DESCRIPTION AND FUNCTIONAL ANATOMY OF THE QUADRATE OF THE DICRAEOSAURID SAUROPOD BAJADASAURUS PRONUSPINAX FROM THE LOWERMOST CRETACEOUS OF PATAGONIA

JUAN PABLO GARDERES; PABLO ARIEL GALLINA; JOHN ANDREW WHITLOCK; NÉSTOR TOLEDO

Congreso; 2020 SVP Virtual Meetings; 2020

The quadrate is an element with outsized importance to sauropod paleobiology. As in most nonmammalian amniotes, it links skull and mandible (quadrate-articular joint: QAJ), as well as chondrocranium and dermatocranium. The QAJ is the only kinetic arthrosis in the sauropod skull. Adductor musculature crossing this joint puts distinctive stresses on it. In particular, the angulation of these stresses change greatly between the different clades of Neosauropoda: diplodocoids, with their ventrally rotated facial skeleton, would have experienced mAMP-related stresses on the quadrate that are much more aligned with the long axis of the element than would have been present in macronarians. Likely as a result of the combination of differential stresses, the anatomy of the quadrate is often quite distinctive, making it both phylogenetically and functionally important.Here, we describe the quadrates of the Early Cretaceous Bajadasaurus pronuspinax Gallina, Apesteguía, Canale & Haluza (Dicraeosauridae: Diplodocoidea). As in other diplodocoid sauropods, this element presents as a triradiate element with posterodorsally oriented main axis, a posterior shallow fossa and a subtriangular shaped articular surface. Both quadrates of Bajadasaurus pronuspinax are nearly completely preserved. The squamosal capitulum is long, with a triangular section, and presents an elongated articulation surface for the squamosal, more than four times higher than wide, differentiating from the ?swelling? morphology observed in Suuwassea Harris. This capitulum presents a deep posterolateral fossa, with a thick lateral margin. The mandibular capitulum, which presents a triangular shaped articular surface, is connected with the body of the quadrate without a distinctive neck, conversely with the condition observed in Suuwassea. This condition results in the expansion of the attachment surface for the muscle adductor mandibulae posterior (mAMP), despite the lacking of an osteological correlate, as inferred in other dinosaurs. The pterygoid process is short and thins anteriorly. It presents a slight anteromedial concavity, with an overall sigmoid morphology in ventral view. None of the quadrates preserved the medial surface completely. The traits observed in the quadrates of Bajadasaurus could imply a shifting in the angle of the mAMP regarding the other adductor muscles and QAJ, resulting in differences in feeding biomechanics compared with other diplodocoid dinosaurs.