REVISITING HELICOPLACOIDS: ECOLOGY AND TAPHONOMY OF ONE OF THE CAMBRIAN'S ODDEST ECHINODERMS

Helicoplacoids were one of the earliest groups of echinoderms to evolve in the early Cambrian (~521 mya), and also one of the first to go extinct, roughly 7 million years later. Complete specimens are only found in the lower Cambrian Poleta Formation of Westgard Pass, in the White-Inyo Mountains of California and Nevada. And despite their abundance at this location, little is known about how they lived and why they went extinct. Long thought to be sediment stickers, new evidence suggests that they may have potentially attached to hard substrates via smaller plates on the lower pole. Hence, to examine the ecology and taphonomic preservation of this unique group, data was collected from the UC Museum of Paleontology (UCMP) invertebrate collection, which contains the largest collection of helicoplacoids in the world. Specimens were assessed in terms of their preservation via levels of taphonomic degradation, and the best preserved fully articulated specimens were examined to determine any potential attachment structures. In addition, ambulacra of each specimen was assessed to determine if any morphological differences existed between adult and juvenile specimens, in addition to any indicators of an oral region. Preliminary results indicate that in contrast to that of many other museum collections, UCMP’s collection was dominated by mostly- to entirely-disarticulated specimens. This agrees with prior speculation which suggests that complete preservation of helicoplacoids is relatively rare, even at Westgard Pass. Overall, it appears that helicoplacoids were rather geographically restricted to that of Western Laurentia, in addition to being aided by perfect taphonomic conditions in the Poleta Formation at that time.