INTERVERTEBRAL JOINT POLARITY IN SAUROPODS
immobilized relative to the trunk. In this case, caudal opisthocoely would be the stable configuration and procoely comparatively unstable. Although this interpretation is mechanically plausible, there is no anatomical evidence to suggest that Opisthocoelicaudia was more likely to adopt a tripodal stance than other titanosaurs. Apart from caudal opistho- coely, the skeletal characteristics used to sug- gest a tripodal stance for Opisthocoelicaudia are shared with other titanosaurs, which have procoelous caudal vertebrae (Powell 2003). Nopcsa (1930) similarly applied the concept of a reversal of forces to explain presacral pro- coely in crocodylians and squamates. He rea- soned that the sprawling posture and hind limb–driven locomotion of these animals would create forces acting from posterior to anterior. As a result, the head and shoulders would act as the fixed end of the vertebral column, and procoely would be favored. This hypothesis does not account for the diversity of locomotor patterns utilized by crocodylians (e.g., Renous et al. 2002) and squamates (e.g., Russell and Bauer 1992). It might, however, offer an explanation for cervical procoely in crocodylians and pterosaurs. During crocody- lian swimming and pterosaur flight, propul- sion is generated posterior to the neck by the tail or the wings. Thus, the forces acting on the neck should be oriented as proposed by Nopcsa (1930). These explanations remain conjectural in the absence of empirical data on how forces are distributed in the reptilian ver- tebral column during each of the different locomotor behaviors. Rotational Stability in the Appendicular
Skeleton.—The earliest research into concavo- convex joint polarity (e.g., Fick 1845; Henke and Reyher 1874; Fick 1890) wasmotivated to explain the patterns present in the human appendicular skeleton. If the polarity of concavo-convex intervertebral joints determines their rotational stability, it must be considered whether the same is true of appendicular joints. The tetrapod glenohumeral and femoro-acetabular joints exhibit a strong preferential joint polarity; in each, the stylopodium (viz., humerus, femur) bears a proximally facing condyle that fits into a socket on one or more girdle elements. That the cotyle in these joints faces away from the body
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seems, at first, to be at odds with the pattern found in the sauropod neck and tail. However, during standing and in the stance phase of locomotion, the limb is braced against the ground, and the body is free to rotate about it. Thus, the weight-bearing autopodium in contact with the ground is the fixedelement andthe body, which is the more mobile element, bears the concavity. This is mechanically equivalent to the condition in the sauropod neck and tail, in which the distal vertebra in each joint is more mobile and bears the cotyle. Phylogenetic evidence supports the inter-
pretation that the tetrapod glenohumeral and femoro-acetabular joint polarity provides sta- bility when the limbs are braced against the ground, supporting the body. In the extant outgroups of Tetrapoda, the lungfishes and coelacanths, the glenoid and acetabulum are convex (Rosen et al. 1981; Janvier 1996), indicating that the tetrapod condition evolved via a reversal of joint polarity. This reversal can be explained by the changing forces associated with the transition from fins to limbs. The nontetrapod polarity would be more favorable in a swimming animal because the distal end of the fin is mobile relative to the body; it is this mobile element that bears the cotyle. This also explains why the shoulder girdle bears a convexity in the basal actinopterygian Polypterus, another aquatic vertebrate with monobasal fins (Pollard 1892). As tetrapodomorphs began to use their fins in weight support and for terrestrial locomotion, a switch to the “tetrapod-type” joint polarity would have been necessary to maintain joint stability. This interpretation does not explain the retention of the concave glenoid in secondarily aquatic tetrapods. Nonetheless, further investigation of the functional significance of appendicular joint polarity in sarcopterygian evolution is merited. The polarity of the human glenohumeral joint
is sometimes altered surgically with the implantation of a reverse shoulder prosthesis. This structure is intended to compensate for rotator cuff injury by medializing the center of rotation, increasing themechanical advantage of the deltoid (Grammont 1979). This is accom- plished by replacing the glenoid fossa with a convex element and the humeral head with a
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