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“”Eunotosaurus”” is an extinct genus of reptile, possibly a close relative of turtles, from the late Middle Permian Karoo Supergroup of South Africa. It is often considered as a possible “missing link” between turtles and their prehistoric ancestors. Its ribs were wide and flat, forming broad plates similar to a primitive turtle shell, and the vertebrae were nearly identical to those of some turtles. It is possible that these turtle-like features evolved independently of the same features in turtles, though some studies suggest “Eunotosaurus” is a genuine, primitive turtle relative.

“Eunotosaurus” had a broad body formed by nine pairs of widened ribs that overlap each other. The forward-most ribs are angled slightly backward and the backward-most ribs angle slightly forward. The ribs are T-shaped in cross section, each having a broad, flat surface on the top and a narrow ridge running along its length on the bottom. The upper surface is convex, giving the body of “Eunotosaurus” a rounded shape. Each pair of ribs connects to an elongated dorsal or back vertebra. Most ribs are fused to the vertebrae, but some smaller specimens of “Eunotosaurus” have rib pairs that connect with the vertebrae but are not fused to them. There are nine dorsal vertebrae, far fewer than what is seen in other parareptiles. The neck of “Eunotosaurus” is short, consisting of six short cervical vertebrae.

Histological analysis of cross-sections of the ribs indicate that they grew in three different phases as an individual developed. As is the case in most land vertebrates, the first phase involves the growth of a rib primordium that ossifies into a rib bone. The second phase, which deviates from most other land vertebrates, is the development of a shelf of bone above the main shaft of the rib to form the T-shape. The third and final phase is the widening of the lower ridge into a teardrop-like shape, reinforcing the rib. While the third phase is unique to “Eunotosaurus”, the second phase is also seen in modern turtles. In turtles, the shelf of bone that forms from the rib shaft becomes a plate of the shell or carapace. In each rib of “Eunotosaurus”, the posterior surface of the lower ridge has Sharpey’s fibers embedded in it. Sharpey’s fibers help anchor muscles to bone. Most amniotes have Sharpey’s fibers on the posterior and anterior edges of the ribs because the ribs are connected to each other by intercostal muscles, which are muscles that assist in breathing. The lack of Sharpey’s fibers on the anterior side of the ribs of “Eunotosaurus” suggests that it lacked functional intercostal muscles. Turtles also lack intercostal muscles and instead have muscles that connect to the undersides of the ribs for the purpose of locomotion. If “Eunotosaurus” is close to the ancestry of turtles, it may have had similar sets of muscles.

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