The cranial anatomy of the coelophysoid theropod Zupaysaurus rougieri from the Upper Triassic of Argentina

Abstract

Zupaysaurus rougieri is a theropod dinosaur that is known from an almost complete skull and limited postcranial material (e.g., axial elements, scapulocoracoid, tibia, astragalocalcaneum), from the Los Colorados Formation (La Esquina Fauna, Norian), NW Argentina. This taxon was originally interpreted as a basal tetanuran, but is actually a member of the Coelophysoidea. Zupaysaurus differs from other basal dinosaurs in the presence of the following cranial apomorphies: a maxillary-jugal ventral margin describing an obtuse angle in lateral view, a horizontal ramus of the maxilla with parallel dorsal and ventral margins, a notch at the base of the ascending process of the maxilla, and a sigmoid ventral process of the squamosal. Furthermore, the skull of this theropod reveals anatomical novelties (e.g., pneumatic lacrimal, maxillary fenestra, oval orbit, wide contact between squamosal and quadratojugal) absent in any other Triassic dinosaur hitherto described, including remaining coelophysoids. Interestingly, these features resemble morphological aspects of more derived theropods, approaching some traits of tetanuran and ceratosaurian neotheropods. In this regard, a redescription of the skull of Zupaysaurus and comparisons with basal neotheropods, mainly coelophysoids, is presented here in order to provide a better cranial anatomical record for this Triassic theropod and insights to their early cranial evolution.

Keywords:

• Skull
• Theropoda
• Coelophysoidea
• Upper Triassic
• Argentina

Acknowledgements

The access to the type specimen of Zupaysaurus rougieri (PULR 076) was provided by S. Martin (PULR). R. Martínez (PVSJ) provided access to specimens under his care. To F. E. Novas (MACN) for his discussions about the anatomy and phylogenetic relationships of Zupaysaurus and the access to unpublished photographs of several theropods, including that of the Figure 9 of the present contribution. The criticism of F. Agnolin (MACN) was highly appreciated. The careful revision of P. Makovicky substantially improved the quality of this work.