FIRST OCCURRENCE OF PACHYRHIZODUS LEPTOPSIS IN THE TROPIC SHALE (CENOMANIAN-TURONIAN) OF SOUTHERN UTAH

The Tropic Shale is a marine formation representing the western margin of the Western Interior Seaway. It was deposited during the Late Cretaceous (Cenomanian/Turonian) and is exposed today in southern Utah. Researchers have been collecting vertebrate fossils from the Tropic Shale since the early 1990s and have revealed an incredible diversity of organisms, including fish, sharks, turtles, plesiosaurs, and mosasaurs. Prior to this work, the known fish fauna from the Tropic Shale included a pycnodont fish, Ichthyodectes ctenodon, Gillicus arcuatus, and multiple specimens of Xiphactinus audax. A newly discovered specimen (MNA V10651), identified as Pachyrhizodus leptopsis, adds a fifth fish to the known biodiversity of the Tropic Shale and extends the known geographic range of Pachyrhizodus to the west. Pachyrhizodus has previously been documented along the eastern margin of the Western Interior Seaway (in Kansas, Texas, and Alabama) and around the globe, in sediments ranging from the Cenomanian to the Maastrichtian.

MNA V10651 was discovered in Grand Staircase-Escalante National Monument in 2012, 1.8 m below the base of Bentonite C in the Tropic Shale. The specimen was disarticulated and incomplete, consisting of the back of the skull, the right maxilla, the left and right dentaries, some isolated teeth, a single vertebra, and additional bone fragments. It has been identified as belonging to the genus Pachyrhizodus on the basis of the pleurodont dentition, the conical shape of the teeth, the thick roots present on the teeth, the irregularly-shaped parietals, and the angular exoccipitals. The specimen shares characteristics of P. leptopsis, including carinae on the teeth and the U-shaped cross-section of the thick area below the tooth row of the dentaries.