TERRESTRIAL, FRESHWATER, AND MARINE TETRAPODS AND TAPHONOMY IN THE “MESAVERDE” FORMATION (LATE CRETACEOUS, CAMPANIAN), BIGHORN BASIN, WYOMING, USA
Washakie County localities include plesiosaurs, mosasaurs, and hesperornithiform birds, consistent with a marine environment along the Western Interior Seaway. Both Washakie and Park county localities include non-avian dinosaurs and crocodylomorphs. Non-avian theropods include Tyrannosauridae, Dromaeosauridae, and Richardoestia gilmorei; troodontids are absent, and similar taxon abundances occur in the coastal Washakie and inland Park county localities. Hadrosaurids dominate in both areas, followed by ceratopsids and rare ankylosaurs. Squamates include Socognathus (Washakie County) and Scincomorpha indet. (Park Co.), both new records for the formation. Among turtles, trionychids are most common in both regions, in addition to adocids; solemydids are abundant at the inland (Park Co.) locations, along with baenids. The giant alligatoroid Deinosuchus and turtle Lophochelys, to date, have been found only in the marine-influenced sites in the eastern Bighorn Basin. Taphonomically, the sites are broadly similar when comparing parameters of average bone particle size and roundness, although one site in the eastern Bighorn Basin shows significantly greater wear on individual fossils.
The presence of Solemydidae is consistent with a minimum age of middle Campanian for the exposures sampled here. The apparent absence of Troodontidae contrasts with age-equivalent formations in southern North America (e.g., Wahweap Formation), but is consistent with patterns to the north in Alberta and Montana (Foremost and lower Judith River formations). Testing hypotheses around faunal endemism requires additional sampling.