Osteology of the appendicular skeleton of Macrocollum itaquii (Dinosauria: Sauropodomorpha) sheds light on early dinosaur wrist evolution

Abstract

Known from exquisitely preserved specimens from early Norian beds of southern Brazil, Macrocollum itaquii is a crucial taxon for understanding sauropodomorph evolution. This species provides a glimpse at the transition between the earliest sauropodomorphs and the classic ‘prosauropods’. To date, the appendicular anatomy of this taxon has not been described in detail. Therefore, in this contribution we present a detailed description of its appendicular skeleton. As expected from its chronostratigraphic position, the limbs of M. itaquii exhibit a combination of plesiomorphic (e.g. absence of a radial fossa, ossified proximal carpals, iliac acetabular wall not fully open) and derived traits (e.g. metacarpal I inset into the carpus, sub-triangular preacetabular process, straight femur). Phylogenetic analyses support the classification of this taxon, along with two or three other Norian taxa, as members of Unaysauridae, an early radiation of plateosaurian sauropodomorphs. An articulated wrist offers rare insights into this region, confirming that sauropodomorphs (like other dinosaur groups) ancestrally possessed at least nine carpals. This finding enhances our understanding of carpal morphology and evolution within sauropodomorphs. Macrocollum itaquii reveals the presence of a unique third phalanx of manual digit V, raising questions about the confidence we can infer the phalangeal counts of the outermost digits.