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An anapsid is an amniote whose skull lacks one or more skull openings (fenestra, or fossae) near the temples.Pough, F. H. et al. (2002) Vertebrate Life, 6th ed. Upper Saddle River, New Jersey: Prentice Hall. Traditionally, the Anapsida are considered the most primitive subclass of amniotes, the ancestral stock from which Synapsida and evolved, making anapsids paraphyly. It is, however, doubtful that all anapsids lack temporal fenestra as a primitive trait, and that all the groups traditionally seen as anapsids truly lacked fenestra.
Reanalysis of prior phylogenies suggests that they classified turtles as anapsids both because they assumed this classification (most of them were studying what sort of anapsid turtles are) and because they did not sample fossil and extant taxa broadly enough for constructing the cladistics. Testudines is suggested to have diverged from other diapsids between 200 and 279 million years ago, though the debate is far from settled. 3rd edition (2004) . Although Procolophonidae managed to survive into the Triassic, most of the other reptiles with anapsid skulls, including the Millerettidae, Nycteroleteridae, and , became extinct in the Lopingian period by the Permian-Triassic extinction event.
Despite the molecular studies, there is evidence that contradicts their classification as diapsids. All known diapsids excrete uric acid as Metabolic waste (uricotelic), and there is no known case of a diapsid reverting to the excretion of urea (ureotelism), even when they return to semi-aquatic lifestyles. Crocodilians, for example, are still uricotelic, although they are also partly ammonotelic, meaning they excrete some of their waste as ammonia. Ureotelism appears to be the ancestral condition among primitive amniotes, and it is retained by mammals, which likely inherited ureotelism from their synapsid and therapsid ancestors. Ureotelism therefore would suggest that turtles were more likely anapsids than diapsids. The only known uricotelic chelonian is the desert tortoise, which likely evolved it recently as adaptation to desert habitats. Some desert mammals are also uricotelic, so since practically all known mammals are ureotelic, uricotelic adaptation is a likely result of convergence among desert species. Therefore, turtles would have to be the only known case of a uricotelic reptile reverting to ureotelism.
This definition explicitly includes turtles in Anapsida; because the phylogenetic placement of turtles within Amniota is very uncertain, it is unclear what taxa, other than turtles themselves, would be included in such defined Anapsida, and whether its content would be similar to the Anapsida of tradition. Indeed, Gauthier, Kluge and Rowe (1988) themselves included only turtles and Captorhinidae in their Anapsida, while excluding the majority of anapsids in the traditional sense of the word from it.
Temporal openings in traditional anapsids
Tsuji and Müller (2009) noted that the name Anapsida implies a morphology (lack of temporal openings) that is in fact absent in the skeletons of a number of taxa traditionally included in the group. A temporal opening in the skull roof behind each eye, similar to that present in the skulls of , has been discovered in the skulls of a number of members of Parareptilia (the group containing most of reptiles traditionally referred to as anapsids), including lanthanosuchoids, millerettids, Bolosauridae, some nycteroleterids, some procolophonoids and at least some . The presence of temporal openings in the skulls of these taxa makes it uncertain whether the ancestral reptiles had an anapsid-like skull as traditionally assumed or a synapsid-like skull instead.
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