Functional Mechanics of Concavo-convex Articulations and Neurocentral Sutures in the Vertebral Column of Sauropod Dinosaurs.

Abstract

Sauropod dinosaurs achieved the largest body sizes and the most elongate necks and tails of any terrestrial vertebrate. Their necks and tails were held aloft as cantilevers, beams supported at one end and free at the other. Synovial joints between vertebrae provide mobility, and synchondrosial joints within vertebrae facilitate growth. These requirements come at a cost, as the joints are potential sites of dislocation, with deleterious consequences for the living animal. Morphological specializations of sauropod inter- and intravertebral joints provided stability without compromising other functional demands. Sauropod intervertebral joints were characterized by concavo-convex morphology, which has been hypothesized to confer greater flexibility or to stabilize joints against dislocation by translation. Examination of joint mobility in an extant analog, Alligator, reveals that concavo-convex joints do not confer greater flexibility than do planar joints, nor do they inherently limit mobility. Convexity is greatest in regions of the greatest shear, consistent with a stabilizing function. Sauropod intervertebral joints have a consistent polarity in which the concave articular surface faces the body (i.e., cervical opisthocoely, caudal procoely). Physical modeling reveals that this polarity is more stable than its opposite because it inhibits the convex articulation from rotating out of joint. The advantage of the sauropod polarity is enhanced by greater joint mobility, distal loading, and mechanically advantageous ligament insertion sites. This provided stabilization without compromising other functions. The intravertebral (i.e., neurocentral) joints of archosaurs such as sauropods remain unfused to a later age than in most other vertebrates, permitting rapid, sustained growth to large body sizes. The greater susceptibility of the joints to dislocation compared to fused bone may be compensated for by complex, interdigitated sutures that resist compression, rotation, and translation. In the sauropod Spinophorosaurus, variation in sutural complexity along the vertebral column is consistent with the expected stress distribution. Large-scale morphological structures in the sutures are oriented to resist specific regional stresses. The integration of fossil data with studies of extant taxa and model experiments provides a means to answer functional questions about extinct organisms. The results offer insights into skeletal biomechanics that are widely applicable to other vertebrates.