Reappraisal of the early Permian dissorophid Alegeinosaurus from Texas, USA

Abstract

Alegeinosaurus aphthitos is a poorly known dissorophid from the early Permian Arroyo Formation of Texas that is represented by a partial articulated block of postcrania, including an articulated pectoral girdle, portions of the ribcage, and articulated vertebrae with osteoderms. It is one of several dissorophid taxa whose placement within the family is unquestioned, but whose anatomy and taxonomy are too poorly resolved to be relevant for current comparative and phylogenetic studies. A redescription of the holotype is presented here to provide a more thorough characterization of its morphology and to address its taxonomic standing. Various aspects of the postcrania conform to those previously reported from the basal dissorophid Aspidosaurus, in particular the unique nature of a single series of keeled osteoderms fused to transversely expanded dorsal tips of the spines. Alegeinosaurus is herein designated as a junior synonym of Aspidosaurus, thereby resolving the relationships of a poorly known taxon and expanding the characterized postcranial regions of Aspidosaurus for future comparative work on dissorophid postcrania.