While "anapsid reptiles" or "anapsida" were traditionally spoken of as if they were a monophyletic group, it has been suggested that several groups of reptiles that had anapsid skulls may be only distantly related. Scientists still debate the exact relationship between the basal (original) reptiles that first appeared in the late Carboniferous, the various Permian reptiles that had anapsid skulls, and the Testudines (turtles, tortoises, and terrapins). However, it was later suggested that the anapsid-like turtle skull may be due to reversion rather than to anapsid descent. The majority of modern paleontologists believe that the Testudines are descended from diapsid reptiles that lost their temporal fenestrae. More recent morphological phylogenetic studies with this in mind placed turtles firmly within diapsids, some place turtles as a sister group to extant archosaurs or, more commonly within Lepidosauromorpha.
All molecular studies have strongly upheld the placement of turtles within diapsids; some place turtles within Archosauria, or, more commonly, as a sister group to extant archosaurs. However, one of the most recent molecular studies, published in 23 February 2012, suggests that turtles are lepidosauromorph diapsids, most closely related to the lepidosaurs (lizards, snakes, and tuataras). 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 cladogram. Testudines were suggested to have diverged from other diapsids between 200 and 279 million years ago, though the debate is far from settled. Most of the other reptiles with anapsid skulls, including the millerettids, nycteroleterids, and pareiasaurs, became extinct in the late Permian period by the Permian-Triassic extinction event. But the procolophonids managed to survive into the Triassic.
Anapsida is still sporadically recognized as a valid group, but this is not favoured by current workers. Anapsids in the traditional meaning of the word are not a clade, but rather a non-monophyletic amalgamation of early amniotes and anamniote tetrapods grouped together by the absence of temporal openings. Gauthier, Kluge and Rowe (1988) attempted to redefine Anapsida so it would be monophyletic, defining it as the clade containing "extant turtles and all other extinct taxa that are more closely related to them than they are to other reptiles". 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. In addition, 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 synapsids, has been discovered in the skulls of a number of members of Parareptilia (the clade containing most of reptiles traditionally referred to as anapsids), including lanthanosuchoids, millerettids, bolosaurids, some nycteroleterids, some procolophonoids and at least some mesosaurs. 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.
^ abRieppel O, DeBraga M (1996). "Turtles as diapsid reptiles". Nature384 (6608): 453–5. doi:10.1038/384453a0.
^Li, Chun; Xiao-Chun Wu, Olivier Rieppel, Li-Ting Wang & Li-Jun Zhao (2008-11-27). "An ancestral turtle from the Late Triassic of southwestern China". Nature456 (7221): 497–501. doi:10.1038/nature07533. PMID19037315.Cite uses deprecated parameters (help)
^Constanze Bickelmann, Johannes Müller and Robert R. Reisz (2009). "The enigmatic diapsid Acerosodontosaurus piveteaui (Reptilia: Neodiapsida) from the Upper Permian of Madagascar and the paraphyly of ‘‘younginiform’’ reptiles". Canadian Journal of Earth Sciences49: 651–661. doi:10.1139/E09-038.
^Katsu, Y.; Braun, E. L.; Guillette, L. J. Jr.; Iguchi, T. (2010-03-17). "From reptilian phylogenomics to reptilian genomes: analyses of c-Jun and DJ-1 proto-oncogenes". Cytogenetic and Genome Research127 (2–4): 79–93. doi:10.1159/000297715. PMID20234127.
^Crawford, N. G.; Faircloth, B. C.; McCormack, J. E.; Brumfield, R. T.; Winker, K.; Glenn, T. C. (2012). "More than 1000 ultraconserved elements provide evidence that turtles are the sister group of archosaurs". Biology Letters. doi:10.1098/rsbl.2012.0331.edit
^ abcdLinda A. Tsuji and Johannes Müller (2009). "Assembling the history of the Parareptilia: phylogeny, diversification, and a new definition of the clade". Fossil Record12 (1): 71–81. doi:10.1002/mmng.200800011.
^ abGauthier, J.A.; Kluge, A.G.; Rowe, T. (1988). "The early evolution of the Amniota". In Benton, M.J. (ed.). The Phylogeny and Classification of the Tetrapods1. Oxford: Clarendon Press. pp. 103–155. ISBN0198577052.
^ abGraciela Piñeiro, Jorge Ferigolo, Alejandro Ramos and Michel Laurin (2012). "Cranial morphology of the Early Permian mesosaurid Mesosaurus tenuidens and the evolution of the lower temporal fenestration reassessed". Comptes Rendus Palevol11 (5): 379–391. doi:10.1016/j.crpv.2012.02.001.