The sauropodomorph hand evolution: morphological changes in the carpus

LEARDI, JUAN MARTÍN

Mendoza

Congreso; 4th International Paleontological Congress; 2014

IANIGLIA

The structure of the anterior autopodium has always been considered one of the hallmarks of Sauropodomorpha. The main changes recognized in this structure are those associated with the modification of the first digit, which is strongly hypertrophied and its first phalanx rotated outwards and the loss of the ossification in carpal elements. The anterior autopodium is further modified in derived sauropods, as they lack any ossified carpal elements and the metacarpals adopt a columnar disposition. Besides the recognition of the absence of ossified carpals in derived sauropods (titanosauriforms) few studies on this transition were made, which is surprising considering the great fossil record of sauropodomorph anterior autopodia. The aim of this contribution is to evaluate the changes in the carpus of Sauropodomorpha trough the phylogeny of the clade. As the carpal elements are rarely preserved, this study is based mostly on articulated and well specimens where the complete anterior autopodium was preserved. The absence of ossified proximal carpals is widely distributed among sauropodomorphs, with the exception of Efraasia where the radiale, ulnare and a centrale are present. Basal sauropodomorphs like Plateosaurus show a reduction in the distal carpals, as the distal carpal 4 is the smallest one of the carpus unlike the large distal carpal 4 of basal dinosaurians (Eoraptor and Herrerasaurus), where it is the largest distal carpal. A similar reduction is observed on distal carpal 3. On the other hand, the distal carpal 1 is laterally expanded and as a proximolateral process that covers the proximal end of the distal carpal 2. Massopods also continue with the reduction of the distal carpal row, as the distal carpal 4 is absent in any known member of the clade (e.g.: Massospondylus, Adeopapposaurus, Seitaad). The structure of the carpus of non-sauropod sauropodiformes is uncertain as no complete carpus has been recovered for these taxa. A higher reduction is observed in the distal carpal row of sauropods and eusauropods, as a medial large single distal carpal is known in derived taxa (i.e.: Camarasaurus). Thus, a gradual reduction is evidenced in the elements of the carpus, starting with the proximal carpals and continuing from the lateral most to the medial distal carpals. Even though, this process seems not to be devoid of any homoplasies as the case of the chinese eusauropod Shunosaurus which has three distal carpals unlike other known members of this derived clade.