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An anapsid is an
amniote Amniotes (from Greek ἀμνίον ''amnion'', "membrane surrounding the fetus", earlier "bowl in which the blood of sacrificed animals was caught", from ἀμνός ''amnos'', "lamb") are a clade A clade (; from grc, , ''klados'', "bran ...
whose skull lacks one or more skull openings (fenestra) near the
temple A temple (from the Latin ) is a building reserved for spiritual rituals and activities such as prayer and sacrifice. Religions which erect temples include Christianity (whose temples are typically called church (building), churches), Hinduism (w ...
s. Traditionally, the Anapsida are the most primitive subclass of
reptile Reptiles, as most commonly defined, are the animals in the Class (biology), class Reptilia , a paraphyletic grouping comprising all amniotes except synapsids (mammals and their extinct relatives) and Aves (birds). Living reptiles comprise turtl ...

reptile
s, the ancestral stock from which
Synapsid Synapsids are a group of animals that includes mammals and every animal more closely related to mammals than to the other members of the amniote clade, such as reptiles and birds. Unlike other amniotes, they have a Skull#Fenestrae, temporal fenes ...

Synapsid
a and
Diapsid Diapsids ("two arches") are a group of amniote tetrapods that developed two holes (temporal fenestra) in each side of their skulls about 300 million years ago during the late Carboniferous period. The diapsids are extremely diverse, and include all ...

Diapsid
a evolved, making anapsids
paraphyletic In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and all descendants of that ancestor excluding a few—typically only one or two—Monophyly, monophyletic subgroups. The group is said to be paraphyleti ...
. 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

While "anapsid reptiles" or "anapsida" were traditionally spoken of as if they were a
monophyletic 300px, A cladogram of the primates, showing a ''monophyletic'' taxon: ''the simians'' (in yellow); a ''paraphyletic'' taxon: ''the prosimians'' (in cyan, including the red patch); and a ''polyphyletic'' group: ''the night-active primates, i.e., ...

monophyletic
group, it has been suggested that several groups of reptiles that had anapsid skulls might be only distantly related. Scientists still debate the exact relationship between the basal (original) reptiles that first appeared in the late
Carboniferous The Carboniferous ( ) is a geologic period A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These periods form elements of a hierarchy of divisi ...
, the various
Permian The Permian ( ) is a and which spans 47 million years from the end of the Period million years ago (Mya), to the beginning of the Period 251.902 Mya. It is the last period of the Era; the following Triassic Period belongs to the Era. The c ...
reptiles that had anapsid skulls, and the
Testudines Turtles are reptile Reptiles are tetrapod Tetrapods (; from Greek 'four' and 'foot') are four-limbed animals constituting the superclass Tetrapoda . It includes extant and extinct amphibians, reptiles (including dinosaurs and ther ...
(
turtle Turtles are an of s known as Testudines, characterized by a developed mainly from their ribs. Modern turtles are divided into two major groups, the s and which differ in the way the head retracts. There are 360 living and recently extinct ...

turtle
s,
tortoise Tortoises () are reptiles of the family Testudinidae of the order Testudines (Latin: tortoise). They are particularly distinguished from other Turtle, turtles (which includes the order Chelonia) by being exclusively land-dwelling, while many (t ...

tortoise
s, and
terrapin Terrapins are one of several small species of turtle Turtles are reptiles of the Order (biology), order Chelonia or Testudines . They are characterized by a special bone, bony or cartilage, cartilaginous animal shell, shell developed fro ...
s). However, it was later suggested that the anapsid-like turtle skull is due to reversion rather than to anapsid descent. The majority of modern paleontologists believe that the Testudines are descended from
diapsid Diapsids ("two arches") are a group of amniote tetrapods that developed two holes (temporal fenestra) in each side of their skulls about 300 million years ago during the late Carboniferous period. The diapsids are extremely diverse, and include all ...

diapsid
reptiles that lost their temporal fenestrae. More recent morphological
phylogenetic In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, ...

phylogenetic
studies with this in mind placed turtles firmly within
diapsid Diapsids ("two arches") are a group of amniote tetrapods that developed two holes (temporal fenestra) in each side of their skulls about 300 million years ago during the late Carboniferous period. The diapsids are extremely diverse, and include all ...

diapsid
s,. or, more commonly, within Archelosauria.


Phylogenetic position of turtles

All
molecular A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral group of two or more atom An atom is the smallest unit of ordinary matter In ...
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 of the most recent molecular studies, published on 23 February 2012, suggests that turtles are lepidosauromorph diapsids, most closely related to the lepidosaurs (
lizard Lizards (suborder Lacertilia) are a widespread group of Squamata, squamate reptiles, with over 6,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The group is paraphyletic as it excludes the s ...

lizard
s,
snake Snakes are elongated, limbless, carnivore, carnivorous reptiles of the suborder Serpentes . Like all other Squamata, squamates, snakes are ectothermic, amniote vertebrates covered in overlapping Scale (zoology), scales. Many species of snakes ...

snake
s, and
tuatara Tuatara (''Sphenodon punctatus'') are reptiles Endemism, endemic to New Zealand. Although resembling most lizards, they are part of a distinct lineage, the order Rhynchocephalia. Their name derives from the Māori language, and means "peaks on ...

tuatara
s). 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 A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics Cladistics (, from Greek language, Greek , ''kládos'', "branch") is an approach to Taxonomy (biology), biological classification in whi ...

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
procolophonid Procolophonidae is an extinct family (biology), family of parareptiles from the Permian and Triassic periods. They were shaped like stocky lizards, with broad-cheeked skulls. Their cheeks sported a stout backward-pointing spike. ''Hypsognathus'', ...
s managed to survive into the
Triassic The Triassic ( ) is a geologic period The geologic time scale (GTS) is a system of chronological dating that classifies Geology, geological strata (stratigraphy) in time. It is used by geologists, paleontology, paleontologists, and other earth ...

Triassic
, most of the other reptiles with anapsid skulls, including the millerettids,
nycteroleterid Nycteroleteridae is a family (biology), family of procolophonian parareptilians (extinct early reptiles) from the Middle Permian, Middle to Late Permian of Russia and North America. They are sometimes classified as a sister group to pareiasaurids ...
s, and
pareiasaur Pareiasaurs (meaning "cheek lizards") were a clade of parareptiles comprising the family In human society, family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or ...
s, became extinct in the
Late Permian Late may refer to: * LATE, an acronym which could stand for: ** Limbic-predominant age-related TDP-43 encephalopathy, a proposed form of dementia ** Local-authority trading enterprise, a New Zealand business law ** Local average treatment effect, ...
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 this is not favoured by current workers. Anapsids in the traditional meaning of the word are not a clade, but rather a
paraphyletic In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and all descendants of that ancestor excluding a few—typically only one or two—Monophyly, monophyletic subgroups. The group is said to be paraphyleti ...
group composed of all the early reptiles retaining the primitive skull morphology, 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 Captorhinidae (also known as cotylosaurs) is one of the earliest and most basal reptile families, all members of which are extinct. Description '' Captorhinids are a clade of small to very large lizard-like reptiles that date from the late Carb ...
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
synapsid Synapsids are a group of animals that includes mammals and every animal more closely related to mammals than to the other members of the amniote clade, such as reptiles and birds. Unlike other amniotes, they have a Skull#Fenestrae, temporal fenes ...

synapsid
s, has been discovered in the skulls of a number of members of
Parareptilia Parareptilia ("at the side of reptiles") is a subclass or clade A clade (; from grc, , ''klados'', "branch"), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic—that is, composed o ...
(the clade containing most of reptiles traditionally referred to as anapsids), including lanthanosuchoids, millerettids,
bolosaurids Bolosauridae is an extinct family (biology), family of ankyramorph parareptiles known from the latest Carboniferous (Gzhelian) or earliest Permian (Asselian) to the early Guadalupian Epoch (geology), epoch (latest Roadian stage) of North America, ...
, some nycteroleterids, some procolophonoids and at least some
mesosaur Mesosaurs ("middle lizards") were a group of small aquatic reptiles that lived during the early Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous period milli ...
s. 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.


See also

* Euryapsida


References


External links


Introduction to Anapsida
from UCMP {{Taxonbar, from=Q331974 Reptile taxonomy Paraphyletic groups