292-8 A Diverse Vertebrate Assemblage from the Upper Cretaceous (Campanian) Neslen Formation, Utah, USA and the Utility of Isolated Remains in Reconstructing Vertebrate Faunas

Abstract:

The Campanian Neslen Formation, exposed in the Book Cliffs of eastern Utah, is a geologic unit that historically has not been heavily prospected for paleontological resources when compared to contemporaneous fluvial strata across the Western Interior Basin. The skull and skin impressions of the hadrosaur Rhinorex and a partial tyrannosaur hindlimb represent the only two vertebrates formally described from the formation. Ten expeditions to the Book Cliffs led by the Prehistoric Museum have produced an abundance of vertebrate remains. Although most of those are isolated bones and other fragmentary specimens, these data can be applied to large-scale paleobiologic and biogeographic questions. 

Here, we describe the vertebrate body fossil assemblage using an approach that focuses on apomorphic characters to avoid circularity in identification and provide the most accurate representation of the fauna. One partial hybodontiform shark tooth, comparable to Meristodonoides, was discovered in estuarine deposits low in the formation. Turtles include members of the Adocidae (c.f. Adocus), Nanhsiungchelyidae (c.f. Basilemys), Baenidae indet., a plastomenine (Hutchemys), and two other trionychid morphotypes. Hutchemys (Derrisemys) was previously identified from the Campanian Kaiparowits Formation but is otherwise only known from the Maastrichtian and Paleocene. We recognize two morphotypes of crocodyliform teeth, including one assignable to the Mesoeucrocodylia. Elements from dinosaurs discovered thus far include dromaeosaur and tyrannosaur teeth as well as bones from saurolophine hadrosaurs, nodosaurid ankylosaurs, and ceratopsians. One ceratopsian specimen is an isolated postorbital horn that curves dorsolaterally from the orbital margin. Though not confidently assignable to any named taxa on its own, it shares some resemblance to Judiceratops from the Judith River Formation of Montana.  

Our results 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. Hutchemys represents a southern Campanian taxon that disperses northward during the Maastrichtian, whereas the published tyrannosaur was hypothesized to share traits with species from Montana and Alberta. The presence of a ceratopsian similar to Judiceratops also supports some northern Laramdian influence in the Neslen, though we use caution in assigning the Neslen specimen to that taxon. The fossil assemblage of the Neslen Formation underscores the utility of even isolated remains in providing data on paleoecology and paleobiogeographic patterns. 

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