Eunotosaurus have an unusually broad upper body made up of nine pairs of extremely widened ribs that overlap one another. The most front ribs were angled towards the back, while the farthest ribs from the front were angled towards the front. The cross section in the ribs are T-shaped, each having a wide, flat surface on top and a slim ridge running along the length on the opposite side. The surface of the dorsal side is convex, giving the Eunotosaurus its rounded shape. In most cases the ribs and the vertebrae are fused, but in some smaller specimens of the genus, the rib pairs are connected with the vertebrae but do not fuse with them. They also have nine dorsal vertebrae, which is a lot fewer than what is normally seen in other parareptiles.
…show more content…
The first phase is the growth of a rib primordium that turns into a rib bone during its embryotic development. The second phase, which differs from most land vertebrates, is the development of a “shelf of bone” above the shaft of the rib bone to form the unique T-shape. The final phase is the development of its teardrop-like shape, which is caused by the widening of the lower ridge of the ribs, which also reinforces the rib. History has shown that the third phase is unique only to the Eunotosaurus, but the second phase is seen in turtles we see today. In turtles, the “shelf of bone” that is formed by the rib shaft becomes a plate of the shell or “carapace”, instead of being within the specimen like in Eunotosaurus. In each rib of Eunotosaurus, the lower ridge has Sharpey's fibers embedded within the posterior surface, which helps anchor muscles to bone. Most amniotes have these fibers on their posterior and their anterior edges of the ribs, reason being the ribs are connected by intercostal muscles (muscles that assist with breathing). The lack of Sharpey's fibers on the anterior edges of the ribs suggested that it lacked functioning intercostal muscles, which, in fact, Turtles lack, and instead have muscles that connect underneath the ribs for the purpose of
The first phase is the growth of a rib primordium that turns into a rib bone during its embryotic development. The second phase, which differs from most land vertebrates, is the development of a “shelf of bone” above the shaft of the rib bone to form the unique T-shape. The final phase is the development of its teardrop-like shape, which is caused by the widening of the lower ridge of the ribs, which also reinforces the rib. History has shown that the third phase is unique only to the Eunotosaurus, but the second phase is seen in turtles we see today. In turtles, the “shelf of bone” that is formed by the rib shaft becomes a plate of the shell or “carapace”, instead of being within the specimen like in Eunotosaurus. In each rib of Eunotosaurus, the lower ridge has Sharpey's fibers embedded within the posterior surface, which helps anchor muscles to bone. Most amniotes have these fibers on their posterior and their anterior edges of the ribs, reason being the ribs are connected by intercostal muscles (muscles that assist with breathing). The lack of Sharpey's fibers on the anterior edges of the ribs suggested that it lacked functioning intercostal muscles, which, in fact, Turtles lack, and instead have muscles that connect underneath the ribs for the purpose of