The evolution of forelimb architecture in early sauropodomorpha

Abstract

During the evolutionary history of Sauropodomorpha, it is thought that they underwent a change from small-bodied, possibly bipedal, animals to large-bodied, quadrupedal animals. To study this transition, this thesis used photogrammetric and geometric morphometric techniques to investigate shape changes at key muscle insertion points in the humerus bone of various sauropodomorph taxa. There is noticeable shape change in the humerus from more basal to more derived Sauropodomorpha. A developmental sequence of Massospondylus forelimbs shows little similarity with the macroevolutionary trajectory along the sauropod stem. The former indicates that morphologies characteristic of Sauropoda began to evolve relatively early in the sauropodomorph lineage, before obligate quadrupedality appeared. The latter suggests that the quadrupedal stance of derived sauropods is not a paedomorphic retention of a juvenile state. The evolution of quadrupedality may have been a stepwise process, possibly involving some functional tradeoffs.