Onshore/Offshore Trends in Preservation of the Cidaroid Subfamily Cidarinae

April 14th, 2007

Echinoid tests are composed of a series of plates, interlocked with adjacent plates, and reinforced by soft tissue. Regular echinoids disintegrate in a generally predictable manner: spines disarticulate, followed by oral and aboral membranes, the lantern, and finally the corona which will break apart progressively. Cidaroid echinoids are characterized by having a small degree of interlocking between plates, which means that disintegration of the soft tissue will likely be followed promptly by disarticulation of the corona. Rapid burial is thus expected to be a requirement for exceptional preservation. In this study, we assess whether an environmental control on the preservation of cidaroid echinoids has operated since Late Triassic time.

Original monographs containing descriptions of type species assigned to the Subfamily Cidarinae were examined to determine the quality of preservation. Specimens were ranked according to completeness of specimen, in a spectrum ranging from a whole corona with intact spines, to fragmentary corona material or disarticulated spines. Depositional environment was inferred for each specimen based on the primary literature. To date, depositional environment has been determined for 125 specimens. Our preliminary data reveal a non-random pattern to preservation. In onshore environments, 6 of 24 examined specimens (25%) are described only from disarticulated spines or coronal plates. In offshore environments, 76 of 101 examined specimens (75%) are described from disarticulated spines or coronal plate. We hypothesize that the disproportionate number of intact specimens in onshore environments results from rapid burial associated with storm beds. Macroevolutionary studies of cidaroid echinoids must be coupled with similar assessments of preservation bias.

Kristopher Rhodes, ’08 Mt. Vernon, IA
Major: Geology

Sponsor: Benjamin Greenstein

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