Reassessment of Unaysaurus tolentinoi (Dinosauria: Sauropodomorpha) from the Late Triassic (early Norian) of Brazil, with a consideration of the evidence for monophyly within non-sauropodan sauropodomorphs

posted on 10.01.2020 by Blair W. McPhee, Jonathas S. Bittencourt, Max C. Langer, Cecilia Apaldetti, Átila A. S. Da Rosa

We present a systematic revision of the ‘prosauropod’ dinosaur Unaysaurus tolentinoi. Past phylogenetic treatments of this taxon have presented it as a key constituent of the low-diversity Plateosauridae. This clade, along with Massospondylidae, is currently regarded as a relatively non-controversial example of monophyly within non-sauropodan sauropodomorphs – an otherwise pectinate grade of animals from which the apically nested Sauropoda is derived. The inclusion of Unaysaurus within Plateosauridae is primarily dependent on similarities shared with the Northern Hemisphere taxon Plateosaurus, with a sister-taxon relationship between these taxa indicating the earliest known instance of Pangaean cosmopolitanism among early sauropodomorphs. However, an in-depth exploration of the character states that in the past have produced this relationship shows that many either present a complex distribution throughout early Sauropodomorpha, or are subject to ambiguities in the formulation and scoring of primary homologies. An updated cladistic analysis finds weak continued support for a sister-taxon relationship between Unaysaurus and Plateosaurus; however, this result is supported by synapomorphies that are either labile with respect to taxon inclusion, not exclusively restricted to the clade, or represented by character-state dichotomies of relatively subtle differentiation. These uncertainties aside, the validity of Unaysaurus tolentinoi remains supported by several unambiguous autapomorphies. Although strong anatomical evidence for monophyletic divergences amongst early sauropodomorphs remains elusive, the relatively low stratigraphical occurrence of Unaysaurus demonstrates that the typical ‘prosauropod’ bauplan – and hence habitual herbivory – had evolved by the early Norian.