PHYLOGENETIC RELATIONSHIPS OF THE TRIASSIC THEROPOD Zupaysaurus rougieri FROM NW ARGENTINA

EZCURRA, M. D.; NOVAS, F. E.

Rio de Janeiro

Congreso; II Congresso Latinoamericano de Paleontologia de Vertebrados; 2005

Museu Nacional

The knowledge of Late Triassic theropods from South America has been recently enlarged with the description of Zupaysaurus rougieri. This taxon comes from the Los Colorados Formation (Late Triassic, La Rioja Province, NW Argentina). Zupaysaurus was interpreted as the oldest known member of Tetanurae. This interpretation was based on the following characters that Zupaysaurus purportedly shares with Tetanurae: lacrimal recess pneumatizated, presence of a maxillary fenestra within the antorbital fossa, maxillary tooth row rostral to the orbit, and distal tibia caudolaterally concave and transversely expanded. Although the first two characters may be valid, the remaining features are problematic. For example, in Zupaysaurus a lacrimal horn is absent, and the last maxillary tooth is situated ventrally to the rostral margin of the orbit, as it occurs in basal theropods (e.g., Herrerasaurus, Coelophysis, Syntarsus); the tibia of Zupaysaurus is not transversely expanded at the degree seen in Tetanurae (e.g., Torvosaurus, Allosaurus), but keeps the same proportions as in the coelophysoid Liliensternus; also, the caudolateral concavity on distal tibia is not a tetanuran but a neotheropodan attribute. A review of the available materials of Zupaysaurus demonstrates its membership to the basal neotheropod clade Coelophysoidea, exhibiting the following synapomorphies of this clade: skull (premaxilla-quadrate) length more than 3 times caudal skull height, ventral margin of maxilla curved dorsally on its rostral portion, minimum length of internal antorbital fenestra 25% or greater than maximum skull (premaxilla-quadrate) length, lacrimal rostral ramus longer than the ventral ramus, and fang-like teeth in rostral end of the dentary. Moreover, Zupaysaurus shares the following apomorphic features with Liliensternus+(Coelophysis + Syntarsus): more than 18 maxillary tooth, and maxilla with lateral alveolar ridge above tooth row. Finally, Zupaysaurus shares the following features with Coelophysis+Syntarsus: rostral margin of maxillary antorbital fossa square shaped, frontal elongate and triangular, and infratemporal fenestra rostrocaudally compressed. Finally, the Argentine taxon apomorphically resembles Syntarsus in the three-pronged condition of the rostral process of jugal. We disagree with previous interpretations in that the distal end of tibia and tarsus of Zupaysaurus is "morphologically intermediate between basal theropods (Herrerasaurus and ceratosaurs) and more derived tetanurans". On the contrary, the distal tibia and astragalocalcaneum of Zupaysaurus are almost identical to the coelophysoid Liliensternus. Both taxa share an outer malleolus of distal tibia that is polygonal-shaped, contrasting with the lobular-shaped present in the remaining saurischians. Also, the ascending astragalar process of Zupaysaurus clearly differs from those of tetanurans (e.g., Torvosaurus, Allosaurus) in which the ascending process is more laminar (e.g., craniocaudally flattened). Another point of discussion concerns with the purported presence of "parasagital crests" in the skull roof of Zupaysaurus, interpreted as a diagnostic feature of this taxon. Close examination of the holotype specimen reveals that the right nasal is rotated outwards from its natural position, thus resembling a longitudinal crest. The absence of parasagital crests in Zupaysaurus invites to review the skull of Syntarsus kayentakatae, for which these crests were also described. In sum, Zupaysaurus represents the first coelophysoid yet recorded in South America. Within Coelophysoidea, the Argentine taxon seems to be more closely related to Coelophysidae (= Coelophysis + Syntarsus) rather than to the basal coelophysoid Dilophosaurus. Phylogenetic analysis performed in this study depicts Zupaysaurus as forming an unresolved polytomy with Gojirasaurus, Liliensternus and Coelophysidae. Within the previously described phylogenetic context, those derived features shared with Tetanurae (e.g., lacrimal recess with multiple pneumatic foramina, and horizontal ramus of lacrimal tapering rostrally) are better interpreted as homoplasies rather than as indicative of the tetanurine nature of Zupaysaurus.