A large gracilisuchid from the lower Upper Triassic beds of the Chañares Formation of NW Argentina

Abstract

The lower member of the Chañares Formation yields one of the richest Middle-early Late Triassic continental tetrapod faunas worldwide and is subdivided in two assemblage zones (AZ): the stratigraphically lower Tarjadia AZ and the upper Massetognathus-Chanaresuchus AZ. The former is represented by dicynodonts, cynodonts, rhynchosaurs, and pseudosuchian archosaurs, whereas the latter is composed of dicynodonts, cynodonts, proterochampsians, and dinosauromorph, pterosauromorph, and pseudosuchian archosaurs. The pseudosuchian record consists of the ‘rauisuchian’ Luperosuchus fractus, indeterminate suchians, and the erpetosuchid Tarjadia ruthae in the Tarjadia AZ and the gracilisuchid Gracilisuchus stipanicicorum in the Massetognathus-Chanaresuchus AZ. Here we revisit the taxonomy of a putative juvenile specimen of L. fractus (PULR-V 057) consisting of a partial skull, but was subsequently preliminary reinterpreted as taxonomically distinct. We recognize that most of the diagnostic features of Luperosuchus fractus are absent in PULR-V 057. By contrast, the presence of the following character-states allows us to reinterpret PULR-V 057 as a Gracilisuchidae: premaxilla with posterodorsal process that fits into slot on lateral surface of nasal; nasal forms part of dorsal border of antorbital fossa; and frontal with anterior portion that tapers anteriorly along midline. Additionally, PULR-V 057 shares with G. stipanicicorum, but not with other gracilisuchids, a straight posterior edge of the postorbital process of jugal, absence of palatal teeth, and horizontal process of the maxilla with parallel dorsal and ventral margins. In particular, the first feature has been proposed as an autapomorphy of G. stipanicicorum. Nevertheless, PULR-V 057 differs from G. stipanicicorum in the presence of a maxilla with an ascending process slanting more posterodorsally, lower maxillary tooth count, absence of ornamentation on external surface of nasal and frontal, and thicker tuberosity on the main body of jugal. A preliminary phylogenetic analysis recovered PULR-V 057 as the sister taxon to G. stipanicicorum. It is noteworthy that PULR-V 057 is twice larger than known specimens of G. stipanicicorum and comes from the same AZ because of its preservational features, making it the largest archosaur specimen described for the Massetognathus-Chanaresuchus AZ. Thus, future research will focus on determining if PULR-V 057 is a more mature specimen of G. stipanicicorum or a different gracilisuchid species. In conclusion, we expand the ecomorphological diversity of gracilisuchids in the Chañares Formation to include medium-sized predatory forms, we agree with previous authors to restrict the hypodigm of L. fractus only to its holotype (PULR-V 04) but also restrict its stratigraphic range to the Tarjadia AZ.