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Vertebrate trace fossils are common in Upper Triassic deposits across the American southwest. These ichnofauna are dominated by Grallator, Brachychirotherium, 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, Utah, vertebrate trace fossils have not been previously reported from Comb Ridge, Utah. This is significant considering that lithostratigraphic work has been ongoing at Comb Ridge since the 1990s in the elsewhere fossiliferous ‘Big Indian Rock Beds’, in the US Highway 163 roadcut that transects Comb Ridge. 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 Triassic 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 Brachychirotherium. 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 current body fossil record.
We incorporated changes suggested by several people who viewed and commented on the first version of the preprint. Some of these changes are changes to the figures while others are changes to the structure and grammar of the text itself.
Location of MNA Loc. 1776 in the State of Utah
Figure 1: Location of MNA Loc. 1776 in the State of Utah. Tan area indicates Bears Ears region.
Figure 2: Generalized geologic map of MNA Loc. 1776, which is located in the ‘Bread Bowl’ area (small dashed line). Dashed and dotted line indicates the ridgeline of Comb Ridge. Trcmb, Monitor Butte Member, Chinle Formation; Trccr, Church Rock Member, Chinle Formation; TrJw, Wingate Sandstone; Jk, Kayenta Formation; Jn, Navajo Sandstone; Qa; Alluvium; Qc; Colluvium. Scale bar = 0.1 km, north is to top of image.
Area in which MNA V10984 and V10985 were discovered
Figure 3: A) Photograph of Bread Bowl area with ledge forming units visible. BIRB; “Big Indian Rock beds”; CL; coquina layer; LLFU; lower ledge-forming unit; TrM, Moenkopi Formation, TrCcr, Church Rock Member, Chinle Formation; TrJW, Wingate Sandstone; Qa; Alluvium. B) generalized stratigraphic column of the same area. C) Vertebrate burrows in the lower ledge forming unit.
Figure 4: A) MNA V10984, Grallator trace and mudcracks. B) MNA V10984, Grallator trace (outlined) with mudcracks. Dashed lines indicates where DII was present when discovered and the approximate presumed extent of DIV.