[N] 2006 Evolution of a quadrupedal limb posture in Melanorosaurus
Bonnan, M. (2006) The forelimb of the basal Sauropodomorph Melanorosaurus and the evolution of pronation, manus shape and quadrupedalism in sauropods. JVP 26(3) Abstracts pp.44
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 semi-circular 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 manus shape and pronation in sauropods. The osteology of a complete forelimb from Melanorosaurus, as well as partial referred specimens, show that 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 semi-circular as in eusauropods; instead its metacarpals were arranged closer to the orientation more typical of theropods and basal sauropodomorphs. A recurved, medially-divergent pollex claw and straighter, blunter claws on digits II and III were present, yet it appears that there was phalangeal reduction in the central three digits of the manus.
We suggest that the characteristic U-shaped manus of eusauropods and neosauropods may have evolved through a process of mosaic evolution. The forelimb morphology of Melanorosaurus suggests manus pronation occurred early in basal sauropods through a change in antebrachial morphology, but that changes to manus morphology followed later in eusauropods, perhaps due to selective pressures such as refining the pronation and weight-bearing efficiency of the manus. We conclude that changes to antebrachial morphology and manus morphology were not temporally linked in sauropods and constitute separate phylogenetic events.