Halgaitosaurus gregarius, a New Upper Carboniferous Araeoscelidian (Reptilia: Diapsida) from the Halgaito Formation, Bears Ears National Monument, Utah, USA

ABSTRACT Halgaitosaurus gregarius is a new genus and species of araeoscelidian diapsid reptile based on numerous specimens from the Upper Carboniferous (Virgilian, Gzhelian) Birthday bonebed, Halgaito Formation, Valley of the Gods, Bears Ears National Monument, Utah, USA. An ontogenetic series is represented by juvenile to fully ossified adult specimens. halgaitosaurus gregarius is distinguished from other araeoscelidians by relative sizes of maxillary teeth, a small contribution of the jugal to the ventral margin of the skull, and eight cervical vertebrae. A phylogenetic analysis suggests that halgaitosaurus and Araeoscelis Williston, 1910, form a clade in a monophyletic Araeoscelidia, with Petrolacosaurus Lane, 1945, and Zarcasaurus Brinkman, Berman, and Eberth, 1984, forming successive sister taxa to this clade. Spinoaequalis deBraga and Reisz, 1995, places as a member of Neodiapsida. halgaitosaurus was an abundant component of the vertebrate fauna that inhabited the coastal plain on the southwestern border of the Paradox Basin in western Pangea during a relatively short period when intermittent to probable perennial streams and rivers traversed it during an otherwise semi-arid to arid climate.