Early Jurassic Batrachopus-rich track assemblages from interdune deposits in the Wingate Sandstone, Dolores Valley, Colorado, USA

Abstract Late Triassic and Early Jurassic tetrapod track assemblages are abundant and quite well documented from the respective Chinle and Glen Canyon groups of the Dolores Valley region of western Colorado and adjacent regions, especially eastern Utah. However, gaps still exist in our knowledge of the spatial and temporal distribution of representative ichnogenera. The stratigraphic distribution of track assemblages in the cliff-forming Wingate Sandstone Formation, the lower unit of the Glen Canyon Group, underlying the Kayenta and Navajo formations, is difficult to determine because almost all Wingate assemblages are known only from fallen blocks, originating from precipitous, vertical cliffs, which yield both dune and interdune ichnofaunas. Among the characteristic Lower Jurassic biochron ichnogenera Grallator , Eubrontes , Otozoum , Anomoepus and Batrachopus , in the Dolores Valley only Grallator is known abundantly, while Otozoum , and Batrachopus were previously known only from single occurrences, and Eubrontes and Anomoepus have yet to be reported. We here report two large, Batrachopus -rich assemblages that give new insight into the interdune paleoenvironments frequented by the Batrachopus trackmaker, traditionally regarded as a basal crocdylomorph, and the theropodan trackmaker of Grallator . Both assemblages contain the best-preserved and most abundant Batrachopus currently known, locally in high densities. Where individual trackways are clearly visible they indicate that the trackmakers had variable gaits. A third new assemblage containing Batrachopus from Salt Creek is also described, as is a fourth new assemblage from the Uncompahgre Plateau which yields moderately large theropod tracks (cf. Eubrontes ), but no Batrachopus . Although diagnostic Late Triassic biochron ichnogenera like Brachychirotherium occur in basal units of the Wingate Sandstone, Batrachopus which is considered an Early Jurassic biochron ichnotaxon, occurs within the formation. Thus, vertebrate ichnotaxa help place the Triassic–Jurassic boundary within the Wingate Formation, shed light on their spatio-temporal distribution, and underscore their utility for paleoecological interpretation, in a unit devoid of tetrapod body fossils.