ENVIRONMENT OF DEPOSITION AT THE EXCAVATION SITE OF THE DINOSAUR NOTHRONYCHUS SP. (THERIZINOSAURIA, THEROPODA) FROM THE UPPER CRETACEOUS TROPIC SHALE IN SOUTHERN UTAH FROM SEDIMENTOLOGICAL AND MICROFAUNAL ANALYSES

The surprise discovery of a skeleton of Nothronychus sp., a large therizinosaurid dinosaur in the lower beds of the Tropic Shale (Late Cretaceous, Cenomanian-Turonian) of southern Utah resulted in the recovery of a nearly complete skeleton in 2001. The geological setting indicates an offshore occurrence in open marine water, at least 100 km from the nearest shore of the Western Interior Seaway in the early phase of the Greenhorn Cyclothem. The carcass of this bulky, one-ton theropod dinosaur with highly specialized feeding and locomotor habits fell to the sea bottom in soft mud, with only minor displacement of the elements of the skeleton before burial amongst well preserved marine fauna including ammonoids and bivalves. Because the site lacks any indication of terrestrial sediments or fauna, the best hypothesis for transport of the carcass is the “bloat and float” explanation. However, a large carcass at sea for a long time should have been subject to scavenging by plesiosaurs, sharks, and fish that inhabited the same waters. In an effort to test that hypothesis, we have initiated sedimentological and microfaunal analyses to more thoroughly examine the environment of deposition of the dinosaur site and several plesiosaur sites in the same area.

Preliminary foraminifera biostratigraphy results indicate a normal, marine environment. Shallow marine sections of Cretaceous epicontinental seas are characterized by planktonic foraminifera species of Heterohelix and Gumbelitra, both genera present throughout the therizinosaurid section. These genera are the principal opportunistic planktonic foraminifers of the mid-Cretaceous, inhabiting near surface waters and having wider ecologic tolerances than other mid-Cretaceous genera. Other planktonic foraminifera present in the section include species of Hedbergella and Globigerinelloides, genera present in shallow epicontinental or marginal seas. The combination of genera present indicates a water depth of less than 100 meters. Normal marine salinity is indicated by the presence of calcareous benthic foraminifera including species of Gavelinella and Lingulogavelinella. The lack of significant shifts in clay mineralogy percents for the section demonstrates the stability of the shoreline during this time.