Phylogenetic reassessment of Pisanosaurus mertii Casamiquela, 1967, a basal dinosauriform from the Late Triassic of Argentina

Abstract

Pisanosaurus mertii was originally described on the basis of an incomplete skeleton from the early Late Triassic (Carnian)of northern Argentina. It is consistently regarded by most authors as a very basal ornithischian, the sister group ofremaining members of the clade. The referral to Ornithischia is based mainly on tooth-bearing bones and toothmorphology. On the other hand, the postcranium is recognized as strikingly plesiomorphic for ornithischians, and even fordinosaurs. The recent description of non-dinosaurian dinosauriforms of the clade Silesauridae having ornithischian-likedentition invites a review of the phylogenetic affinities of Pisanosaurus. In this regard, an overview of the holotypespecimen allows a reanalysis of previous anatomical interpretations of this taxon. The phylogenetic analysis presented heresuggests that Pisanosaurus may be better interpreted as a member of the non-dinosaurian Silesauridae. It shares withsilesaurids reduced denticles on the teeth, teeth fused to maxilla and dentary bone, sacral ribs shared between two sacralvertebrae, lateral side of proximal tibia with a fibular flange, and dorsoventrally flattened pedal ungual phalanges. Thepresent analysis indicates that Pisanosaurus should be removed from the base of the Ornithischia and should no longer beconsidered the oldest representative of this dinosaurian clade.