Abstract

The spheroidal edrioasteroid echinoderm Totiglobus, preserved in the middle Cambrian Chisholm Shale of eastern Nevada, was one of many interesting morphologies to develop during the period of rapid evolutionary diversification in the early and middle Cambrian. To understand more about how the unusual morphology of Totiglobus evolved, a detailed examination of fossil specimens and the strata in which they are preserved was performed. A total of 263 specimens were examined and placed into taphonomic categories based on their preserved orientation relative to the seafloor. Seventy-two well-preserved specimens were used for statistical analysis. Of these, 49% were preserved with the aboral surface down. The preference for preservation in this category, with the aboral surface (which contains a suctorial attachment disc) oriented directly on top of the sediment, was statistically significant (p < 0.025). These data support the hypothesis that Totiglobus lived attached to the seafloor through suction. Rock samples were also collected from seven localities in the Chisholm Shale and the bioturbation levels were determined using the ichnofabric index (ii) method. A total of 48 samples, comprising 4.09 m of strata, were collected and x-radiographed. Ichnofabric index (ii) data recorded from these rocks revealed extremely low bioturbation levels (∼ii  =  1) with no mixed layer development (1.01 average ii). These results likewise support the sediment attachment hypothesis for Totiglobus. This study indicates that some unusual Cambrian morphologies, including that of Totiglobus, evolved partly in response to the presence of nonactualistic seafloor conditions and are therefore not simply early evolutionary experiments.

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