Partial analyses of humeral shape in sauropodomorph dinosaurs highlight a hidden modularity and the differential evolution of sauropod bauplan‐related traits

Sauropods are the largest‐known terrestrial animals, characterized by their columnar limbs and obligate quadrupedality. They are nested within Sauropodomorpha, a clade whose earliest representatives were, however, small bipeds. The early evolution of gigantism and quadrupedality within sauropodomorphs remains debated, since several non‐sauropods reached large sizes before the emergence of the sauropod bauplan, and are frequently interpreted as quadrupedal. Recently, a study of sauropodomorph limb long bones highlighted a differential evolutionary pattern between the forelimb and hindlimb, with sauropod‐related traits appearing abruptly in the former and gradually in the latter. However, conclusions were limited for the humerus by poor preservation of this bone in large non‐sauropodan sauropodomorphs. We aimed to clarify how humeral morphology evolved with the emergence of the sauropod bauplan. Using 3D geometric morphometrics, we analysed humeral proximal and distal halves, adding numerous key but incomplete specimens. We show that humeral halves evolved differently: proximally, locomotor groups are overlapping, reflecting morphological similarities between those groups, whereas distally, quadrupedal sauropods are markedly distinct. This highlights a decoupled evolutionary pattern within the humerus itself, with sauropod‐related traits appearing gradually proximally, and abruptly distally as previously found for the antebrachium. This congruence suggests the existence of a morphological module centred around the elbow. It corroborates that sauropodomorph forelimb evolution was more complex than a progressive transition towards the sauropod condition, questioning how quadrupedality was feasible in non‐sauropods. Forelimb evolution may be regionalized around joints rather than bone elements, similarly to distantly‐related mammalian groups, suggesting a more widespread trend in amniotes.