The mid-Cretaceous Wayan Formation of Idaho consists of floodplain-deposited siltstones and mudstones, with subordinate fluvial sandstones and conglomerates. Deposition occurred in a tectonic foredeep at the toe of the Sevier thrust complex. Measurement of two incomplete and one complete section indicates a thickness of roughly 1,344 meters in the study area. No laterally extensive marker beds are present but the lower Wayan contains distinct chert pebble conglomerates, and the middle Wayan may contain a higher relative abundance of fluvial sandstone. Known fossil localities are limited to the lower and middle portions of the formation. The Wayan Formation, as currently defined, has a flora consisting of the ferns Gleichenia and Anemia, conifers, and angiosperms. This flora compares best with the Cenomanian aged Dakota Sandstone flora of Westwater, Utah. The vertebrate fauna is overwhelmingly dominated by the small burrowing ornithopod Oryctodromeus with other vertebrates being rare, consisting of a small dromaeosaurid theropod, iguanodontid grade ornithopods, and a large crocodilian similar to Deinosuchus. The fauna is most similar to the Cenomanian Blackleaf Formation of Montana. Prior to this report, only fragmentary dinosaurian remains had been reported from Idaho. The recovery of 10+ individuals of Oryctodromeus, some reasonably complete and partially articulated, provide significant insights into this genus, including an elongate femoral head projecting at roughly 40° from the femoral shaft, elongate cervical and dorsal centra, and abundant ossified tendons in the dorsal and caudal columns. Taphonomic evidence supports previous suggestions of adult/juvenile social groups and burrowing behavior in this taxon. New detrital zircon U-Pb dates and palynological analyses support a latest Albian to Cenomanian age for the Wayan Formation, but provide little additional age resolution. The tightest age resolution is provided by faunal correlation using Oryctodromeus, a taxon previously known only from the Cenomanian-age Blackleaf Formation of southwestern Montana. Similar tectonic settings, ages, lithologies, and the presence of Oryctodromeus in both faunas suggest synonymy of the Wayan and Blackleaf Formation, with the Wayan having precedence.



College and Department

Physical and Mathematical Sciences; Geological Sciences



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Wayan, Blackleaf, ornithopod, Cretaceous, Idaho, Oryctodromeus

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Geology Commons