PALEONTOLOGY, PALEOECOLOGY, AND STRATIGRAPHY OF THE MIDDLE PORTION OF THE CHUGWATER GROUP, CENTRAL WYOMING

The tripartite Chugwater Group of Wyoming is one of the least studied continental Triassic intervals in North America. The Lower Triassic Red Peak Formation long has been thought to be devoid of evidence of past life and often assigned a marine deposition. Our new efforts demonstrated robust vertebrate and invertebrate ichnocoenoses, and an unambiguous continental depositional environment for the upper portion of the Red Peak Formation. Building upon that work, recent ⁸⁷Sr/⁸⁶Sr values fit to the global marine strontium curve indicated a depositional age assignment for the overlying Alcova Limestone to the late Olenekian (Spathian). That age constraint redated the only vertebrate fossil from the Alcova Limestone, Corosaurus alcovensis, making it the oldest pistosaurid sauropterygian from North America. Furthermore, it provided a minimum age for those vertebrate ichnoassemblage from the underlying Red Peak Formation.

To date, few fossils have been reported from the enigmatic strata that separate the Lower Triassic Alcova Limestone from the Upper Triassic Popo Agie Formation, namely the Crow Mountain, ‘unnamed redbeds’, and Jelm formations. The paucity of fossils has, in part, hindered refinement of the temporal range for those units. However, recent discoveries may help to elucidate the depositional ages of these strata. These include 11 rhynchosaur cranial elements (maxillae and dentaries) and one archosaur quadrate from sections near Alcova Reservoir (central Wyoming). In addition to body fossils, new vertebrate tracks have been observed, including a grallatoroid trace from the southern Bighorn Mountains (‘unnamed redbeds’), which extends the known range of Dinosauriformes into the Middle Triassic of North America. This new vertebrate assemblage is comparable to Middle Triassic strata of South America and Europe and expands the biogeographic range of these clades.