The oldest vertebrate trace fossils from Comb Ridge (Bears Ears Region, southeastern Utah)

Abstract

Vertebrate trace fossils are common in Upper Triassic deposits across the American southwest. These ichnofauna are dominated by Grallator, Brachycheirotherium, and Pseudotetrasauropus and lack ichnotaxa traditionally considered to be Early Jurassic in age, such as Eubrontes and Anomoepus. While known from Indian Creek and Lisbon Valley, vertebrate trace fossils have not been previously reported from Comb Ridge. This is despite lithostratigraphic work having been conducted at the elsewhere-fossiliferous “Big Indian Rock Beds” at the US Highway 163 roadcut since the 1990s. 2016 fieldwork by the Museums of Western Colorado: Dinosaur Journey recovered two sandstone slabs that had been dislodged from a river channel sand in the Church Rock Member of the Chinle Formation. The slabs preserve the first documented vertebrate trace fossils from Comb Ridge: a single pes impression of the ichnogenus Grallator, and several manus and at least one pes impression of a small archosaur. We tentatively refer this second track set to the ichnogenus Brachychierotherium. Taken together, these specimens provide evidence for a more diverse vertebrate fauna in the Church Rock Member of the Chinle Formation at Comb Ridge than indicated by the body fossil record.