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Concept
Eunotosaurus
Summary
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.
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.
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