A new taxon of phytosaur (Archosauria: Pseudosuchia) from the Late Triassic (Norian) Sonsela Member (Chinle Formation) in Arizona, and a critical reevaluation of Leptosuchus Case, 1922

Abstract

Leptosuchus Case, 1922 (Reptilia: Phytosauria) from the Late Triassic of the American West is represented by many specimens. Here, I present complete morphological descriptions of the skull material of a new taxon from the Sonsela Member (Chinle Formation) of Petrified Forest National Park, Arizona, with the first rigorous phylogenetic analysis focused on the interrelationships of Leptosuchus. The new taxon is recovered as the sister taxon to Pseudopalatinae. It possesses one unambiguous synapomorphy (the 'septomaxillae' form part of the lateral borders of the nares) and shares the presence of a subsidiary opisthotic process with Pseudopalatinae. The new taxon does not fall within the restricted clade Leptosuchus. In my analysis, the previously proposed, but undemonstrated, sister taxon relationship between Angistorhinus and Rutiodon is not supported, Paleorhinus is recovered as paraphyletic, and a subset of taxa traditionally included within Leptosuchus are found to be more closely related to Pseudopalatinae, rendering Leptosuchus paraphyletic. 'Leptosuchus' adamanensis emerges as sister taxon to Smilosuchus gregorii and is here referred to as Smilosuchus adamanensis nov. comb., and 'Machaeroprosopus' lithodendrorum is also transferred to Smilosuchus lithodendrorum nov. comb. Documentation of the variation present within Phytosauria, and specifically within Leptosuchus sensu lato, demonstrates higher diversity within Phytosauria than previously appreciated and places the character states previously proposed for Pseudopalatinae into a broader context of shared characters.

The Early Evolution of Archosaurs: Relationships and the Origin of Major Clades

“…However, I have recently observed a weakly flexed tibial facet in an isolated phytosaur astragalus (TMM 31100-466). The astragalus is likely from either Paleorhinus or Angistorhinus, two primitive members of Phytosauria (Ballew, 1989;Stocker 2010). Even if a flexed tibial facet is the ancestral condition of Phytosauria, my analysis shows that a flexed facet is also present in basal dinosauromorphs (e.g., Dromomeron romeri).…”

“…As with proterochampsians, Euparkeria (SAM 5867) has a similar arrangement of the antorbital fossa. Within phytosaurs, the antorbital fossa is present on the dorsal process of the maxilla and the lacrimal in the primitive phytosaurs Parasuchus (Chatterjee, 1978) and 'Paleorhinus' scurriensis (TTU-P 00539; Langston, 1949;Stocker, 2010) but absent on the maxilla and lacrimal in Smilosuchus (UCMP 27200) and Pseudopalatus pristinus (NMMNH 31292). In aetosaurs (Aetosaurus, SMNS 5770), Riojasuchus (PVL 3827), Gracilisuchus (MCZ 4117), Turfanosuchus (IVPP V 3237), Ticinosuchus (PIZ T2817), ''rauisuchians,'' crocodylomorphs, and dinosauriforms (e.g., Silesaurus ZPAL Ab III/361/26; Herrerasaurus PVSJ 407), the antorbital fossa is located on the lacrimal, the dorsal process of the maxilla, and nearly the entire dorsal margin of the posterior process of the maxilla.…”

“…The maxilla of phytosaurs lacks an antorbital fossa on the dorsolateral margin of the posterior process of the maxilla (the ventral border of the antorbital fenestra). Basal phytosaurs such as Parasuchus (Chatterjee, 1978) and 'Paleorhinus' scurriensis (TTU-P 00539; Langston, 1949;Stocker, 2010) have a clear antorbital fossa on the lacrimal and the dorsal process (5 ascending process) of the maxilla as in Euparkeria (SAM 5867) . More derived phytosaurs such as Smilosuchus (USNM 18313) and Pseudopalatus (UCMP 27235) lack an antorbital fossa on any of the elements surrounding the antorbital fenestra (pers.…”

A new phytosaur (Archosauriformes, Phytosauria) from the Lot’s Wife beds (Sonsela Member) within the Chinle Formation (Upper Triassic) of Petrified Forest National Park, Arizona

“…Morphological differences in phytosaurs are often interpreted in one of two ways, either as the result of individual variation within few taxa (Gregory, 1962;Ballew, 1989) or as taxonomically informative phylogenetic characters among many taxa (Hungerbühler, 2002;Parker and Irmis, 2006;Stocker, 2010). The former strategy favors temporally long lineages and wide geographic distributions following the work of Gregory (1962) on Phytosaurus and Ballew (1989) on Rutiodon.…”

“…The latter, specimen-based approach allows questions regarding ontogeny, diversity, and biogeography in the fossil record to be addressed based on repeatable observations of diagnostic morphological characters (Nesbitt and Stocker, 2008). At the center of each of these strategies are the phytosaur specimens themselves, and recent fieldwork has resulted in a dramatic increase in the number of specimens available for analysis and reinterpretation as part of current investigations into the evolution of the clade (Parker, 2006;Stocker, 2010).…”