Osteology of the axial skeleton of Macrocollum itaquii (Dinosauria: Sauropodomorpha) and its implications for early sauropodomorph cervicalization

Abstract Macrocollum itaquii represents one of the most complete early-diverging sauropodomorphs ever described. The skeletons of Macrocollum itaquii were excavated from the Early Norian beds of the Candelária Sequence, southern Brazil. Therefore, its stratigraphic position, between the earliest Carnian forms and the typical sauropodomorphs of the Late Norian, provides a rare glimpse into a crucial evolutionary moment for the group. Despite its completeness, the axial skeleton of Macrocollum has not been described in detail until now. Given the significant role of the axial skeleton in the evolutionary history of sauropodomorphs, the osteology of the axial column of Macrocollum itaquii is described here. Congruent with its stratigraphic position, the axial skeleton combines derived (e.g. highly elongated neck, pneumatic features, and convex projection of dorsal neural spine) and plesiomorphic (e.g. nine cervical vertebrae and three sacral vertebrae) traits. The remarkable preservation of the type series allows for the evaluation of the third record of an articulated gastral apparatus in non-sauropod sauropodomorphs. An updated phylogenetic analysis reinforces the position of Macrocollum and unaysaurids as part of an earlier radiation than the typical ‘prosauropods’ and outside Plateosauria, yet branching later than all Carnian sauropodomorphs.