VERTEBRATE BODY AND TRACE FOSSILS FROM THE UPPER CRETACEOUS (CAMPANIAN) NESLEN FORMATION, UTAH, USA: IMPLICATIONS FOR LARAMIDIAN BIOGEOGRAPHY AND PALEOECOLOGY

The Western Interior Basin of North America preserves numerous penecontemporaneous fossil assemblages from the Campanian stage. These have served as a study system for understanding paleobiogeography and evolutionary patterns, providing data supporting some level of provinciality and endemism in certain clades. We present new contributions to the vertebrate body and trace fossil assemblage from the Campanian (~75 Ma) Neslen Formation of east-central Utah. The Neslen Formation represents deposition in fluvial to paralic settings along the Laramidian margin of the Western Interior Seaway. Compared to other Campanian formations, the Neslen Formation has not been rigorously surveyed for its paleontological resources. Thus far, only two vertebrates – the hadrosaur Rhinorex and a partial tyrannosaur hindlimb – are published from the formation.

Four field seasons in the Neslen Formation by the Prehistoric Museum have produced over 150 new cataloged specimens of plants, invertebrates, and vertebrates, along with numerous localities preserving vertebrate tracks. The lower portion of the formation is the most fossiliferous, especially the Palisades Coal Zone and overlying estuarine deposits. Though most of the vertebrate body fossils are isolated elements, many are diagnostic to the generic level. Turtles include Adocus, Basilemys, c.f. Neurankylus, a plastomenine (c.f. Hutchemys), and two other trionychids. Additionally, we recognized the ichnogenera Emydhipus and Chelonipus, both hypothesized to represent turtle trace makers. One partial hybodontiform shark specimen was discovered in estuarine deposits. At least two morphotypes of crocodyliform teeth were discovered, including one assignable to the Mesoeucrocodylia based on apomorphies. Additionally, large tracks that may be attributable to the group were discovered on the bases of bayhead delta deposits. New dinosaur remains are represented by dromaeosaur and tyrannosaur teeth, as well as bones from hadrosaurs, nodosaurid ankylosaurs, and ceratopsians. One ceratopsian specimen is a postorbital horn that exhibits a morphology unique amongst known ceratopsians, curving dorsolaterally then dorsally directly from the orbital margin.

Results from body fossils support the presence of components from both the hypothesized northern and southern Campanian biogeographic provinces, indicating either a latitudinal gradient or zone of mixing between two provinces. Plastomenines, including the genus Hutchemys, are more common to the south (southern UT and NM). The published tyrannosaur from the formation possesses features that may be similar to northern (MT and AB) tyrannosaurs. The fossil assemblage of the Neslen Formation underscores the utility of even fragmentary remains in providing data on paleoecology and paleobiogeographic patterns.