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Taphonomy and Environmental Distribution of Helicoplacoid Echinoderms

¹ Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089

Helicoplacoids are Early Cambrian echinoderms with triradiate ambulacra that are covered by helically-arranged columns of calcite plates. They are abundant only in shales of the Middle Member of the Lower Cambrian Poleta Formation (Atdabanian) at Westgard Pass in the White-Inyo Mountains of California. To identify and understand the unique taphonomic conditions that led to their preservation, 146 helicoplacoid specimens were examined along with the rocks in which they are preserved. Considering their loosely articulated skeletal construction, together with their common occurrence at the base of cm-scale graded beds, helicoplacoids most likely were preserved during obrution events. A majority (69%) of helicoplacoid specimens are partially disarticulated, probably indicating that most helicoplacoids underwent some combination of pre-burial and post-burial decay. Because most (73%) helicoplacoid specimens are preserved on the same bedding plane as at least one other individual, and many (39%) are preserved on bedding planes containing at least 10 individuals, it appears that helicoplacoids were gregarious and frequently were preserved in mass mortality obrution deposits. Low levels of bioturbation, possible microbial stabilization of the sediment, a shallow redox boundary, and a normally calm depositional environment capable of preserving obrution deposits are all factors that aided in the preservation of helicoplacoids. Additionally, the presence of helicoplacoids in several facies of the Middle Member of the Poleta Formation indicates that they lived in a wider range of paleoenvironments than those represented by the shales, where they are found most commonly. The exceptional preservation of helicoplacoids, therefore, is most likely narrowly restricted stratigraphically and geographically because the proper balance of energy regimes, together with the factors mentioned above, was achieved only rarely during the Early Cambrian, not because helicoplacoids were restricted to living in one paleoenvironment.