INTERVERTEBRAL JOINT POLARITY IN SAUROPODS The concavo-convex joints of the sauropod
neck and tail show strong, conservative patterns in the polarization of the articular surfaces. Most vertebrae can be classified into two polarities, depending upon which end of the centrum bears the convex articulation (i.e., the condyle) and which the concave articulation (i.e., the cotyle). Opisthocoelous centra (Fig. 1A) have the condyle at the anterior end and the cotyle at the posterior end. Procoelous centra (Fig. 1B) have the reverse anatomical orientation, with the cotyle facing forward and the condyle facing backward. In sauropods, opisthocoely is characteristic of the presacral spine (Fig. 1C). Only the most basally diverging sauropods (i.e., Antetonitrus, Lessemsaurus, Gongxianosaurus) retain the ancestral condition of amphiplatyan (flat- ended) cervical centra, and these taxa are not known from complete cervical series. In all other sauropods, all postaxial cervical centra are invariantly opisthocoelous, as are at least some centra in the pectoral region (e.g., Tazoudasaurus [Allain et al. 2004]; Shunosaurus [Zhang 1988]). This pattern
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extends to the sacrum in macronarians (Wilson and Sereno 1998). Procoelous anterior caudal vertebrae
(Fig. 1C) evolved independently three or four times in sauropods according to our current understanding of their interrelationships: (1) in the Late Jurassic basal eusauropod Mamenchisaurus (Young 1954; Xing et al. 2015); (2) within the diplodocoids (Calvo and Salgado 1995); (3) within the titanosaurs (McIntosh 1990; Wilson and Sereno 1998; Upchurch et al. 2004); and (4) in a tail of uncertain taxonomic position from the Late Jurassic of Tanzania (Bonaparte et al. 2000). In some Late Cretaceous titanosaurs, procoely extends to middle and distal caudal vertebrae (McIntosh 1990; Wilson 2002; Upchurch et al. 2004). Opisthocoelous anterior caudal vertebrae are known in only one sauropod, the titanosaur Opisthocoelicaudia (Borsuk- Bialynicka 1977). Other patterns occurring in the first caudal vertebra and the distal caudal vertebrae are summarized in J. A. Fronimos and J. A. Wilson (unpublished). Opisthocoelous cervical centra also occur in
some theropod and ornithopod dinosaurs and many ungulates, and procoelous caudal vertebrae occur in crocodylians, squamates, and a few other reptiles. Cervical opisthocoely and caudal procoely occur together only in sauropods (see Nopcsa 1930: p. 19; J. A. Fronimos and J. A. Wilson unpublished: Table 1) and trionychid turtles (Williams 1950; Hoffstetter and Gasc 1969). Sauropods are distinct from the other taxa listed,inthatboththe neck andtailare long, heavy cantilevers. Although cervical opisthocoely and caudal
FIGURE 1. Sauropod centrum articular morphology and the anatomical distribution of centrum types. A, Opisthocoelous cervical vertebra of the titanosaur Trigonosaurus. B, Procoelous caudal vertebra of the titanosaur Trigonosaurus. The serial distribution of opisthocoely and procoely is depicted on a sauropod silhouette (C) with schematic representations of the two concavo-convex joint polarities. In both the neck and the tail, the concave articular surfaces of the centra face toward the body. Vertebrae are shown in left lateral view. Dashed lines indicate missing regions of vertebrae. Scale bar, 10cm (A) and 6 cm (B). Silhouette modified from Wilson and Sereno (1998).
procoely have the opposite anatomical orienta- tion, they are mechanically equivalent; that is, in each case the concave articulation faces toward the body and the convex articulation faces away (Fig. 1C). In the intervertebral joints of the sauropod neck and tail, the forces of weight support are transferred proximally from cotyle to condyle, not from condyle to cotyle (with the exception of Opisthocoelicaudia mentioned above).
Previous Functional Studies of Concavo-Convex
Joints.—The consistent mechanical polarity of concavo-convex joints in the sauropod neck
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