Palæos:		Unit 200: Anapsida
The Vertebrates		Overview

Anapsida: Overview

REPTILOMORPHA
|--SYNAPSIDA
`--+--EUREPTILIA
   |   
   Anapsida
   |--Bolosauridae 
   `--Procolophonia
      |--Procolophonoidea 
      `--+--Hallucicrania 
         |  |--Lanthanosuchidae 
         |  `--Pareiasauria 
         `--Testudines
            |--Pleurodira
            `--Cryptodira

The term Anapsida ("no arch") originally referred to all those reptile groups that lack skull openings behind the eyes. Originally most of the primitive ancestral reptiles were included here, as well as the turtles.  It is now known that this is a rather artificial group.  The lizard-like Permo-Carboniferous captorhinids - originally placed here, are now believed to represent a seperate group.  There is also some controversy about whether the Turtles, for a long time included among the anapsids, belong here.  That still leaves a diverse selection of Permo-Triassic reptiles, from small insectivorous species superficially similar to living lizards to great cumbersome armoured herbivores (such as the Scutosaurus, shown above).  The alternative term "Parareptilia" is sometimes used to describe this extinct group (assuming here that turtles are not anapsids).

The Anapsids appeared suddenly during the later part of the early Permian - probably evolving from near the captorhinids.  They diversified quickly, supplanting the captorhinids as a lizard-like ecomorph.  During the Late Permian they evolved huge, heavily armored herbivores, the pareiasaurs.  These early ankylosaur or glyptodont analogues often had widely-flared skulls, ornamented with irregular blobs of bone, looking like half-melted wax. The overall appearance is so bizarre that MSY Lee has aptly named the pareiasaur - lanthanosuchid taxon the Hallucicrania.  Lee (1997) has also argued forcefully that turtles evolved from a dwarf lineage of pareiasaurs. Apart from the question of turtle relationships, it seems only a single lineage of anapsids, the procolophonids - survived into the Triassic.  There they flourished and diversified, before dying out at the end of the period to be replaced by lepidosaurian sphenodonts (Rhynchocephalia).

Definitive turtles are only known from the Late Triassic,  Proganochelys is the first well-known turtle . By that era, all other known turtles had diversified into two clades, the pleurodire (side-neck) turtles and the cryptodire ("hidden"-neck) turtles.  These differ in a number of ways, including the manner in which the pelvis is attached to the shell, the method used for retracting the neck, and the use of different "pulley" mechanisms to give additional mechanical advantage to the muscles closing the jaws.  Curiously neither group had any mechanism for withdrawing the neck under the shell in these basal forms.  Both groups independently evolved this defense mechanism much later, during the Cretaceous.  Today, cryptodires dominate the turtle population in most of the world. Pleurodires are still found in the Gondwanan continents of Africa and South America, and are the only turtles native to Australia, another Gondwanan land. (ATW & MAK)

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