Palaeos		Anapsida
Vertebrates		Eunotosaurus

Anapsida: Eunotosaurus

REPTILIOMORPHA
|--SYNAPSIDA
`--+--EUREPTILIA
   |   
   ANAPSIDA
   |--+--Millerettidae
   |   `--Eunotosauridae
   |--Mesosauridae
   `--+--Lanthanosuchoidea
      `--+--Bolosauridae
         `--Procolophonia
            |--+--Nycteroleteridae
            |   `--Pareiasauridae
            `--Procolophonoidea  
               |--Owenettidae 
               |?--TESTUDINES
               `--Procolophonidae

Life restoration of Eunotosaurus africanus. illustration by Smokeybjb (Wikipedia)

Descriptions

Eunotosaurus africanus Seeley, 1892

Phylogeny: Anapsida : Mesosauridae + (Bolosauridae + Procolophonia) + Millerettidae + *. 

Range: Late Middle Permian (Capitanian) of Southern Africa . 

Comments: A Tapincopehalus Assemblage Zone index fossil (Rubidgeetal99). Once considered the ancestor of the Chelonia, it is now known that this small animal independently evolved turtle like characteristics. The semi-rigid, turtle-like ribcage implies a tortoise-like fashion of walking (Sumida & Modesto) . Beyond being a member of the anapsida, evolutionary relationships were for a long time uncertain, which is not surprsiing for a highly derived taxon (the same thing is the case with Mesosaurus). But recent cladistic studies (Müller & Tsuji 2007, Cisneros et al 2008) show Eunotosaurus to be be a Millerettid. More unexpected (and unlikely?) is the study by Lyson et al 2010 that even restores the chelonian ancestor hypothesis. One problem here is that Eunotosaurus is too early to be a good ancestor, some 45 million years separate it from Odontochelys, the oldest known turtle (there's a similar problem with the time difference between "Protoavis" and Archaeopteryx; a major adaptive radiation such as birds, turtles, or placental mammals would be expected to be immediately evident in the fossil record) MAK120324

Links: Eunotosaurus And Turtle Evolution - All About Reptiles Eunotosaurus - Wikipedia