Paleobiology, 42(4), 2016, pp. 624–642 DOI: 10.1017/pab.2016.16


Polarity of concavo-convex intervertebral joints in the necks and tails of sauropod dinosaurs


John A. Fronimos, Jeffrey A. Wilson, and Tomasz K. Baumiller


Abstract.—The highly elongated necks, and often tails, of sauropod dinosaurs were composed of concavo-convex vertebrae that provided stability without compromising mobility. Polarities of these concavo-convex joints in the neck and tail are anatomically opposite one another butmechanically equivalent. Opisthocoelous cervical vertebrae and procoelous caudal vertebrae have the convex articular face directed away from the body and the concave articular face directed toward the body. This “sauropod-type” polarity is hypothesized to be (1)more resistant to fracturing of the cotylar rim and(2) better stabilized against joint failure by rotation than the opposite polarity. We used physical models to test these two functional hypotheses. Photoelastic analysis ofmodel centra loaded as cantilevers reveals that neither polarity better resists fracture of the cotylar rim; strain magnitude and localization are similar in both polarities. We assessed the rotational stability of concavo-convex joints using pairs of concavo-convex centra loaded near the joint. Sauropod-type joints withstood significantly greaterweight before failure occurred, a patternwe interpret to be dependent on the position of the center of rotation, which is always within the convex part of the concavo-convex joint. In sauropod-type joints, the free centrum rotates about a center of rotation that lies within the more stable proximal centrum. In contrast, the opposite polarity results in a free centrum that rotates about an internal point; whenthe condyle rotates down and out of joint, the distal endrotates back toward the body, unopposed by ligamentous support. Sauropod-type joints remained stable with greater mobility, more mechanically advantageous tensile element insertions, and greater distal loads than the opposite polarity. The advantages conferred by this joint polarity would have facilitated the evolution of hyperelongated necks and tails by sauropods. Polarity of concavo-convex joints of the appendicular skeleton (e.g., hip, shoulder) is also consistent with the demands of rotational stability.


John A. Fronimos, Jeffrey A. Wilson, and Tomasz K. Baumiller. Museum of Paleontology and Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, Michigan 48109-1079, U.S.A. E-mail: jfronimo@umich.edu, wilsonja@umich.edu, tomaszb@umich.edu


Accepted: 14 March 2016 Published online: 27 June 2016 Supplemental material deposited at Dryad: http://doi.org/10.5061/dryad.1p0q9


Introduction Sauropod dinosaurs achieved the largest


body sizes (Alexander 1989) and the longest necks of any terrestrial vertebrate (Taylor and Wedel 2013). The long necks and tails are hypothesized to have been held aloft (Coombs 1975) because trackways lack tail drag marks (e.g., Bird 1941). Sauropod necks and tails were loaded like cantilevers, beams supported at only one end (i.e., the body) and free at the other end. As a result, the forces of weight support were likely experienced as compression of the vertebral centra and tension of the muscles, tendons, and ligaments attaching to the neural arches, as in mammals (Slijper 1946) and extant reptiles (Hoffstetter and Gasc 1969). Unlike a rigid cantilevered beam, however, necks and tails are segmented, with joints between vertebrae held together in life by muscles, tendons, ligaments, and cartilage. Intervertebral joints permit


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mobility but are also potential sites of catastrophic dislocation. In order for sauropods to achieve their characteristic large body sizes and long necks, the intervertebral joints required stabilization against increas- ingly large stresses without compromising the extent of mobility required for survival. One way in which sauropod intervertebral joints may have been stabilized without compromising flexibility is by the evolution of concavo-convex centra (e.g., opisthocoely, procoely) in the neck and, in some lineages, in the tail (J. A. Fronimos and J. A. Wilson unpublished). Previous research on concavo- convex intervertebral joint function in croco- dylians has suggested they resist dislocation by shear due to the nesting of one centrum in another and the increased surface area of contact relative to planar joints (Salisbury and Frey 2001; J. A. Fronimos and J. A. Wilson unpublished).


0094-8373/16


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