Body Size Evolution and Locomotion in Sauropodomorpha: What the South American Record Tells Us

ALEJANDRO OTERO; JOHN HUTCHINSON

South American Sauropodomorph Dinosaurs: Record, Diversity and Evolution

Springer

Año: 2022; p. 443 - 472

The transition from early sauropodomorphs to sauropods is of special interest given that a shift from obligatory or facultative bipedalism to an obligatory quadrupedalism is evident. In this chapter, we review and discuss the biological mechanisms underpinning such evolutionary transformations. The discovery of the South American sauropodomorph Mussaurus patagonicus has helped elucidate changes in the upper forelimb from bipedality to quadrupedality. Shoulder range of motion studies has shown that Mussaurus could not protract its forelimb past vertical, which suggests that quadrupedal locomotion could not have been possible, although it might have been if the elbow was habitually strongly flexed, as has been hypothesized for ornithischian quadrupeds. Yet muscle moment arm studies indicated that Mussaurus could not straighten its elbow, suggesting it did not have columnar forelimbs like later, fully quadrupedal sauropods. Quadrupedal locomotion first evolved in the adult forms of the sauropodomorphs closest to Sauropoda (e.g., Melanorosaurus). However, it has been suggested that already some early sauropodomorphs (e.g., Massospondylus) were quadrupedal during early ontogenetic stages and adopted a bipedal stance, at least facultatively, as adults. The postural shifting that some sauropodomorphs experienced during their ontogeny has important implications for understanding evolutionary processes that caused those shifts. Available ontogenetic series of Mussaurus provide additional insight into these evolutionary developmental transitions. The body?s center of mass of this species moved from a position in the mid-thorax to a more posterior position close to the pelvis, consistent with a shift from quadrupedalism to bipedalism at a young age. This postural modification could be the product of the relative enlargement of the tail and the reduction of the neck during ontogeny, challenging previous studies, which emphasized that that transformation would have been linked to a relative enlargement of the forelimbs. Viewed in a phylogenetic context, the South American sauropodomorph record provides key information regarding the evolution of body size and limb mechanics in this group. An anterior center of mass shift occurred during the evolution of quadrupedalism in the Late Triassic, followed by a more striking anterior shift in Late Jurassic?Cretaceous titanosauriforms, a phenomenon apparently closely linked with locomotion (e.g., weight distribution; reduced athleticism) and environment. As South American titanosaurs included the largest land animals ever, these also inform us about the constraints on terrestrial gigantism and the surprising diversity of giant forms that can exist despite these biomechanical and other constraints.