COASTAL AND INLAND ASSEMBLAGES OF CHONDRICTHYES AND ACTINOPTERYGII DOCUMENT PALEOENVIRONMENTS ACROSS THE "MESAVERDE" FORMATION (LATE CRETACEOUS, CAMPANIAN), BIGHORN BASIN, WYOMING, USA

The “Mesaverde” Formation, exposed in the Wind River and Bighorn basins of Wyoming, preserves a significant Campanian-aged vertebrate assemblage. Previously, chondricthyans were well documented in presumed estuarine deposits from the Teapot Sandstone Member in the eastern Bighorn Basin of Washakie County. Ongoing fieldwork is resampling these localities, in addition to identifying new sites in the western Bighorn Basin of Park County. These work essentially creates an environmental transect spanning a coastal/marine assemblage (eastern Bighorn Basin) and an inland assemblage (western Bighorn Basin).

The marine assemblage of Washakie County includes abundant lamniform sharks, with Archaeolamna kopingensis, Carcharias steineri, and Scapanorhynchus texanus particularly well represented, alongside the sclerorhynchid Ischyrhiza mira and the hybodonts Meristodonoides montanensis and M. wyomingensis. Batoids include abundant myledaphines (Pseudomyledaphus and Cristomylus) as well as Rhinobatos casieri and Protoplatyrhina renae. Actinopterygians are dominated by Enchodus spp. (E. petrosus and E. gladiolus), in addition to Paralbula casei, Protosphyraena sp., Amiidae, and Lepisosteidae.

A newly identified inland assemblage from Oregon Basin of Park County is dominated by orectolobiformes, particularly Cantioscyllium bighornensis, as well as the myledadphines Pseudomyledaphus and Cristomylus. The hybodont Lonchidion also occurs in the Oregon Basin assemblage; unlike the marine assemblages, Meristodonoides is incredibly rare inland. Actinopterygians are most commonly represented by Lepisosteidae, in addition to Amiidae (including Vidalamiinae and Amiinae) and Elopiformes. This compares closely with other assemblages across the Western Interior Basin.

The occurrence of Pseudomyledphus and Cristomylus and apparent absence of Myledaphus bipartitus is most consistent with a minimum age of middle Campanian (~80.6 to 76.4 Ma), in concordance with the occurrence of solemydid turtles. Biogeographically, the presence of Lonchidion is shared with southern faunal assemblages of the Campanian, and other taxa are shared with both northern and southern assemblages. This is potentially consistent with the “Mesaverde” Formation being a mixing zone between regions, although additional sampling is needed to test this hypothesis.