Anapsid

Source: Wikipedia, the free encyclopedia.
For the extinct fish, see
Anaspid
.

Anapsids
Temporal range:
Ma
Range possibly includes turtles
Anapsid skull
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Informal group: Anapsida
Williston, 1917
Subgroups

An anapsid is an amniote whose skull lacks one or more skull openings (fenestra, or fossae) near the temples.[1] Traditionally, the Anapsida are 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

Testudines (turtles, tortoises, and terrapins). 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 reptiles that lost their temporal fenestrae. More recent morphological phylogenetic studies with this in mind placed turtles firmly within diapsids,[2][3][4][5][6] or, more commonly, within Archelosauria.[7]

Phylogenetic position of turtles

All

lepidosaurs (lizards, snakes, and tuataras).[14] However, in a later paper from the same authors, published in 2014, based on more extensive data, the archosauromorph hypothesis is supported.[7]

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

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.[citation needed]

Anapsida in modern taxonomy

Anapsida is still sporadically recognized as a valid group, but this is not favoured by current workers.[17][18] 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.[17][18] 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".[19]

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.[19]

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.

synapsids, 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, bolosaurids, some nycteroleterids, some procolophonoids and at least some mesosaurs.[18][20][21] 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.[21]

See also

References

  1. ISBN 0-13-041248-1
  2. doi:10.1111/j.1096-3642.1997.tb01280.x
    .
  3. doi:10.1002/mmng.200800011
    .
  4. S2CID 83693335
    .
  5. ^ Evans, Susan E. (2009). "An early kuehneosaurid reptile (Reptilia: Diapsida) from the Early Triassic of Poland" (PDF). Paleontologica Polonica. 65: 145–178..
  6. ^ Borsuk−Białynicka, Magdalena; Evans, Susan E. (2009). "A long−necked archosauromorph from the Early Triassic of Poland" (PDF). Paleontologica Polonica. 65: 203–234.
  7. ^
    PMID 24798503
    .
  8. PMID 10508547
    .
  9. ^
    PMID 9826682
    .
  10. PMID 15625185
    .
  11. PMID 17719245
    .
  12. S2CID 12116018
    .
  13. PMID 22593086
    .
  14. PMID 21775315
    .
  15. S2CID 4264378
    .
  16. ISBN 0-632-05637-1
    .
  17. ^
    PMID 15545258
    .
  18. ^
    doi:10.1002/mmng.200800011
    .
  19. ^
    ISBN 978-0198577058
    .
  20. PMID 15306328
    .
  21. ^
    doi:10.1016/j.crpv.2012.02.001
    .

External links

Wikispecies has information related to Anapsida.
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