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

We present a systematic revision of the ‘prosauropod’ dinosaur <i>Unaysaurus tolentinoi</i>. 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 <i>Unaysaurus</i> within Plateosauridae is primarily dependent on similarities shared with the Northern Hemisphere taxon <i>Plateosaurus</i>, 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 <i>Unaysaurus</i> and <i>Plateosaurus</i>; 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 <i>Unaysaurus tolentinoi</i> remains supported by several unambiguous autapomorphies. Although strong anatomical evidence for monophyletic divergences amongst early sauropodomorphs remains elusive, the relatively low stratigraphical occurrence of <i>Unaysaurus</i> demonstrates that the typical ‘prosauropod’ bauplan – and hence habitual herbivory – had evolved by the early Norian.