PRELIMINARY TAPHONOMIC ANALYSIS OF A COELOPHYSOID THEROPOD DINOSAUR BONEBED IN THE EARLY JURASSIC NUGGET SANDSTONE OF UTAH

Early stages of excavation at new dinosaur bonebed in NE Utah within a wet-phase horizon near the middle of the upper eolian unit of the Nugget Sandstone has produced over 2700 bones and bone fragments. The vast majority of the bones pertain to new coelophysoid theropod characterized by a lightly-built foot with a splint-like metatarsal II fused proximally to metatarsal III. This taxon is represented by uncrushed, disarticulated remains of at least 12 individuals with scapulocoracoid lengths of 6-24 cm. Other fossils include a moderate-sized theropod (teeth), a diminutive sphenodontid, and probable cycad foliage impressions.

The fossils occur in two superimposed beds of massive, well-sorted, sandstones with uncommon ripple mark relics. Most bones are parallel to bedding in hydraulically stable positions. Aside from co-ossified bones, none are articulated, but they are commonly closely associated indicating partial to complete loss of soft tissues prior to very minor transport and burial. The upper bone bed yields the greatest concentration of bones, making it difficult to recognize associated individuals. Its elongate bones (n= 462) are strongly oriented NNE-SSW. The lower bed contains fewer bones but associated skeletons are easily recognized. Its elongate bones (n=506) are more randomly oriented, but with a NNW-SSE trend. The bonebeds correlate laterally with decimeter scale, green clayey siltstones with overlying tridactyl theropod tracks.

The green beds represent shallow lacustrine environments. The large quantity of bones suggest gregarious habits as reported in other coelophysoids. The large bone accumulations developed in an interdunal area, possibly at times of drought or alkaline lake conditions. How the bones were buried in the sand remains unknown but we attribute the re-orientation of elongate bones to wave action along a shoreline during a highstand of an ephemeral lake. The near absence of structure in most of the sandstones is attributed to invertebrate bioturbation.

The site provides insights into an interdunal environment and a wet-phase climate of the widespread Early Jurassic erg of what is now western North America. Future work on the superbly preserved bones will permit detailed osteological and phylogenetic analysis of the new theropod and other faunal members.