Sauropod haemal arches: morphological variability and phylogenetic aspects

ALEJANDRO OTERO; PABLO GALLINA; JUAN CANALE; ALEJANDRO HALUZA

San Juan

Congreso; 4° Congreso Latinoamericano de paleontología de Vertebrados; 2011

Museo de Ciencias Naturales, Universidad Nacional de San Juan

Sauropod haemal arches (‘chevrons’) are caudal bony structures that traditionally have been classified in two different morphological types observed in different anatomical views (i.e., anterior vs. lateral): forked and Y-shaped (Upchurch et al., 2004). Forked chevrons are characterized by the possession of distal cranial and caudal processes and are present in middle and posterior caudal vertebrae of several non-titanosaurifom eusauropods (e.g., basal eusaropods, diplodocids). On the other hand, Y-shaped chevrons display two dorsal rami that close the haemal canal ventrally, as well as a ventral or distal blade, typical of most anterior region of the tail of all sauropods and present along the tail of most titanosauriforms. Instead of this traditional classification, this research proposes one using the combination of information observed in both anterior and lateral views: the ‘Y-shaped’ and the ‘V-shaped’ chevrons. Four types of ‘Y-shaped’ and six types of ‘V-shaped’ chevrons were recognized. Complete chevron series in some eusauropods allows the comparison of topological equivalent structures along the tail and also among taxa. A basal titanosaur from Patagonia, Argentina exhibits a peculiar mid-caudal haemal arch morphology in which more than one cranial and caudal process is present. The projections rising from the distal blade are topologically equivalent to those seen in other taxa, because the unequivocal correlate is the distal blade. Nonetheless, the projections rising from the proximal rami do not seem to be topologically correlative to other projections seen before in a sauropod dinosaur since they are linked to a different osteological correlate (i.e., proximal rami). The morphological variability seen in sauropod chevrons along the tail could be in close relationship with the development and distribution of M. caudofemoralis longus, as seen in extant crocodiles and as previously proposed for non-avian theropods (Persons and Currie, 2011) and caudal centra and transverse processes of sauropod dinosaurs (Salgado and García, 2002; Gallina and Otero, 2009). Two new characters devoted to middle chevrons are here proposed, in which the transitional morphology is described.