Anapsid

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Paraphyletic grouping of reptiles

title: "Anapsid" type: doc version: 1 created: 2026-02-28 author: "Wikipedia contributors" status: active scope: public tags: ["reptile-taxonomy", "paraphyletic-groups"] description: "Paraphyletic grouping of reptiles" topic_path: "general/reptile-taxonomy" source: "https://en.wikipedia.org/wiki/Anapsid" license: "CC BY-SA 4.0" wikipedia_page_id: 0 wikipedia_revision_id: 0

::summary Paraphyletic grouping of reptiles ::

| name = Anapsids | fossil_range = Late Carboniferous to Late Triassic

Protorothyrididae
Parareptilia
Testudines (Turtles, tortoises & terrapins) An anapsid is an amniote whose skull lacks one or more skull openings (fenestra, or fossae) near the temples. Traditionally, the Anapsida are considered the most primitive subclass of amniotes, the ancestral stock from which Synapsida and Diapsida evolved, making anapsids paraphyletic. 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.

Anapsids and the turtles

::figure[src="https://upload.wikimedia.org/wikipedia/commons/3/31/Caretta_carettaZZ.jpg" caption="pmid=24798503}}"] ::

Phylogenetic position of turtles

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. One molecular study, published in 2012, suggests that turtles are lepidosauromorph diapsids, most closely related to the lepidosaurs (lizards, snakes, and tuataras). However, in a later paper from the same authors, published in 2014, based on more extensive data, the archosauromorph hypothesis is supported.

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 is suggested to have diverged from other diapsids between 200 and 279 million years ago, though the debate is far from settled. Although procolophonids managed to survive into the Triassic, 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.

Despite the molecular studies, there is evidence that contradicts their classification as diapsids. All known diapsids excrete uric acid as nitrogenous 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.

Anapsida in modern taxonomy

Anapsida is still sporadically recognized as a valid group, but is not favoured by current workers. Anapsids in the traditional meaning of the word are not a clade, but rather a paraphyletic group composed of all the early reptiles retaining the primitive skull morphology, grouped together by the absence of temporal openings.

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. 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.

References

Pough, F. H. et al. (2002) ''Vertebrate Life'', 6th ed. Upper Saddle River, New Jersey: Prentice Hall. {{ISBN. 0-13-041248-1
(1997). "Reptile phylogeny and the interrelationships of turtles". Zoological Journal of the Linnean Society.
(2009). "Assembling the history of the Parareptilia: phylogeny, diversification, and a new definition of the clade". Fossil Record.
(2011). "Amniotes through major biological crises: faunal turnover among Parareptiles and the end-Permian mass extinction". Palaeontology.
Evans, Susan E.. (2009). "An early kuehneosaurid reptile (Reptilia: Diapsida) from the Early Triassic of Poland". Paleontologica Polonica.
(2009). "A long−necked archosauromorph from the Early Triassic of Poland". Paleontologica Polonica.
(July 2014). "Toward consilience in reptile phylogeny: miRNAs support an archosaur, not lepidosaur, affinity for turtles: Reptile phylogeny from miRNAs". Evolution & Development.
(October 1999). "Molecular evidence for a clade of turtles". [[Molecular Phylogenetics and Evolution]].
(1998). "Complete mitochondrial genome suggests diapsid affinities of turtles". [[Proceedings of the National Academy of Sciences]].
(2004-12-29). "Sister group relationship of turtles to the bird-crocodilian clade revealed by nuclear DNA-coded proteins". [[Molecular Biology and Evolution]].
(November 2007). "Extended mitogenomic phylogenetic analyses yield new insight into crocodylian evolution and their survival of the Cretaceous–Tertiary boundary". [[Molecular Phylogenetics and Evolution]].
(2010-03-17). "From reptilian phylogenomics to reptilian genomes: analyses of c-Jun and DJ-1 proto-oncogenes". Cytogenetic and Genome Research.
(2012). "More than 1000 ultraconserved elements provide evidence that turtles are the sister group of archosaurs". Biology Letters.
(2012-02-23). "MicroRNAs support a turtle + lizard clade". Biology Letters.
(1996). "Turtles as diapsid reptiles". Nature.
Benton, M. J.. (2000). "Vertebrate Paleontology". Blackwell Science.
(2004). "The phylogenetic definition of Reptilia". Systematic Biology.
(2009). "Assembling the history of the Parareptilia: phylogeny, diversification, and a new definition of the clade". Fossil Record.
(1988). "The Phylogeny and Classification of the Tetrapods". Clarendon Press.
(2004). "A procolophonoid reptile with temporal fenestration from the Middle Triassic of Brazil". Proceedings of the Royal Society B: Biological Sciences.
(2012). "Cranial morphology of the Early Permian mesosaurid ''Mesosaurus tenuidens'' and the evolution of the lower temporal fenestration reassessed". Comptes Rendus Palevol.

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