Revision of Redondasaurus (Archosauria: Parasuchidae) from the Upper Triassic (Apachean) of the American southwest

Abstract

A recent restudy of Apachean-age phytosaurs, primarily from the Chinle Group of the American Southwest, has allowed for the recognition of at least six new skulls of the Apachean Redondasaurus, which when added to previously known cranial material, brings the total number of recognized Redondasaurus skulls to ~ 12. With this larger sample size, new taxonomically informative characters have been recognized that further distinguish Redondasaurus from closely-related phytosaur taxa (e.g., Pseudopalatus, Nicrosaurus). The primary diagnostic character of Redondasaurus has traditionally been supratemporal fenestrae that are hidden in dorsal view. However, now, with a greater sampling of skull characters, the following features are also recognized as diagnostic of Redondasaurus: reduced antorbital fenestrae; a prominent pre-infratemporal shelf at the anteroventral margin of the lateral temporal fenestra; septomaxillae that wrap around the outer margin of the external narial opening; a thickened orbital margin (distinguishing it from all other phytosaur except Coburgosuchus); and an inflated posterior nasal behind the external narial opening. Based on these newly recognized diagnostic characters Redondasaurus is demonstrably distinct from Pseudopalatus and arguments for their synonymy should be abandoned. Further, we interpret Redondasaurus as sexually dimorphic, with the males possessing an overall more robust skull with a crest that extends the length of the snout and females with a more gracile skull that lacks a crest, analogous to the sexual dimorphs recognized in Pseudopalatus. Given the sexual dimorphism in Redondasaurus, the two named species should be synonymized, as the only difference between the two species were sexually dimorphic characters of the crest. Thus, the holotype of Redondasaurus gregorii is the female morph and the holotype of R. bermani is the male morph. Given R. gregorii’s priority, R. bermani should be treated as a junior subjective synonym. The newly recognized diagnostic characters of Redondasaurus will hopefully allow additional specimens to be identified and increase the biostratigraphic utility of this taxon.