A small aetosaur skeleton collected in 1939 from the Tecovas Formation of Texas and assigned to Desmatosuchus is reassigned to a new taxon, Sierritasuchus macalpini. Phylogenetic analysis suggests that Sierritasuchus is a member of the Desmatosuchinae. It can be distinguished from other desmatosuchines by two autapomorphies: (1) recurved spines on the lateral plates that are triangular in cross-section with a sharply ridged anterior edge; and (2) the presence of a sharp, ventrally oriented ridge on the posterior faces of the dorsal eminences of the paramedian plates, as well as a unique combination of characters including the presence of an anterior bar on the paramedian and lateral plates, a random pattern of ornamentation on the paramedian plates, and a dorsal eminence that contacts the posterior plate margin of the paramedian plates. Histological study of the holotypic plates in combination with comparison to a growth series in Typothorax, and size-independent growth indicators such as neurocentral suture closure suggests that the specimen is neither a young juvenile nor a fully-grown adult. The recognition of morphologically distinct specimens such as the holotype and referred material of Sierritasuchus demonstrates that past practices of assigning aetosaur specimens to known taxa based on superficial resemblance has masked diversity in this clade. Voucher specimens for biochronologic and biogeographic analyses should be carefully investigated before being used for such studies.
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