VERTEBRATE FOSSIL DISCOVERIES PROMPT MAJOR SHIFT IN MESOZOIC STRATIGRAPHY OF PICKETWIRE CANYONLANDS, COMANCHE NATIONAL GRASSLAND, SOUTHEASTERN COLORADO

Recent discovery of Triassic tetrapod fossils in southeastern Colorado necessitates large scale changes to the currently-accepted stratigraphy of the area. The bone-bearing strata lie atop a thick eolianite historically mapped as, and referred to, the Middle Jurassic Entrada Sandstone. Fossils consist of cm-scale broken, and often rounded, bone fragments and teeth recovered from thin (m-scale) discontinuous channels of limestone-pebble conglomerate deposited in a high energy fluvial environment. Fossil taxa include archosaurs, temnospondyls, and a single ganoid fish scale. Temnospondyl bones consist of pitted fragments consistent with skull and/or pectoral girdle elements of metoposaurs. Phytosaur fossils include type C and B teeth, skull and jaw fragments, and osteoderms. Aetosaurs are known from osteoderms, principally pieces of anterior bars of dorsal paramedians and fragmentary lateral osteoderms from “spiked” taxa with strongly flexed laterals. All identifiable tetrapods pertain to taxa known only from strata of Late Triassic age elsewhere, but none constrain the assemblage more precisely, although the assemblage is reminiscent of lower Chinle Group faunas historically thought to be Carnian in age. The two most reasonable solutions to this conundrum are that the Triassic strata of this area have been mistakenly correlated with the Entrada Sandstone, or else the fossils are reworked into dramatically younger (Mid-Late Jurassic) sediments. The conglomerates are lithologically dissimilar from other Jurassic units regionally, but similar to those in the Upper Triassic of Wyoming and New Mexico. Therefore, we consider the fossils to be in autochthonous deposits. The most parsimonious solution is to tentatively correlate the eolianite underlying the fossiliferous strata to the Middle Triassic Jelm Formation in northern Colorado and Wyoming, the conglomerates to the Chinle, and the overlying gypsiferous strata between the conglomerates and the Morrison Formation ~30 m higher to the Ralston Creek Formation.