LATE PALEOZOIC TETRAPOD BIOCHRONOLOGY IN THE WESTERN UNITED STATES

In the western US, late Paleozoic tetrapod body fossils are known principally from the Four Corners states of Utah, Colorado and New Mexico, and can be correlated, in part, to the classic Early Permian tetrapod assemblages from Texas-Oklahoma. Three land-vertebrate faunachrons (A, B, C) encompass the Four Corners record. The oldest faunachron (A) begins with the FAD (first appearance datum) of Edaphosauridae, which marks a substantial change in the tetrapod fauna with the FADs of Eryopidae, Cochleosauridae, Dissorophidae, Trimerorhachidae, Diadectidae, Petrolacosauridae, Varanopseidae, Sphenacodontidae and others. The edaphosaur Ianthasaurus is an index taxon. The Badger Creek assemblage from the Sangre de Cristo Formation near Howard, CO, and most of the Cutler Group assemblage from El Cobre Canyon, NM, belong to this Late Pennsylvanian faunachron. The FAD of Eryops begins faunachron B, and new data from the mixed marine-nonmarine Bursum Formation of central NM indicate this FAD is late Virgilian. Faunachron B encompasses assemblages with diverse temnospondyls, few microsaurs and nectrideans, anthracosaurs (Archeria), the diadectomorph Diadectes, the captorhinomorphs Protorothyris and Romeria and diverse pelycosaurs (especially Dimetrodon, Edaphosaurus and Stereophallodon). The captorhinids Romeria and Proterothyris are index taxa, as is the temnospondyl Edops. Most tetrapods from the Abo Formation in NM, and those from the upper part of the Cutler Group in SW CO and Halgaito Shale in SE UT are of faunachron B age. Faunachron C begins with the FAD of Seymouria. It includes assemblages with temnospondyls similar to faunachron B (but without Edops and Neldasaurus), the microsaurs Carrolla and Pantylus, few nectrideans, Archeria, Diadectes and Seymouria, the captorhinomorphs Eocaptorhinus and Procaptorhinus, diverse pelycosaurs (including the FADs of Secodontosaurus and Varanosaurus) and the diapsid Araeoscelis. The assemblage from the Organ Rock Shale in SE Utah is of faunachron C age. The Four Corners late Paleozoic tetrapods provide an important record across the Pennsylvanian-Permian boundary not matched in other late Paleozoic tetrapod successions.