Meeting Abstract

P3.45  Jan. 6  Reconstruction and possible evolutionary changes of muscle and ligament attachments in the occipital region and atlas-axis complex in non-avian theropod dinosaurs TSUIHIJI, T.*; WITMER, L.M.; Ohio University, Athens; Ohio University, Athens tsuihiji@ohio.edu

Various muscles and ligaments connect the occipital region and vertebral column in reptiles, supporting the head on the neck. In the present study, attachment sites of such soft tissues on the occiput and atlas-axis complex were examined in various non-avian theropod dinosaurs by using the Extant Phylogenetic Bracket approach based on the anatomy of extant diapsids. Some muscles and ligaments have well-developed osteological correlates for their attachments: for example, a prominent process or deep depression on the supraoccipital and/or parietal marks the insertion of m. spinalis capitis or its homolog. Based on such features, muscle and ligament attachments could be reconstructed rather robustly. The results suggest fairly large differences and possible evolutionary changes in sizes of these soft tissues among theropods, especially large-bodied ones. The presence of a strong nuchal crest in more basal taxa such as ceratosaurians and allosaurids suggests that the supraspinal ligament inserting on it would have been well-developed and played a prominent role in supporting the head. In addition, these taxa have a large attachment area for m. rectus capitis posterior arising from the atlas-axis. The more derived (= closer to Aves) tyrannosaurids, on the other hand, show a large area of fleshy insertion of m. spinalis capitis on the parietal crest. The nuchal crest, however, is poorly developed compared to that in more basal forms. In Aves, the supraspinal ligament does not reach the occiput unlike the plesiomorphic diapsid condition. The apparently weak development of this ligament attaching to the skull in tyrannosaurids suggests that these dinosaurs were already on the way toward the acquisition of the avian condition.