The oldest vertebrate trace fossils from Comb Ridge (Bears Ears Region, southeastern Utah)
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Abstract
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.
Cite this as
2017. The oldest vertebrate trace fossils from Comb Ridge (Bears Ears Region, southeastern Utah) PeerJ Preprints 5:e2662v2 https://doi.org/10.7287/peerj.preprints.2662v2Author comment
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.
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Supplemental Information
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.
Generalized geologic map of MNA Loc. 1776
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.
MNA V10984 and outline of track
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.
Overview of MNA V10985
Figure 5: A) MNA V10985. B) MNA V10985 with tracks outlined. M1, manus print 1; M2, manus print 2; P1, pes print 1; P2, pes print 2.
Close up of Pes 1, MNA V10985
Figure 6: A) Close-up of P1, MNA V10985. B) Close-up of P1, MNA V10985 with the trace outlined. Scale bar = 1 cm.
Close up of Manus 1, MNA V10985
Figure 7: A) Close-up of M1, MNA V10985. B) Close up of M1 with the track and features outlined, MNA V10985. Scale bar = 1 cm.
Close up of Manus 2, MNA V10985
Figure 8: A) Close up of M2, MNA V10985. B) Close of M2 with track outlined, MNA V10985. Scale bar = 1 cm.
Pes of MNA V10985 compared to Brachychirotherium
Figure 9: A comparison between an aetosaur/Brachycheirotherium pes track (modified from Heckert et al., 2010) (A) and P1 of MNA V10985 (B). Scale bar 1 = cm for each track.
MNA V10985 compared to a Brachychirotherium manus
Figure 10: A comparison between an aetosaur/Brachycheirotherium manus track (modified from Heckert et al., 2010) (A) and M1 of MNA V10985 (B). Scale bar 1 = cm for each track
Additional Information
Competing Interests
The authors declare that they have no competing interests.
Author Contributions
Robert J Gay conceived and designed the experiments, performed the experiments, analyzed the data, contributed reagents/materials/analysis tools, wrote the paper, prepared figures and/or tables, reviewed drafts of the paper.
Xavier A Jenkins performed the experiments, analyzed the data, wrote the paper, reviewed drafts of the paper.
Taormina Lepore performed the experiments, analyzed the data, wrote the paper, reviewed drafts of the paper.
Field Study Permissions
The following information was supplied relating to field study approvals (i.e., approving body and any reference numbers):
The specimens described here were collected under Bureau of Land Management paleontology permit UT14-001S issued to RG and are reposited at the Museum of Northern Arizona.
Data Deposition
The following information was supplied regarding data availability:
The research in this article did not generate, collect or analyse any raw data or code.
Funding
The fieldwork to discover and collect the specimens described here was funded by a grant from the Canyonlands Natural History Association. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
