NEW RECORDS OF VERTEBRATES FROM THE LATE CRETACEOUS TROPIC SHALE OF SOUTHERN UTAH

The Tropic Shale is comprised primarily of open marine, clastic-dominated shales that crop out in central southern Utah. These deposits lie stratigraphically between the underlying Dakota Formation and overlying Straight Cliffs Formation and are equivalent to the well studied portion of the Mancos Shale exposed at Black Mesa, AZ, about 100 km southeast of our study area. The Tropic Shale was deposited along the western margin of the Cretaceous Interior Seaway, and it spans the late Cenomanian Sciponoceras gracile NA Ammonite Zone through the mid-Turonian Prionocyclis hyatti Zone. Although the invertebrate megafauna of the Tropic Shale has been studied in detail, particularly in terms of ammonite and inoceramid biostratigraphy, the vertebrate fauna has not been given similar attention. This lack of attention is due primarily to what heretofore has been perceived as a near absence of vertebrates in the unit (with the exception of chondrichthyans), and because studies of vertebrates have focused more on the terrestrial units lying above and below the marine shales. Non-fish vertebrates previously reported from the Tropic Shale are limited to one series of associated plesiosaur vertebrae, other isolated plesiosaur elements, and a marine turtle recovered from equivalent exposures at Black Mesa. Our recent work has significantly altered the perception that the Tropic Shale preserves few vertebrates, and indicates that this unit has a diverse and important extra-chondrichthyan vertebrate fauna that includes osteichthyans, a variety of plesiosaurs and marine turtles, and even a terrestrial dinosaur. The dinosaur represents one of the first two therizinosaurs ever reported outside of Asia. The detailed biostratigraphic framework previously constructed for the Tropic Shale and the equivalent portion of the Mancos Shale at Black Mesa, together with numerous prominent bentonite and limestone concretion marker beds in the sequence, allows us to place all of our finds in a high resolution chronostratigraphic context. These new finds add significantly to our understanding of the vertebrate fauna of the Cretaceous Interior Seaway during the Cenomanian-Turonian (Greenhorn Cyclothem) interval.