OPHIUROIDS IN THE AFTERMATH OF THE END-PERMIAN BIOTIC CRISIS - NEW FOSSILS FROM NORTH AMERICA AND ITALY

Fully articulated ophiuroids are extremely rare in the fossil record and much of their evolutionary history is based on the study of isolated rock slabs containing a few well-preserved individuals. By contrast, Lower Triassic sediments in eastern Nevada and the Italian Dolomites contain a rich accumulation of ophiuroid body fossils, disarticulated ossicles, and trace fossils (ichnogenus Asteriacites). The well-preserved body fossils from eastern Nevada are the first Triassic ophiuroids reported from North America.

Ophiuroid specimens are found in a wide range of lithologies. Body fossils occur in silty limestones and fine grained calcareous sandstone beds, interpreted as rapidly deposited sediment flows. Deposition took place during brief, high energy (storm?) events in an otherwise low-energy environment below normal wave base. Disk diameters range from 2 to 4.5 mm and delicate arms structures such as tentacle pores, spines, and dorsal and ventral shields are preserved. Fossils are found alone or in association with a low diversity community of bivalves and, rarely microgastropods. Disarticulated ossicles are much more common and occur in bioclastic storm beds throughout the shallow marine, mixed carbonate-siliciclastic ramp environments of the Werfen Formation (Italy) and indicate that ophiuroids were a significant component of the epifaunal community. The ossicles co-occur with disarticulated bivalve fragments, conodont elements and common microgastropods, and in storm gutter casts, may be the dominant bioclast. Ophiuroid trace fossils are limited to fine grained, siliciclastic shallow marine redbeds. They are often associated with bedding-parallel trace fossils produced by a low diversity, shallow penetrating, deposit feeding community. Ophiuroid body and trace fossils have been found in deposits of Dienerian to Spathian age only, while disarticulated ossicles occur from the Griesbachian (isarcica zone), and show that ophiuroids were present throughout the Early Triassic. However, these Early Triassic ophiuroids have smaller disk diameters than specimens from the Late Permian and Middle/Late Triassic. Thus ophiuroids, along with other organisms such as gastropods, bivalves and fish exhibit unusually small body sizes after the end-Permian mass extinction.