A new description of the forelimb of the basal sauropodomorph Melanorosaurus:: Implications for the evolution of pronation, manus shape and quadrupedalism in sauropod dinosaurs

The evolution of a quadrupedal limb posture is characteristic of the earliest sauropod dinosaurs and involved secondarily modifying a non-supporting forelimb into a pronated support column with a semicircular metacarpus. Melanorosaurus readi is a basal sauropodomorph phylogenetically close to the earliest sauropods, and the morphology of its forelimb sheds additional light on the origins of the unique manus shape of sauropods and the initial stages of manus pronation. We describe the osteology of a complete forelimb of Melanorosaurus (NM QR3314), as well as partial referred specimens (SAM-PK-K3449, SAM-PKK3532), and show that the forelimb elements of this taxon comprise a mosaic of basal sauropodomorph and basal sauropod characteristics. The humerus retains the plesiomorphic morphology of basal sauropodomorphs. However, like sauropods, the forearm of Melanorosaurus clearly shows the development of a proximal craniolateral process on its ulna and a shift in the position of the radius to a more cranial orientation relative to the ulna. The manus of Melanorosaurus was not a semicircular colonnade as in sauropods: instead its metacarpals were arranged closer to the orientation more typical of theropods and basal sauropodomorphs. A recurved, medially divergent pollex claw and straighter but blunter claws on digits II and III were present. We suggest that the characteristic U-shaped manus of eusauropods and neosauropods may have resulted from mosaic evolution. The forelimb morphology of Melanorosaurus suggests that pronation of the manus occurred early in basal sauropods through a change in antebrachial morphology, but that changes to the morphology of the manus followed later in eusauropods, perhaps related to further manus pronation and improved stress absorption in the metacarpus. Thus, we conclude that changes to antebrachial morphology and manus morphology were not temporally linked in sauropods and constitute separate phylogenetic events.