Morphological variability of haemal arches within Sauropoda

OTERO, A.; GALLINA, P. A.; CANALE, J. I.; HALUZA, A.

Congreso; Congreso Latinoamericano de Paleontología de Vertebrados; 2011

Sauropod haemal arches (?chevrons?) are caudal bony structures that traditionally have been enclosed 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 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 titanosauriforms. Instead of this traditional classification, we propose one using the combination of information observed in both anterior and lateral views. This way, we grouped the chevron types in two major categories, reflecting their morphology in anterior view: the ?Y-shaped? and the ?V-shaped? chevrons. Four types of ?Y-shaped? and five types of ?V-shaped? chevrons were recognized. The presence of almost complete chevron series in many eusauropods allows the identification and comparison of equivalent structures along the tail and also between taxa. In this regard, the proximal rami of a single chevron seem to be constant in their placement and arrangement along the caudal series of the tail. The distal blade may display a different morphology along the caudal series, although its recognition passing distally along the series can also be tracked. 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 equivalent to those seen in other taxa, because the unequivocal correlate is the distal blade. Nonetheless, the projections rising from the proximal rami seem to not be correlative to other projections seen before in a sauropod dinosaur since they are linked to a different osteological correlate (i.e., proximal rami). Caudal remains are frequent in the sauropod record, however, most of them are isolated elements and complete articulated series are rare. Therefore, focusing in comprehensive studies on the anatomy of this particular region will provide more knowledge to the anatomical diversity and the systematic of the group.