Taphonomy and environmental distribution of helicoplacoid echinoderms

Helicoplacoids are Early Cambrian echinoderms with tri-radiate ambulacra th at are covered by helically-arranged columns of calcite plates. They are ab undant only in, shales of the Middle Member of the Lower Cambrian Poleta Fo rmation (Atdabanian) at Westgard Pass in the White-Inyo Mountains of Califo rnia. 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 articulat ed 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 preburial and post-burial decay. Because most (73%) helico placoid specimens are preserved on the same bedding-plane as at Least one o ther 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 shall ow 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 li ved in a wider range of paleoenvironments than those represented by the sha les, where they are found most commonly The exceptional preservation of hel icoplacoids, therefore, is most likely narrowly restricted stratigraphicall y and geographically because the proper balance of energy regimes, together with the factors mentioned above, was achieved only rarely during the Earl y Cambrian, not because helicoplacoids were restricted to Living in one pal eoenvironment.