A taxonomic and biostratigraphic re-evaluation of the Post Quarry vertebrate assemblage from the Cooper Canyon Formation (Dockum Group, Upper Triassic) of southern Garza County, western Texas

Abstract

• The Post Quarry, within the lower part of the type section of the Upper Triassic Cooper Canyon Formation in southern Garza County, western Texas, contains a remarkably diverse vertebrate assemblage. The Post Quarry has produced: the small temnospondyl Rileymillerus cosgriffi; the metoposaurid Apachesaurus gregorii; possible dicynodonts and eucynodonts; a clevosaurid sphenodontian; non-archosauriform archosauromorphs (Trilophosaurus dornorum, simiosaurians, and possibly Malerisaurus); the phytosaur Leptosuchus; several aetosaurs (Calyptosuchus wellesi, Typothorax coccinarum, Paratypothorax, and Desmatosuchus smalli); the poposauroid Shuvosaurus inexpectatus (“Chatterjeea elegans”); the rauisuchid Postosuchus kirkpatricki; an early crocodylomorph; several dinosauromorphs (the lagerpetid Dromomeron gregorii, the silesaurid Technosaurus smalli, a herrerasaurid, and an early neotheropod); and several enigmatic small diapsids. Revised lithostratigraphic correlations of the lower Cooper Canyon Formation with the Tecovas Formation, the occurrence of Leptosuchus, and the overall composition of the assemblage indicate that the Post Quarry falls within the Adamanian biozone, and not the Revueltian biozone. Stratigraphic subdivision of the Adamanian biozone may be possible, and the Post Quarry may be correlative with the upper part of the Adamanian biozone in Arizona. The age of the Post Quarry assemblage is possibly late Lacian or earliest Alaunian (late early Norian or earliest middle Norian), between 220 and 215 Ma.