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Major Subgroups

A simplified history of Anapsida

A simplified picture of the history of the Anapsida, showing the ranges of constituent groups. Some of these groups are argued to be elsewhere within Reptilia: see the note below. Note that most of the groups originated in, and went extinct at the end of, the PERMIAN. Time periods: Pen, Pennsylvanian (Carboniferous); Per, Permian; Tri, Triassic; Jur, Jurassic. Used with permission: UCMP


Millerettidae

The millerittids originated by the upper Permian in South Africa. They were small and lightly built, with sharp conical teeth suggesting an insectivorous or carnivorous diet.  Their hearing was still rather basic. Some millerittids possessed temporal fenestrae.

Millerosaurus

Millerosaurus pricei, a millerittid. The skull is around 50mm long, with temporal fenestrae. It would have lived somewhat like a modern lizard. Watson 1957


Acleistorhinus
This is the earliest known anapsid, from the lower Permian of North America. It was small in size, and already showed signs of being quite advanced, but its teeth were still fairly simple. See figure A below. It is most closely related to the lanthanosuchids. It therefore is not an ancestral form for the anapsids, indicating an earlier origin for the group - probably in the mid to late Carboniferous.

Lanthanosuchidae
This is an enigmatic group from the late Permian of Russia. They are closely related to Acleistorhinus, but distant relatives of turtles. Because of their strange anatomy, their position within Anapsida has been debated: they could be part of a more ancient group of  early tetrapods. Their skulls were broad (around 15cm wide by 20cm long), and possessed temporal fenestrae, and bony ridges to add strength. They were also very flat, so flat that the jaw musculature did not fit in the skull and had to be attached outside it, through special openings behind the eyes. They may have used their flat skulls to push through leaf litter, feeding on insects and grubs, or they could have been aquatic.

Nyctiphruretia
This group is poorly known. They may be the group from which the Procolophonoidae evolved, as they appear to be quite primitive.

Pareiasauria

The position of the pareiasaurs within Anapsida is debated, but they may be the closest relatives of the turtles. They are a well known and diverse group from the upper Permian, and have been found in Europe, Asia and Africa. They included the largest terrestrial anapsids that ever lived, typically 2-3m long. The Russian Scutosaurus for example had massive elephantine limbs, and a broad, heavy skull (see B below) with bumps and frills, characteristic of the pareiasaurs. All pareiasaurs had bony scales ('scutes') over at least part of their bodies, and these have been suggested to be the beginnings of the turtle shell. They also had advanced hearing, and multi-cusped, leaf shaped teeth, indicating herbivory.

Scutosaurus

Scutosaurus. Benton 2000

Procolophonoidae
The procolophonids are the only extinct group to have survived the end Permian extinction, suviving from late Permian, for 50 million years through to the end of the Triassic. They are possibly, if not the pareiasaurs, the closest relative of the turtles. They were very diverse, and have been found on nearly every continent. Procolophonids were quite small, and had well developed hearing. Most had sharp teeth for eating meat and insects, but later forms, in the late Triassic, had more bulbous teeth suggesting herbivory (see C below).


Testudines
This group includes the only living members of the anapsids, the turtles (sometimes known as the Chelonia). Testudines originated in the late Triassic and radiated to include 25 families. One of the oldest known turtles is Proganochelys from the upper Jurassic, by which time the teeth, common to all previous anapsids, had been lost, and the familiar shell had been gained. Its skull was exceptionally strong, and it was most likely terrestrial.

Example anapsid skulls


Various anapsid skulls, with corresponding bones coloured: A, the Lower Permian genus Acleistorhinus. B, the pareiasaur Scutosaurus. C, the Triassic procolophonoid Procolophon. D, Proganochelys, one of the oldest known turtles. Scale bars equal 1 cm. Used with permission: UCMP


Note on anapsid classification
Traditionally, many taxa (animals) inferred to be basal (ancient) reptilian forms have been placed in the Anapsida, largely based upon the lack of temporal fenestration. As this is a primitive (ancestral) character for amniotes and cannot be used to constrain them, they are technically an artificial group.

This has caused many problems when new characters of the members of Anapsida have come to light: they may be moved within the group, or out of it altogether. When a basal group undergoes cladistic (relational) reshuffling, this inevitably has wide reaching consequences -  for anything inferred to have evolved from it, i.e. the turtles, and in parallel to it, i.e. the rest of the reptiles, and even beyond.


The relationships between the groups within Anapsida are still hotly debated
, depending on which characteristics are used to define these relationships, and how much importance is placed upon them. This has resulted in controversy on whether groups in the subclass, particularly the turtles, belong with Anapsida or elsewhere in the larger class of Reptilia.


Recent studies have found that turtles are most closely related to the diapsids - either the archosaurs (crocodiles and birds), or lepidosuars (lizards and snakes). There is as yet no consensus on where they truly belong. (If turtles are excluded from Anapsida, the alternative term of Parareptilia is sometimes used for the remaining members of the group.)