TAPHONOMY AND BODY SIZE RANGE OF HELICOPLACOID ECHINODERMS: A UCMP PERSPECTIVE

Helicoplacoids were early suspension feeding echinoderms that lived during the “Cambrian Substrate Revolution”, roughly 520 million years ago, when the ocean substrate changed from mainly mat-dominated to a more mixed substrate-water surface. Complete specimens are only found in the Lower Cambrian Poleta Formation of the White-Inyo Mountains of California, and despite their abundance at this location, after a short period of time they went extinct likely due to their lack of adaptation to the increased amount of bioturbation over time. However, much is still unknown about helicoplacoids and why they are only found preserved completely at one locality. To examine the body size and taphonomy of Helicoplacoids, we collected data from the UC-Museum of Paleontology (UCMP), which contains the largest collection of helicoplacoids in the world. We compared this data to that of the helicoplacoid collections of USC and LACMNH using data from Dornbos and Bottjer, 2001.This data was analyzed quantitatively by measuring helicoplacoid sizes and assessing their preservation via levels of taphonomic degradation. Our results indicate that in contrast to that of the southern CA collections, UCMP’s collection was dominated by mostly- to entirely-disarticulated specimens. An exception to this was our type collection, of which had excellent preservation, indicating that there is potentially a bias towards better preservation and hence more low energy conditions in type collections. In addition, we also found that helicoplacoids typically ranged in size from 1 to 7 cm in length and 0.5 to 3 cm in width and likely didn’t get much larger than this. Overall, it appears that the paleoenvironment preserved in the Poleta Formation represents a very special case of taphonomic preservation in which helicoplacoids were exceptionally fossilized.