Editing Anapsid (section)

==Phylogenetic position of turtles==
All [[Molecular phylogenetics|molecular]] studies have strongly upheld the placement of turtles within diapsids; some place turtles within [[Archosauria]],<ref name = "Mannen">{{Cite journal|last1 = Mannen|first1 = Hideyuki|last2=Li |first2=Steven S.-L. |title = Molecular evidence for a clade of turtles|journal = [[Molecular Phylogenetics and Evolution]]|volume = 13|issue = 1|pages = 144–148| date = October 1999|doi = 10.1006/mpev.1999.0640 | pmid = 10508547| bibcode=1999MolPE..13..144M }}</ref> or, more commonly, as a sister group to extant archosaurs.<ref name = "Zardoya">{{cite journal| last1=Zardoya| first1=R.| last2=Meyer |first2=A.| year=1998|title=Complete mitochondrial genome suggests diapsid affinities of turtles| journal=[[Proceedings of the National Academy of Sciences]]| issn=0027-8424| volume=95| issue=24| pages=14226–14231| doi=10.1073/pnas.95.24.14226| pmid=9826682| pmc=24355| bibcode=1998PNAS...9514226Z| doi-access=free}}</ref><ref name = "Iwabe">{{Cite journal | last1 = Iwabe | first1 = N. |last2=Hara |first2=Y.|last3=Kumazawa |first3=Y.|last4=Shibamoto |first4=K.|last5=Saito |first5=Y.|last6=Miyata |first6=T. |last7=Katoh |first7=K. | title = Sister group relationship of turtles to the bird-crocodilian clade revealed by nuclear DNA-coded proteins | journal = [[Molecular Biology and Evolution]] | volume = 22 | issue = 4 | pages = 810–813 | date = 2004-12-29 | doi = 10.1093/molbev/msi075 | pmid = 15625185 | doi-access = free }}</ref><ref name = "Roos">{{Cite journal | last1 = Roos | first1 = Jonas |last2= Aggarwal |first2=Ramesh K. |last3=Janke |first3=Axel | title = Extended mitogenomic phylogenetic analyses yield new insight into crocodylian evolution and their survival of the Cretaceous–Tertiary boundary | journal = [[Molecular Phylogenetics and Evolution]] | volume = 45 | issue = 2 | pages = 663–673 | date = November 2007 | doi = 10.1016/j.ympev.2007.06.018 | pmid = 17719245| bibcode = 2007MolPE..45..663R }}</ref><ref name = "Katsu">{{Cite journal | last1 = Katsu | first1 = Y. |last2= Braun |first2=E. L. |last3= Guillette |first3=L. J. Jr. |last4= Iguchi |first4=T. |title = From reptilian phylogenomics to reptilian genomes: analyses of c-Jun and DJ-1 proto-oncogenes|journal = Cytogenetic and Genome Research|volume = 127|issue = 2–4|pages = 79–93|date = 2010-03-17|doi = 10.1159/000297715 | pmid = 20234127| s2cid = 12116018 }}</ref><ref name=Crawford12>{{Cite journal|last1 = Crawford|first1 = N. G.|last2 = Faircloth|first2 = B. C.|last3 = McCormack| first3 = J. E.|last4 = Brumfield|first4 = R. T.|last5 = Winker|first5 = K.|last6 = Glenn|first6 = T. C.|title = More than 1000 ultraconserved elements provide evidence that turtles are the sister group of archosaurs|doi = 10.1098/rsbl.2012.0331|journal = Biology Letters|year = 2012|pmid =  22593086| pmc = 3440978 | volume=8 | issue=5 | pages=783–786}}</ref> One molecular study, published in 2012, suggests that turtles are [[lepidosauromorph]] diapsids, most closely related to the [[lepidosaur]]s ([[lizard]]s, [[snake]]s, and [[tuatara]]s).<ref name ="turtlelizard">{{Cite journal |first1=Tyler R. |last1=Lyson |first2=Erik A. |last2=Sperling |first3=Alysha M. |last3=Heimberg |first4=Jacques A. |last4=Gauthier |first5=Benjamin L. |last5=King |first6=Kevin J. |last6=Peterson | title = MicroRNAs support a turtle + lizard clade | journal = Biology Letters | volume = 8 | issue = 1 | pages = 104–107 | doi = 10.1098/rsbl.2011.0477 | pmid=21775315 | pmc=3259949|date=2012-02-23 |bibcode=2012BiLet...8..104L }}</ref> However, in a later paper from the same authors, published in 2014, based on more extensive data, the archosauromorph hypothesis is supported.<ref name="ReferenceA"/>

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|cladogram]]. Testudines is suggested to have diverged from other diapsids between 200 and 279 million years ago, though the debate is far from settled.<ref name = "Rieppel">{{cite journal |vauthors=Rieppel O, DeBraga M |title=Turtles as diapsid reptiles |journal=Nature |volume=384 |issue= 6608|pages=453–455 |year=1996 |doi=10.1038/384453a0 |bibcode=1996Natur.384..453R |s2cid=4264378 |url=http://doc.rero.ch/record/16242/files/PAL_E3477.pdf }}</ref><ref name = "Zardoya"/><ref>{{cite book|last=Benton|first=M. J.|title=Vertebrate Paleontology|edition=2nd|publisher=Blackwell Science|location=London|year=2000|isbn=978-0-632-05614-9|title-link=Vertebrate Paleontology (Benton)}} 3rd edition (2004) {{ISBN|0-632-05637-1}}.</ref> Although [[Procolophonidae|procolophonid]]s managed to survive into the [[Triassic]], most of the other reptiles with anapsid skulls, including the [[Millerettidae|millerettid]]s, [[Nycteroleteridae|nycteroleterid]]s, and [[pareiasaur]]s, became extinct in the [[Lopingian|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 [[Metabolic waste|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|date=August 2020}}