Eunotosaurus (Latin: stout-backed lizard) is an extinct genus of reptile of disputed affinities. It is known from the late Middle Permian (Capitanian stage) of the Karoo Supergroup of South Africa and the Mwesia Beds of Malawi. Its ribs are wide and flat, forming broad plates similar to a primitive turtle shell, and the vertebrae are comparable to those of some turtles. Accordingly, Eunotosaurus has often been 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 fe
Eunotosaurus (Latin: stout-backed lizard) is an extinct genus of reptile of disputed affinities. It is known from the late Middle Permian (Capitanian stage) of the Karoo Supergroup of South Africa and the Mwesia Beds of Malawi. Its ribs are wide and flat, forming broad plates similar to a primitive turtle shell, and the vertebrae are comparable to those of some turtles. Accordingly, Eunotosaurus has often been 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 Eunotosaurus may instead have been a parareptile, an early-diverging neodiapsid unrelated to turtles, or a synapsid.
==History of study== Eunotosaurus was named in 1892 based on a specimen (now NHMUK PV R 1968 in the Natural History Museum, London) that he had obtained from Mr. L. Pienaar at the farm Weltevreden near Beaufort West, during Seeley's visit to South Africa in 1889. Seeley was uncertain of the systematic position of Eunotosaurus, but postulated that it was likely referable to the Mesosauria, based on the pubis anatomy. 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.
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