Rib
Ribs are classed as flat bones which usually have a protective role in the body. Humans have 24 ribs, in 12 pairs. All are attached at the back to the thoracic vertebrae, and are numbered from 1–12 according to the vertebrae they attach to. The first rib is attached to thoracic vertebra 1 (T1). At the front of the body most of the ribs are joined by costal cartilages to the sternum. The ribs connect to the vertebrae with two joints, the costovertebral joints.
Eunotosaurus
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.