M.F. Bonnan, J.M. Parrish, K.A. Stevens, J.P. Graba, and P. Senter
The feeding range of sauropod dinosaurs was constrained by the height of the base of the neck, which was itself constrained by the nature of the articulation between the scapulocoracoid and the trunk. Sauropod scapulocoracoid orientation and mobility remain controversial because no tight, bony articulations were present between these bones and the vertebral column, and few osteological markers are available to constrain shoulder orientation. Previous hypotheses of scapulocoracoid position in sauropods were inferred from death poses or through simple goodness-of-fit criteria, but were not developed within a phylogenetic context. We examined the scapulocoracoids, ribcage, and sterna of several neosauropods, basal sauropodomorphs, extant archosaurs, and lepidosaurs in order to: 1) assess the soft tissue contributions to scapulocoracoid orientation and mobility; and 2) determine osteological correlates associated with shoulder position. Archosaurian scapulae vary in position from nearly parallel to the vertebral column (birds) to nearly vertical (crocodilians) but in all cases the scapular blade is nearly parallel to the top of the neural spines along its distal extent, suggesting a similar orientation in dinosaurs. Positions and homologies of the musculature supporting the scapulocoracoid were conservative within taxa. Scapulocoracoid movements against the sternum, affected by the Mm. serratus, levator scapulae, and sternocoracoideus groups, become more restricted in archosaurs, particularly in birds. Flattened areas on the external surfaces of the dorsal ribs ("facets") are present in birds, sauropods, and other dinosaurs but are absent in crocodilians. Dissection and CT-scan data show that the scapular blade bows away from the dorsal ribs in Alligator whereas it lies in close contact with the dorsal ribs in birds, which may explain the lack of "facets" in crocodilians. Rib facets correlate with the neutral orientation of the scapular blade in birds. We suggest the presence of rib "facets" and the more restricted movements of the scapulocoracoid in diapsid outgroups support a constrained, sub-vertical orientation of the pectoral girdle in sauropods.
The following images show one potential placement for the sternal plates, from the poster. View the PDF version of the poster here.
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