articulated skeletons represent a restricted, taphonomically biased part of the ophiuroid fossil record.