PROGRESS TOWARD CONSTRAINING CARBONIFEROUS-PERMIAN VERTEBRATE BIOCHRONOLOGY IN WESTERNMOST PANGEA

Collecting in the underexplored Carboniferous-Permian (C-P) along the Colorado-Utah border has shed light on C-P vertebrate succession in western Pangea. This transition was marked by brief cooling and aridification that restricted aquatic freshwater habitats across the western part of the supercontinent. A better understanding of vertebrate biochronology would permit inter-basinal comparisons of community responses to large-scale climate change. Here, I present new data and review the current state of knowledge of C-P vertebrates in the Four Corners states, with emphasis on the understudied Paradox and Eagle basins of Utah and Colorado. Channel deposits in Utah’s lower Cutler beds are known to preserve diverse ‘fishes,’ dissorophoid amphibians, diadectomorphs, synapsids and reptiles. Co-occurrences of the dipnoan Sagenodus copeanus, limnoscelids, and the synapsids Ophiacodon navajovicus and Sphenacodon constrain portions of the lower Cutler beds to latest Pennsylvanian (Coyotean land vertebrate faunachron [LVF]). New conodonts from marine incursions include the basal Permian Adetognathus n. sp. B, its first local occurrence in the middle of the Halgaito Formation, supporting prior speculation that this unit encompasses the C-P boundary (CPB) and that its contact with the underlying Rico beds is time transgressive. In the Eagle Basin, Colorado, new fossils in basal limestones of the Maroon Formation show similarities to those of other late Pennsylvanian localities in western North America, most notably in New Mexico (Kinney Brick) and Kansas (Hamilton). These include xenacanths, a hybodont with affinities to Hamiltonichthys, platysomids, Sagenodus cf. S. hlavini, and an amphibamiform tetrapod. In summary, as few as three and up to four C-P LVFs are represented by the Utah-Colorado sites: (1) a Cobrean assemblage is known from the Sangre de Cristo Formation of central Colorado; (2) a basal Maroon assemblage with late Pennsylvanian elements in western Colorado is newly described; (3) Coyotean assemblages in the undivided Cutler of southwestern Colorado and the lower Cutler beds of eastern Utah are shown here to span the CPB; and (4) younger Seymouran assemblages in the Organ Rock Formation of southern Utah preserve an early Permian terrestrial fauna as in the Abo Formation, New Mexico.