OBSERVATIONS ON A NEW ORNITHOPOD FROM THE MID-CRETACEOUS WAYAN FORMATION OF EASTERN IDAHO

The mid-Cretaceous (Albian-Cenomanian) Wayan Formation of Idaho contains a vertebrate assemblage overwhelmingly dominated by an undescribed basal ornithopod. Specimens commonly occur in variegated, caliche nodule bearing paleosols representing a semiarid environment. Specimens commonly include articulated caudal vertebrae and complete articulated feet, with elements having undergone little or no abrasion or pre-depositional weathering, all indicative of relatively quick burial. Two localities have produced remains of multiple closely associated individuals of various sizes.

The taxon has robust pes phalanges, moderately divergent metatarsals, and an expanded distal tibia as in Zephyrosaurus. Similarities with Orodromeus and related ornithopods include anterior caudal vertebrae with the transverse processes originating near the neurocentral suture, and distal caudal vertebrae with hexagonal ends on the centra. Like Oryctodromeus and Orodromeus, the Wayan form has an enlarged coracoid. A notable difference from Oryctodromeus is the presence of abundant epaxial and hypaxial tendons. In addition, the Wayan form has a less robust deltopectoral crest in comparison to Orodromeus. Preliminary estimates suggest an animal of roughly two meters in length.

Our preliminary work suggests this form is closely allied with the Zephyrosaurus + Oryctodromeus + Orodromeus clade. This unnamed taxon has potential to elucidate relationships with the above clade, and to enhance knowledge of the evolution and distribution of Cretaceous “hypsilophodontid” grade ornithopods in western North America. Recent studies have suggested the above clade may have been specialized for fossorial behavior. However, no evidence of Oryctodromeus-like burrows has yet been recognized in the Wayan.