Eunotosaurus lived in the late Middle Permian (Capitanian stage) and fossils can be found in the Karoo Supergroup of South Africa and Malawi.

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 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.

Moreover, a juvenile specimen also shows upper temporal fenestrae, meaning the skull demonstrates a fully diapsid condition.

[9] Eunotosaurus was named in 1892 for 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.

[11] 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.

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.

This marked the first occurrence of Eunotosaurus outside South Africa, and confirm that the Mwesia Beds correspond with the Tapinocephalus and Pristerognathus zones.

[8] However, analyses which also include genetic data from living reptiles strongly support the idea that turtles fall within a group called Diapsida, as close relatives of either lizards (in which case they would be lepidosauromorphs) or birds and crocodiles (making them archosauromorphs).

[17] However, the discovery of Pappochelys, a prehistoric species whose fossil remains show a mixture of features found in Eunotosaurus and the toothed stem-turtle Odontochelys, helped to resolve the issue.

Though an analysis which included data from Pappochelys found weak support for the idea that Eunotosaurus was a parareptile, it found stronger support for the hypothesis that Eunotosaurus was itself a diapsid closely related to turtles, and that its apparently primitive, anapsid skull was probably developed as part of the turtle lineage, independently of parareptiles.

[24][7] Turtles have recently been considered diapsids on the basis of genetic and phylogenetic evidence, and thus more closely related to modern lizards, snakes, crocodiles, and birds than parareptiles.

This study claimed that Eunotosaurus shared derived features of its ribs and vertebrae with the earliest turtles, thus making it a transitional form.

[25] Brazilosaurus sanpauloensis Mesosaurus tenuidens Stereosternum tumidum Eunotosaurus africanus Milleretta rubidgei Broomia perplexa "Millerosaurus" nuffieldi Milleropsis pricei Millerosaurus ornatus Australothyris smithi Microleter mckinzieorum Ankyramorpha The cladogram below follows the most likely result found by another analysis of turtle relationships, published by Rainer Schoch and Hans-Dieter Sues in 2015.

[5] Acleistorhinidae Millerettidae Mesosaurus Procolophonia Younginiformes Eunotosaurus Coelurosauravus Lepidosauromorpha Archosauromorpha Pappochelys Odontochelys Kayentachelys Proganochelys