VERTEBRATE TRACE FOSSILS FROM THE NESLEN FORMATION (UPPER CRETACEOUS: CAMPANIAN) IN EASTERN UTAH

Across the Western Interior of North America, fluvial basins preserve a penecontemporaneous record of vertebrate biodiversity during the Campanian stage of the Late Cretaceous. Research has focused primarily on the vertebrate body fossil record of these basins, especially the biogeographic patterns of organisms across Laramidia, the narrow landmass west of the interior seaway. The occurrences and implications of vertebrate trace fossils have received much less attention, but offer a window into the behavior of organisms across this landmass. Here, we provide a preliminary report on vertebrate ichnology from the basal Neslen Formation in the Book Cliffs of eastern Utah. The Neslen Fm records the gradual regression of the Western Interior Seaway from the region, with the fossiliferous base of the formation being dominated by paralic deposits.

We documented vertebrate trace fossils using traditional photography and LiDAR scanning. Most of these tracks and trackways are at or near the bases of sandstone packages interpreted as bayhead delta deposits. Thus far, we have identified seven morphotypes. Small (2-4 cm) morphotypes include anteriorly-tapering tetradactyl manus and pes tracks (in two size classes) and crescentic tetradactyl tracks, both of which are represented in trackways. A third small track is tridactyl, elongate, and isolated from other tracks. A mid-sized (~9 cm) track is tetradactyl and tapers anteriorly, with the manus and pes overlapping. A larger (~13 cm) didactyl track is represented by elongate, widely-spaced impressions. Finally, a large (> 20 cm), oval-shaped, tetradactyl track exhibits visible webbing between digit impressions.

Four of the traces are assignable to the ichnogenera Emydhipus and Chelonipus, which likely represent turtle track makers. This is consistent with the fresh to brackish-water setting of the basal Neslen Formation. Three of the trace morphotypes remain unassigned to ichnogenera, and require further investigation for a confident ichnotaxonomic assignment. The vertebrate trace fossil record of the Neslen Formation offers a window into the ecology and behaviors of organisms thus far poorly documented from the Campanian of Laramidia and adds to a growing understanding of this ecosystem near the margin of the Western Interior Seaway.