Built for bulk: posture, microanatomy and soft tissue shaped gigantism in Sauropoda.

Sauropodomorph dinosaurs show how appendicular skeletal architecture accommodated extreme increases in body mass. Although sauropod gigantism has been linked to columnar limbs, wide-gauge posture, thick articular cartilage and tubular long-bone organization, the biomechanical interaction among these components is poorly understood. Here, we examine how femo ral shape, internal bone architecture and articular soft tissues influenced mechanical scaling across Sauropodomorpha. We generated three-dimensional femoral models spanning early sauropodomorphs, narrow-gauge diplodocoids, intermediate-gauge titanosauriforms, wide-gauge titanosaurs and an African elephant control. Finite element analyses compared theoretical solid models with taxon-specific native configurations, incorporating medullary cavities and articular cartilage caps. Mechanical performance was assessed using 95th-percentile von Mises stress, strain energy density and structural stiffness. Femoral performance did not scale uniformly with body mass. The two sampled narrow-gauge taxa, Diplodocus and Amargasaurus, defined steeper scaling trajectories for stress and strain energy density, whereas wide-gauge titanosaurs generally displayed flatter trajectories, comparatively lower stress and strain energy density, and elevated stiffness in several large-bodied forms. Targeted sensitivity analyses further showed that the mechanical effects of medullary architecture and cartilage depended on their interaction with taxon-specific femoral geometry. These findings suggest that sauropod gigantism was sustained by lineage-specific combinations of femoral geometry, internal architecture and articular soft tissues.