Evolution of the titanosaur metacarpus

SEBASTIÁN APESTEGUÍA

Thunder-Lizards: the Sauropodomorph Dinosaurs

Indiana University Press

Lugar: Bloomington & Indianapolis; Año: 2005; p. 321 - 345

The manus in Neosauropoda early acquired a tubular configuration. Although basal titanosauriforms already shown a trend to metacarpal lengthening and finger’s loss, titanosaurs were farther. Their homogeneous, long and robust metacarpals devoid of digits are unique. The metacarpals were vertically arranged into a completely tubular and homogeneous structure, only connected by the proximal and distal epiphyses and directly reaching the ground with no phalanx interference, reducing the fragility of the manus and making the titanosaur manus one of the most graviportal structures in the natural world. The distal metacarpal ends are consequently square, bigger laterally, profusely pitted, and not biconvex, in regard to the loss of the phalanges. The achievement of a complete eugraviportal forelimb structure, added a new segment to the fore limb. The process involved the enlargement of the fifth metacarpal to be rather equal to the first, the reduction and disappearing of digits related to the flat distal metacarpal trochlea, the reduced proximal and intermetacarpal contacts, the distal metacarpal interaction flanges, and the bowed first metacarpal in some taxa, in relation to an asymmetrical and short first phalanx and perhaps the maintaining of a first ungual phalanx, both lost in derived species. The first metacarpal is bowed forward in several titanosaurs, such as Andesaurus and Argyrosaurus, and might be a synapomorphic character for Titanosauria, reversed in derived forms. If true, the persistence of a large ungual in the digit I of some species and the bowed metacarpal could be related to each other both phylogenetically and functionally. The long fifth metacarpal faced the first in posterior view and closed the completely tubular metacarpus. Distal intermetacarpal contacts developed covering flanges in the second and third metacarpals. In proximal view, the first metacarpal varies from large ‘D’-shaped to flat outlines; the second and third are robust and wedge-shaped; the fourth a biconcave square-shaped bone; and the fifth, a twisted clepsydra-like bone with flat epiphyses. The titanosaur carpus, on the other side, resemble that of basal titanosauriforms and seems not to be homologous to that of diplodocoids. It comprises both proximal and distal elements. Olecranon reacquisition, laterally expanded ilia, bowed opisthocoelous back, lost of hyposphene-hypantrum articulations, reduction of the forelimbs, sacrum enlargement, and osteoderms make titanosaurs unique among Sauropoda, and those features can be related to the purported nest-excavation capabilities of advanced titanosaur sauropods.