GENUS-LEVEL TAPHONOMIC VARIATION WITHIN CLADID CRINOIDS, UPPER PENNSYLVANIAN BARNSDALL FORMATION, NORTHEASTERN OKLAHOMA

Fossil crinoid remains are frequently used in taphonomic studies due to the delicate nature of their multiplated, soft-tissue-bound endoskeleton and their rapid post-mortem skeletal disarticulation. In many cases, characteristic patterns of disarticulation can be successfully used to recognize distinct physical or biological events responsible for the taphonomic state of the crinoid assemblage under study. With the development of the taphonomic grade concept, it became apparent that different crinoid morphologies are more prone to disarticulation than others; however, this concept has rarely been applied to taxonomic levels below the rank of subclass or order.

A thin mudstone interval in the Upper Pennsylvanian Barnsdall Formation of northeastern Oklahoma features a large crinoid assemblage (over 1000 individuals) in a range of taphonomic states. Although incredibly diverse at lower taxonomic levels (44 genera, 51 species), the majority of these belong to the subclass Cladida, thus allowing investigation of taphonomic patterns among crinoid taxa at finer taxonomic levels than has commonly been done previously. Assessment of articulated and partially articulated crinoid specimens and of fully disarticulated individuals represented by separate ossicles reveals a great deal of taphonomic variation even at the genus level. Genera exhibit differences in styles of compaction as a result of skeletal and presumedly soft-tissue anatomy. Those taxa that are most complete (i.e., feature considerable or complete arms) may not be those with the most strongly sutured cup plates and, in fact, may be among the least strongly sutured taxa as shown by compaction and plate separation. Post-mortem skeletal disarticulation did not occur uniformly for taxa even though recovered from the same horizon, resulting in relative-abundance rankings for articulated and disarticulated individuals that are quite disparate. These genus-level taphonomic variations may reflect scavenger or predator preferences, differences in connective tissues or articulations, or structural integrity or degree of plate suturing. Thus, caution is urged in interpreting taphonomic grades of crinoids utilizing one or two genera to represent a larger taxonomic group.