A NEW PARTIAL SKELETON OF XIPHACTINUS AUDAX WITH A WELL-PRESERVED NEUROCRANIUM FROM THE LATE CRETACEOUS TROPIC SHALE OF SOUTHERN UTAH

The Western Interior Seaway covered the western region of North America during the Cretaceous. The long collection history along the eastern side of the seaway, from formations like the Niobrara Chalk, has led to the discovery of a wide diversity of vertebrate fossils, including an abundance of bony fish. While work along the western side of the seaway has been relatively recent in comparison, the same types of vertebrates are being discovered. Sediments of the Tropic Shale were deposited during the Late Cretaceous (Cenomanian to Turonian) along the western margin of the Western Interior Seaway and are comprised of fine-grained black shale. The shale is separated by laterally continuous bentonite layers ranging from the oldest, Bentonite A, to the youngest, Bentonite E. The Tropic Shale contains a diverse vertebrate fauna including: plesiosaurs, mosasaurs, marine turtles, sharks, bony fish, and a coastal dinosaur. Research in the Tropic Shale has thus far been focused on large marine reptiles including plesiosaurs and a recently identified mosasaur.

MNA V10796 (Museum of Northern Arizona, Flagstaff) is a partial skeleton of Xiphactinus audax discovered in 2012 and excavated in 2014-15 from the Tropic Shale in Glen Canyon National Recreation Area. The specimen was approximately 0.3 meters below the lower couplet of Bentonite D. MNA V10796 was partially articulated, is moderately compressed and heavily fractured. The specimen includes a well-preserved neurocranium, two fragmentary teeth, five abdominal vertebrae, a couple of ribs, and a partial pectoral fin. The skeletal material from MNA V10796 is morphologically similar to previously described specimens of X. audax. The neurocranium, which is the first to be described from a specimen of X. audax from the Tropic Shale, is from the right side of the skull and includes the supraoccipital, epiotic, parietal, pterotic, sphenotic, dermosphenotic, frontal, postfrontal, prefrontal, ethmoid, and the premaxilla. The teeth have glassy, smooth, enamel surfaces, are straight with no carinae, and are oval in cross-section. The vertebrae are strongly amphicoelous, perforated with a small notochordal canal, and are circular in anteroposterior view. This specimen adds to the more limited information known about X. audax along the western side of the Western Interior Seaway.