Earlier we looked at Stephanospondylus (Fig. 1) as the ancestor of turtles like Proganochelys, more primitive than Odontochelys, which represents a splinter off the main line. Unfortunately I never put Stephanospondylus together as a basic silhouette reconstruction, common to all other taxa at reptileevolution.com. Here it is (Fig. 1) along with sister taxa.
Based on the wide ribs, Stephanospondylus had a low, wide torso, some of which were broadened with costal plates, derived from those in Milleretta and destined to form the carapace in turtles. Here (Fig. 1) you can see the evolution of the pectoral girdle, humerus and femur.
Diadectes (Fig. 2) had similar large costal ribs, but these were smooth, not highly textured as in Stephanospondylus. We don’t know where the costal ribs were located in Stephanospondylus. In Proganochelys and Odontochelys the anterior ribs are narrow, not provided with large costal plates (Fig. 1). Stephanospondylus also had a broad, textured interclavicle, the starting point for the plastron.
Note that Stephanospondylus did not have a central neural spine as did Diadectes and Milleretta. That spine, as you know, anchors back muscles. Not having that spine indicates that back muscles were on the way out for Stephanospondylus, one more sign that it was developing a carapace, as in turtles, which also, obviously, lack back muscles.
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