Paleoneurology of the dicraeosaurid sauropod Amargasaurus cazaui Salgado y Bonaparte 1991

PAULINA CARABAJAL, A.

San Juan

Congreso; IV Congreso Latinoamericano de paleontología de vertebrados; 2011

Museo de Ciencias Naturales, Universidad Nacional de San Juan

The braincase of the type species of Amargasaurus cazaui (MACN-N 15) from the Early Cretaceous of Argentina (Salgado and Bonaparte, 1991), is complete and undistorted, and represents the only known neurocranial material in South America for the dicraeosaurid family. It was described in detail by Salgado and Calvo (1992). This study reinterprets two cranial foramina:  cranial nerve (CN) III is reinterpreted as CN III-IV, and CN IV is reinterpreted as the middle cerebral vein foramen. The specimen has been computed tomography (CT) scanned and a three-dimensional digital reconstruction of the endocranium and inner ear has been prepared.  The cranial endocast is complete in that it terminates anteriorly at the opening for the olfactory tract and posteriorly at the foramen magnum, and is 10 cm in total anteroposterior length. The cerebral and pontine flexures are similar to those in Dicraeosaurus hansemanni Janensch 1914. Although Amargasaurus shares with Dicraeosaurus the presence of a large dorsal longitudinal sinus and an enlarged endocranial space related to the postparietal fenestra, the latter seems to be different from that illustrated by Janensch (1936) for Dicraeosaurus. In Amargasaurus, the enlarged  endocranial space is dorsal to the forebrain, being a paired cavity posteriorly but fused into a single cavity strongly related to the olfactory tract area anteriorly. The pituitary fossa descends from the ventral surface of the endocast and is oriented slightly  posteroventrally. CN II has long passages across the laterosphenoid-orbitosphenoid complex. These nerves extend posteromedially to converge at the midline of the anteroventral margin of the main body of the diencephalon. Not only CN VI is anteroventrally projected, but also CN V and III-IV, unlike the laterally projected nerves in titanosaurids (Paulina Carabajal, 2009). Dorsal to the labyrinth of the inner ear, there is a vascular element, probably for the dorsal head vein. The inner ear can be partially reconstructed and is described for the first time for this family of sauropods.