PALEOECOLOGY OF THE MIDDLE CAMBRIAN EDRIOASTEROID ECHINODERM TOTIGLOBUS: IMPLICATIONS FOR UNUSUAL CAMBRIAN BODY PLANS

The edrioasteroid echinoderm Totiglobus, preserved in the Middle Cambrian Chisholm Shale of eastern Nevada, was one of many strange morphologies to develop during the period of rapid evolutionary diversification in the Early and Middle Cambrian. In order to deepen our understanding of how this unusual body plan evolved, detailed examination of fossil specimens and the rocks in which they are preserved was used to test the hypothesis that Totiglobus lived as a sediment attacher on relatively firm soft substrates with limited mixed layer development. 263 specimens were examined and placed into taphonomic categories based on their preserved orientation relative to the seafloor: 1) oral surface down, 2) side down, 3) aboral surface down, and 4) too poorly preserved to tell. Of the 72 specimens that were well-preserved enough to determine orientation, 49% were preserved in category 3 and there is a statistically significant preference (p<0.025) for being preserved in this category with the aboral surface, and therefore the suctorial attachment disc, down. Rock samples were collected from seven localities in the Chisholm Shale and bioturbation levels in the rocks were determined using the ichnofabric index (ii) method. A total of 48 samples, comprising 4.09 meters of stratigraphy, were collected and x-radiographed. Ichnofabric index data was recorded from slabs of the samples and their x-radiographs, as well as from one Chisholm Shale outcrop that had 83.1 cm of stratigraphy with visible primary sedimentary structures. Examination of these rocks reveals extremely low bioturbation levels (~ii1) with no mixed layer development. These results, along with the statistical preference for aboral surface down orientation, support the sediment attachment hypothesis and indicate that some unusual Cambrian body plans, such as Totiglobus, evolved in response to the presence of non-actualistic substrate conditions and are not simply early evolutionary experiments.