Eunotosaurus (Latin: Stout-backed lizard) is an extinct genus of amniote, possibly a close relative of turtles. Eunotosaurus lived in the late Middle Permian (Capitanian stage) and fossils can be found in the Karoo Supergroup of South Africa. Eunotosaurus resided in the swamps of southern Africa. 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. Accordingly, it is often considered as a possible transitional fossil between turtles and their prehistoric ancestors. However, it is possible that these turtle-like features evolved independently of the same features in turtles, since other anatomical studies and phylogenetic analyses suggest that Eunotosaurus may instead have been a parareptile, an early-diverging neodiapsid unrelated to turtles, or a synapsid.
Description
Eunotosaurus reached up to in total body length. It 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.
History of study
Eunotosaurus was named in 1892, but it was not until 1914 that it was proposed to be an ancestor of Chelonia, the turtle order. English zoologist D. M. S. Watson claimed that Eunotosaurus was transitional between cotylosaurs (now referred to as captorhinids) and Chelonia. He compared it to "Archichelone", a name he devised for a hypothetical chelonian ancestor, noting that its ribs appeared to be intermediate between those of turtles and other tetrapods. Watson's "Archichelone" had a pelvic girdle that was pushed back on the vertebral column and placed under the shell. However, fossils of Eunotosaurus show that the pelvis is in the normal tetrapod position and is placed over the ribs rather than within them, as in modern turtles. Many fossils have been found showing a semi-rigid, turtle-like rib cage, one which presumably necessitated a tortoise-like fashion of walking.
Eunotosaurus was considered the ancestor of turtles up until the late 1940s. In his 1956 book Osteology of the Reptiles, American paleontologist Alfred Sherwood Romer claimed that Eunotosaurus could not be included within Chelonia based on the available evidence. He placed it within Anapsida in its own order incertae sedis.