Ichnofaunas from the Triassic-Jurassic Boundary Sequences of the Gateway area, Western Colorado: Implications for Faunal Composition and Correlations with Other Areas

Abstract

Large fossil footprint assemblages have been recovered from the Chinle Group and overlying Wingate Formation, of the Dolores Valley canyonlands near Gateway, Colorado. Current evidence suggests that all the Chinle and many of the lower Wingate Formation assemblages are Late Triassic in age, but that the upper Wingate ichnofaunas are probably Lower Jurassic. Evidence for mammal and non-mammalian synapsid-like tracks in the Chinle is equivocal, but evidence for synapsid tracks in the eolian Wingate is abundant and strongly suggestive of significant facies control (i.e., the synapsid-eolian facies association is similar to that found in older and younger eolian deposits).

Currently a great imbalance exists between the lack of ichnotaxonomic names assigned to northern hemisphere Mesozoic mammal and mammal-like tracks and the plethora of names, some of dubious value, assigned to similar tracks from the southern hemisphere. The problems inherent in this imbalance are discussed in the context of the triple influences of faunal provincialism (including facies controls), provincial taxonomy, and uncertain biostratigraphic dating.

Current evidence suggest that while small theropod tracks (i.e., Grallator) are abundant throughout the Colorado sequence, large theropod tracks (e.g., Eubrontes) do not occur below the Wingate-Kayenta transition zone, which may lie close to the Triassic-Jurassic boundary. Evidence of Brachychirotherium from the lower Wingate and the first reports of large Otozoum from higher in the section (probably from just below the Kayenta Formation) tend to confirm this inference, help differentiate Late Triassic from Liassic ichnofaunas, and facilitate regional and global correlations.

Keywords:

• Wingate Formation
• Triassic-Jurassic boundary
• mammal tracks
• synapsid tracks

ACKNOWLEDGEMENTS

We thank Ken Cart and Robin Prunty (Fruita, Colorado) for finding the Otozoum trackway described herein and bringing it to our attention. We also thank Jennifer Schellenbach (Fruita) and Josh Smith (University of Utah) for their assistance in the field. We also thank John Foster, Museum of Western Colorado, and Erik Kvale, Indiana University, Bloomington, for their reviews of the manuscript.