IDAHO'S FIRST DINOSAUR IDENTIFIABLE TO GENUS LEVEL, ORYCTODROMEUS SP., FROM THE MID-CRETACEOUS WAYAN FORMATION, AND THE GEOLOGICAL AND PALEONTOLOGICAL SETTING

Here we report on Idaho’s first dinosaur identifiable to genus level, the small burrowing ornithopod Oryctodromeus sp., from the mid-Cretaceous age Wayan Formation of eastern Idaho. We also report on the geological, faunal, and floral settings. Partial skeletons and associated remains of Oryctodromeus represent over ten individuals of various ontogenetic stages. The specimens typically exhibit high degrees of articulation and little-to-no bone modification, suggesting rapid post-mortem burial, possibly within burrows as described for the type and paratype specimens from the Blackleaf Formation of Montana. One Idaho find consists of associated remains of three individuals, a juvenile, a subadult, and an adult.

Newly discovered members of the Wayan fauna include: an indeterminate dromaeosaur, known from an isolated tooth; iguanodontids, represented by an isolated tooth and a separate collection of vertebrae and toe elements; as well as a large crocodylian similar to Deinosuchus, represented by a fragmentary partial skull. The flora includes two forms of the fern Gleichenia, the fern Anemia, and conifer and angiosperm foliage. Newly reported palynomorphs include Eucommiidites minor, Liliacidites orbiculatus, and Triporoletes cenomanianus. Some palynomorphs indicate a marine influence in the Wayan fluvial systems.

Deposition of the Wayan Formation occurred in monsoonal inland floodplain and fluvial environments at the toe of the Sevier Thrust System, with the Cretaceous Interior Seaway to the east. Recent detrital zircon U-PB dates for the Wayan Formation indicate an age near the Albian/Cenomanian stage boundary for the base of the formation with a middle Cenomanian age indicated for the middle of the formation. This indicates age equivalence with the Blackleaf Formation of Montana and the uppermost Cedar Mountain Formation (upper Mussentuchit Member) of Utah, and a slightly younger age than the Cloverly Formation of Wyoming and Montana. Measurement of a poorly exposed but apparently undisturbed section of the Wayan indicates a thickness of 1,344 meters in Tincup Canyon of Caribou County. New stratigraphic sections reaffirm that stacked mudstone and siltstone paleosols characterized by calcareous nodule horizons dominate the Wayan, with some coarser intervals.