Editing Turtle (section)

==Systematics and evolution==
{{further |Turtle classification |List of Testudines families}}

=== Fossil history===
[[File:Origin of Turtle Body Plan.jpg|thumb|upright=1.2|Diagram of the origins of the turtle body plan through the [[Triassic]]: isolated bony plates evolved to [[Turtle shell#Origins|form a complete shell]], in a sequence involving ''[[Pappochelys]]'', ''[[Eorhynchochelys]]'', ''[[Odontochelys]]'', and ''[[Proganochelys]]''.<ref name="Schoch Sues 2019"/>|alt=Diagram of evolution of turtle shells showing four fossil species]]

Zoologists have sought to explain the evolutionary origin of the turtles, and in particular of their unique shells. In 1914, [[Jan Versluys]] proposed that bony plates in the dermis, called [[osteoderm]]s, fused to the ribs beneath them, later called the "Polka Dot Ancestor" by Olivier Rieppel.<ref name="Schoch Sues 2019"/><ref>{{Cite book |title=Turtles as Hopeful Monsters: Origins and Evolution<!--what a title, but Rieppel is ok actually--> |last=Rieppel |first=Olivier |isbn=978-0-253-02507-4 |publisher=Indiana University Press |page=195 |oclc=962141060 |year=2017}}</ref> The theory accounted for the evolution of fossil [[pareiasaur]]s from ''[[Bradysaurus]]'' to ''[[Anthodon (reptile)|Anthodon]]'', but not for how the ribs could have become attached to the bony dermal plates.<ref name="Schoch Sues 2019">{{cite journal |last1=Schoch |first1=Rainer R. |last2=Sues |first2=Hans-Dieter |last3=Benson |first3=Roger |title=The Origin of the Turtle Body Plan: Evidence from Fossils and Embryos |journal=Palaeontology |volume=63 |issue=3 |year=2019 |pages=375–393 |issn=0031-0239 |doi=10.1111/pala.12460 |doi-access=free }}</ref>

More recent discoveries have painted a different scenario for the evolution of the turtle's shell. The [[Crown group#Stem groups|stem]]-turtles ''[[Eunotosaurus]]'' of the Middle [[Permian]], ''[[Pappochelys]]'' of the [[Middle Triassic]], and ''[[Eorhynchochelys]]'' of the [[Late Triassic]] lacked carapaces and plastrons but had shortened torsos, expanded ribs, and lengthened dorsal vertebrae. Also in the Late Triassic, ''[[Odontochelys]]'' had a partial shell consisting of a complete bony plastron and an incomplete carapace. The development of a shell reached completion with the Late Triassic ''[[Proganochelys]]'', with its fully developed carapace and plastron.<ref name="Schoch Sues 2019"/><ref name="Lyson2020"/> Adaptations that led to the evolution of the shell may have originally been for digging and a [[fossorial]] lifestyle.<ref name="Lyson2020">{{cite journal |last1=Lyson |first1=Tyler R. |last2=Bever |first2=Gabriel S. |year=2020 |title=Origin and Evolution of the Turtle Body Plan |journal=Annual Review of Ecology, Evolution, and Systematics |volume=51 |issue=1 |pages=143–166 |doi=10.1146/annurev-ecolsys-110218-024746 |s2cid=225486775|doi-access=free }}</ref>

The oldest known members of the Pleurodira lineage are the [[Platychelyidae]], from the [[Late Jurassic]].<ref name=":02">{{Cite journal|last1=Cadena|first1=Edwin|last2=Joyce|first2=Walter G.|date=April 2015|title=A Review of the Fossil Record of Turtles of the Clades Platychelyidae and Dortokidae|url=http://www.bioone.org/doi/10.3374/014.056.0101|journal=Bulletin of the Peabody Museum of Natural History|language=en|volume=56|issue=1|pages=3–20|doi=10.3374/014.056.0101|s2cid=56195415|issn=0079-032X|url-access=subscription}}</ref> The oldest known unambiguous cryptodire is ''[[Sinaspideretes]],'' a close relative of softshell turtles, from the Late Jurassic of China.<ref>{{Cite journal|last1=Evers|first1=Serjoscha W.|last2=Benson|first2=Roger B. J.|date=January 2019|editor-last=Smith|editor-first=Andrew|title=A new phylogenetic hypothesis of turtles with implications for the timing and number of evolutionary transitions to marine lifestyles in the group|url=https://onlinelibrary.wiley.com/doi/10.1111/pala.12384|journal=Palaeontology|language=en|volume=62|issue=1|pages=93–134|doi=10.1111/pala.12384|bibcode=2019Palgy..62...93E |s2cid=134736808}}</ref> Turtles became highly diverse during the Cretaceous, as climatic conditions in this period were favourable for their global dispersal.<ref>{{Cite journal |last1=Nicholson |first1=David B. |last2=Holroyd |first2=Patricia A. |last3=Benson |first3=Roger B. J. |last4=Barrett |first4=Paul M. |date=3 August 2015 |title=Climate-mediated diversification of turtles in the Cretaceous |journal=[[Nature Communications]] |language=en |volume=6 |issue=1 |pages=7848 |doi=10.1038/ncomms8848 |pmid=26234913 |pmc=4532850 |issn=2041-1723 }}</ref> During the [[Late Cretaceous]] and [[Cenozoic]], members of the pleurodire families [[Bothremydidae]] and [[Podocnemididae]] became widely distributed in the Northern Hemisphere due to their coastal habits.<ref>{{Cite journal |last=Pérez-García |first=Adán |date=September 2, 2017 |title=A new Turtle Taxon (Podocnemidoidea, Bothremydidae) Reveals the Oldest Known Dispersal Event of the Crown Pleurodira from Gondwana to Laurasia |url=https://www.tandfonline.com/doi/full/10.1080/14772019.2016.1228549 |journal=Journal of Systematic Palaeontology |volume=15 |issue=9 |pages=709–731 |doi=10.1080/14772019.2016.1228549 |bibcode=2017JSPal..15..709P |s2cid=88840423 |issn=1477-2019|url-access=subscription }}</ref><ref>{{Cite journal |last1=Ferreira |first1=Gabriel S. |last2=Bandyopadhyay |first2=Saswati |last3=Joyce |first3=Walter G. |date=November 15, 2018 |title=A Taxonomic Reassessment of Piramys auffenbergi, a Neglected Turtle from the late Miocene of Piram Island, Gujarat, India |journal=PeerJ |language=en |volume=6 |pages=e5938 |doi=10.7717/peerj.5938 |pmid=30479901 |pmc=6240434|issn=2167-8359 |doi-access=free }}</ref> The oldest known soft-shelled turtles and sea turtles appeared during the [[Early Cretaceous]].<ref>{{cite book |last1=Hirayama |first1=Ren |title=Kappachelys okurai gen. et sp. nov., a New Stem Soft-Shelled Turtle from the Early Cretaceous of Japan |date=2013 |url=http://link.springer.com/10.1007/978-94-007-4309-0_12 |work=Morphology and Evolution of Turtles |pages=179–185 |editor-last=Brinkman |editor-first=Donald B. |place=Dordrecht |publisher=Springer Netherlands |doi=10.1007/978-94-007-4309-0_12 |isbn=978-94-007-4308-3 |last2=Isaji |first2=Shinji |last3=Hibino |first3=Tsuyoshi |series=Vertebrate Paleobiology and Paleoanthropology |editor2-last=Holroyd |editor2-first=Patricia A. |editor3-last=Gardner |editor3-first=James D.}}</ref><ref>{{Cite journal |last1=Evers |first1=Serjoscha W. |last2=Barrett |first2=Paul M. |last3=Benson |first3=Roger B. J. |date=May 2019 |title=Anatomy of Rhinochelys pulchriceps (Protostegidae) and Marine Adaptation During the Early Evolution of chelonioids |journal=PeerJ |volume=7 |pages=e6811 |doi=10.7717/peerj.6811 |pmid=31106054 |pmc=6500378 |issn=2167-8359 |doi-access=free }}</ref> Tortoises originated in Asia during the [[Eocene]].<ref>{{Cite journal |last1=Hofmeyr |first1=Margaretha D. |last2=Vamberger |first2=Melita |last3=Branch |first3=William |last4=Schleicher |first4=Alfred |last5=Daniels |first5=Savel R. |date=July 2017 |title=Tortoise (Reptilia, Testudinidae) Radiations in Southern Africa from the Eocene to the Present |url=http://doi.wiley.com/10.1111/zsc.12223|journal=Zoologica Scripta |volume=46 |issue=4 |pages=389–400 |doi=10.1111/zsc.12223 |s2cid=88712318|url-access=subscription }}</ref> A late surviving group of stem-turtles, the [[Meiolaniidae]], survived in Australasia into the [[Pleistocene]] and [[Holocene]].<ref>{{Cite journal |last1=Poropat |first1=Stephen F. |last2=Kool |first2=Lesley |last3=Vickers-Rich |first3=Patricia |last4=Rich |first4=Thomas H. |date=April 3, 2017 |title=Oldest Meiolaniid Turtle Remains from Australia: Evidence from the Eocene Kerosene Creek Member of the Rundle Formation, Queensland |url=https://www.tandfonline.com/doi/full/10.1080/03115518.2016.1224441 |journal=Alcheringa: An Australasian Journal of Palaeontology |volume=41 |issue=2 |pages=231–239 |doi=10.1080/03115518.2016.1224441 |bibcode=2017Alch...41..231P |s2cid=131795055 |issn=0311-5518|url-access=subscription }}</ref>

=== External relationships===
<!--[[File:Haeckel Chelonia.jpg|thumb|upright|"Chelonia" from [[Ernst Haeckel]]'s ''[[Kunstformen der Natur]]'', 1904 |alt=Historic painting of examples of different families of turtles ]]-->

The turtles' exact ancestry has been disputed. It was believed they were the only surviving branch of the ancient [[evolutionary grade]] [[Anapsid]]a, which includes groups such as [[Procolophonidae|procolophonid]]s and pareiasaurs. All anapsid skulls lack a [[Temporal fenestra|temporal opening]] while all other living amniotes have temporal openings.<ref name="Rieppel">{{cite journal |last1=Rieppel |first1=Olivier |year=1996 |last2=DeBraga |first2=M. |title=Turtles as Diapsid Reptiles |journal=Nature |volume=384 |issue=6608 |pages=453–455 |doi=10.1038/384453a0 |bibcode=1996Natur.384..453R |s2cid=4264378 |url=http://doc.rero.ch/record/16242/files/PAL_E3477.pdf }}</ref> It was later suggested that the anapsid-like turtle skulls may be due to [[backward evolution]] rather than to anapsid descent.<ref name="Zardoya">{{Cite journal |last1=Zardoya |first1=Rafael |last2=Meyer |first2=Axel |title=Complete Mitochondrial Genome Suggests Diapsid Affinities of Turtles |year=1998 |journal=PNAS |volume=95 |issue=24 |pages=14226–14231 |issn=0027-8424 |doi=10.1073/pnas.95.24.14226 |pmid=9826682 |pmc=24355 |bibcode=1998PNAS...9514226Z |url=https://kops.uni-konstanz.de/bitstream/123456789/6719/1/Complete_mitochondrial_genome_1998.pdf |access-date=October 31, 2018 |archive-date=July 24, 2021 |archive-url=https://web.archive.org/web/20210724105253/https://kops.uni-konstanz.de/bitstream/handle/123456789/6719/Complete_mitochondrial_genome_1998.pdf;jsessionid=CDA4382393A7DA330520DCB828E8B86D?sequence=1 |url-status=live|doi-access=free }}</ref> Fossil evidence has shown that early stem-turtles possessed small temporal openings.<ref name="Lyson2020"/>

Some early morphological [[phylogenetics|phylogenetic]] studies have placed turtles closer to [[Lepidosauria]] ([[tuatara]]s, [[lizard]]s, and [[snake]]s) than to [[Archosaur]]ia ([[crocodilian]]s and birds).<ref name="Rieppel"/> By contrast, several [[Molecular phylogenetics|molecular]] studies place turtles either 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 |date=October 1999 |journal=Molecular Phylogenetics and Evolution |volume=13 |issue=1 |pages=144–148 |doi=10.1006/mpev.1999.0640 |pmid=10508547 }}</ref> or, more commonly, as a [[sister group]] to extant archosaurs,<ref name="Zardoya"/><ref name="Iwabe">{{Cite journal |last1=Iwabe |first1=Naoyuki |last2=Hara |first2=Yuichiro |last3=Kumazawa |first3=Yoshinori |last4=Shibamoto |first4=Kaori |last5=Saito |first5=Yumi |last6=Miyata |first6=Takashi |last7=Katoh |first7=Kazutaka |title=Sister Group Relationship of Turtles to the Bird-Crocodilian Clade Revealed by Nuclear DNA-coded Proteins |date=December 2004 |journal=Molecular Biology and Evolution |volume=22 |issue=4 |pages=810–813 |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 }}</ref><ref name="Katsu">{{Cite journal |last1=Katsu |first1=Yoshinao |date=March 2010 |last2=Braun |first2=Edward L. |last3=Guillette |first3=Louis J. Jr. |last4=Iguchi |first4=Taisen |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 |doi=10.1159/000297715 |pmid=20234127 |s2cid=12116018}}</ref> though an analysis conducted by Tyler Lyson and colleagues (2012) recovered turtles as the sister group of lepidosaurs instead.<ref>{{Cite journal |last1=Lyson |first1=Tyler R. |last2=Sperling |first2=Erik A. |last3=Heimberg |first3=Alysha M. |last4=Gauthier |first4=Jacques A. |last5=King |first5=Benjamin L. |last6=Peterson |first6=Kevin J. |title=MicroRNAs Support a Turtle + Lizard Clade |year=2012 |journal=[[Biology Letters]] |volume=8 |issue=1 |pages=104–107 |doi=10.1098/rsbl.2011.0477 |pmid=21775315 |pmc=3259949}}</ref> Ylenia Chiari and colleagues (2012) analyzed 248 [[nuclear gene]]s from 16 vertebrates and suggested that turtles share a [[most recent common ancestor|more recent common ancestor]] with birds and crocodilians. The date of separation of turtles and birds and crocodilians was estimated to be {{Ma|255}} during the Permian.<ref name="Chiari2012">{{Cite journal |last1=Chiari |first1=Ylenia |year=2012 |last2=Cahais |first2=Vincent |last3=Galtier |first3=Nicolas |last4=Delsuc |first4= Frédéric |title=Phylogenomic Analyses Support the Position of Turtles as the Sister Group of Birds and Crocodiles (Archosauria) |journal=BMC Biology |volume=10 |issue=65 |page=65 |doi=10.1186/1741-7007-10-65 |pmid=22839781 |pmc=3473239 |doi-access=free }}</ref> Through [[genomic]]-scale phylogenetic study of [[ultra-conserved element]]s (UCEs) to clarify the placement of turtles within reptiles, Nicholas Crawford and colleagues (2012) similarly found that turtles are closer to birds and crocodilians.<ref name="Crawford 2012">{{Cite journal |year=2012 |last1=Crawford |first1=Nicholas G. |last2=Faircloth |first2=Brant C. |last3=McCormack |first3=John E. |last4=Brumfield |first4=Robb T. |last5=Winker |first5=Kevin |last6=Glen |first6=Travis C. |title=More than 1000 Ultraconserved Elements Provide Evidence that Turtles are the Sister Group to Archosaurs |journal=Biology Letters |volume=8 |issue=5 |pages=783–786 |url=http://faculty.oxy.edu/mccormack/McCormack/picks/Crawford_2012_BioLett.pdf |access-date=September 21, 2014 |doi=10.1098/rsbl.2012.0331 |pmid=22593086 |pmc=3440978 |archive-date=August 10, 2012 |archive-url=https://web.archive.org/web/20120810100341/http://faculty.oxy.edu/mccormack/McCormack/picks/Crawford_2012_BioLett.pdf |url-status=live }}</ref>

Using the draft (unfinished) genome sequences of the green sea turtle and the [[Chinese softshell turtle]], Zhuo Wang and colleagues (2013) concluded that turtles are likely a sister group of crocodilians and birds.<ref name="Wang 2013">{{cite journal |doi=10.1038/ng.2615 |pmid=23624526 |pmc=4000948 |title=The Draft Genomes of Soft-shell Turtle and Green Sea Turtle Yield Insights into the Development and Evolution of the Turtle-specific Body Plan |journal=Nature Genetics |volume=45 |issue=6 |pages=701–706 |year=2013 |last1=Wang |first1=Zhuo |last2=Pascual-Anaya |first2=Juan |last3=Zadissa |first3=Amonida |last4=Li |first4=Wenqi |last5=Niimura |first5=Yoshihito |last6=Huang |first6=Zhiyong |last7=Li |first7=Chunyi |last8=White |first8=Simon |last9=Xiong |first9=Zhiqiang |last10=Fang |first10=Dongming |last11=Wang |first11=Bo |last12=Ming |first12=Yao |last13=Chen |first13=Yan |last14=Zheng |first14=Yuan |last15=Kuraku |first15=Shigehiro |last16=Pignatelli |first16=Miguel |last17=Herrero |first17=Javier |last18=Beal |first18=Kathryn |last19=Nozawa |first19=Masafumi |last20=Li |first20=Qiye |last21=Wang |first21=Juan |last22=Zhang |first22=Hongyan |last23=Yu |first23=Lili |last24=Shigenobu |first24=Shuji |last25=Wang |first25=Junyi |last26=Liu |first26=Jiannan |last27=Flicek |first27=Paul |last28=Searle |first28=Steve |last29=Wang |first29=Jun |last30=Kuratani |first30=Shigeru |display-authors=3}}</ref> The external phylogeny of the turtles is shown in the [[cladogram]] below.<ref name="Crawford 2012"/>

{{clade
|label1=[[Diapsida]]
|1={{clade
   |label1=[[Lepidosauromorpha]]
   |1=[[Squamata]] (lizards, snakes) [[File:Zoology of Egypt (1898) (Varanus exanthematicus).png|70px]]
   |label2=[[Archosauromorpha]]
   |2={{clade
      |1='''Testudines''' [[File:Psammobates geometricus 1872 white background.jpg|60px]]
      |2={{clade
         |1=[[Crocodilia]] (crocodiles, alligators) <span style="{{MirrorH}}">[[File:Deinosuchus riograndensis.png|100px]]</span>
         |2=[[Aves]] (birds) [[File:Spot-billed pelican takeoff white background.jpg|80px]]
         }}
      }}
   }}
}}

===Internal relationships===
Modern turtles and their extinct relatives with a complete shell are classified within the clade [[Testudinata]].<ref>{{Cite journal |last1=Joyce |first1=Walter G. |last2=Anquetin |first2=Jérémy |last3=Cadena |first3=Edwin-Alberto |last4=Claude |first4=Julien |last5=Danilov |first5=Igor G. |last6=Evers |first6=Serjoscha W. |last7=Ferreira |first7=Gabriel S. |last8=Gentry |first8=Andrew D. |last9=Georgalis |first9=Georgios L. |last10=Lyson |first10=Tyler R. |last11=Pérez-García |first11=Adán |date=December 2021 |title=A Nomenclature for Fossil and Living Turtles Using Phylogenetically Defined Clade Names |journal=[[Swiss Journal of Palaeontology]] |language=en |volume=140 |issue=1 |pages=5 |doi=10.1186/s13358-020-00211-x |s2cid=229506832 |issn=1664-2376 |doi-access=free|bibcode=2021SwJP..140....5J |hdl=11336/155192 |hdl-access=free }}</ref> The most recent common ancestor of living turtles, corresponding to the split between Pleurodira (side-necked species) and [[Cryptodira]] (hidden necked species), is estimated to have occurred around {{Ma|210}} during the Late Triassic.<ref>{{cite journal |last1=Böhmer |first1=Christine |last2=Werneburg |first2=Ingmar |year=2017 |title=Deep Time Perspective on Turtle Neck Evolution: Chasing the ''Hox'' Code by Vertebral Morphology |journal=Scientific Reports |volume=7 |issue=1 |page=8939 |doi=10.1038/s41598-017-09133-0|pmid=28827543 |pmc=5566328 |bibcode=2017NatSR...7.8939B }}</ref> Robert Thompson and colleagues (2021) comment that living turtles have low diversity, relative to how long they existed. Diversity has been stable, according to their analysis, except for a single rapid increase around the [[Eocene-Oligocene boundary]] some 30 million years ago, and a large regional extinction at roughly the same time. They suggest that global climate change caused both events, as the cooling and drying caused the land to become arid and turtles to become extinct there, while new continental margins opened up by the climate change provided habitats for other species to evolve.<ref name="Thomson Spinks Shaffer 2021">{{cite journal |last1=Thomson |first1=Robert C. |last2=Spinks |first2=Phillip Q. |last3=Shaffer |first3=H. Bradley |title=A Global Phylogeny of Turtles Reveals a Burst of Climate-associated Diversification on Continental Margins |journal=Proceedings of the National Academy of Sciences |volume=118 |issue=7 |date=February 8, 2021 |issn=0027-8424 |doi=10.1073/pnas.2012215118 |page=e2012215118 |pmid=33558231 |pmc=7896334 |bibcode=2021PNAS..11812215T |doi-access=free }}</ref>

The cladogram, from Nicholas<!--2nd use--> Crawford and colleagues 2015, shows the internal phylogeny of the Testudines down to the level of [[Family (biology)|families]].<ref name="Crawford Parham 2015">{{cite journal |last1=Crawford |first1=Nicholas G. |last2=Parham |first2=James F. |last3=Sellas |first3=Anna B. |last4=Faircloth |first4=Brant C. |last5=Glenn |first5=Travis C. |last6=Papenfuss |first6=Theodore J. |last7=Henderson |first7=James B. |last8=Hansen |first8=Madison H. |last9=Simison |first9=W. Brian |title=A Phylogenomic Analysis of Turtles |journal=Molecular Phylogenetics and Evolution |volume=83 |year=2015 |pages=250–257 |issn=1055-7903 |doi=10.1016/j.ympev.2014.10.021|pmid=25450099 }}</ref><ref name="Knauss Joyce Lyson Pearson pp. 125–142"><!--for Chelydroidea-->{{cite journal |last1=Knauss |first1=Georgia E. |last2=Joyce |first2=Walter G. |last3=Lyson |first3=Tyler R. |last4=Pearson |first4=Dean |title=A New Kinosternoid from the Late Cretaceous Hell Creek Formation of North Dakota and Montana and the Origin of the ''Dermatemys mawii'' Lineage |journal=Paläontologische Zeitschrift |publisher=Springer Science and Business Media LLC |volume=85 |issue=2 |date=September 21, 2010 |issn=0031-0220 |doi=10.1007/s12542-010-0081-x |pages=125–142|s2cid=129123961 }}</ref> The analysis by <!--Robert--> Thompson and colleagues in 2021 supports the same structure down to the family level.<ref name="Thomson Spinks Shaffer 2021"/>

{{clade
|label1='''Testudines'''
|1={{clade
   |label1=[[Pleurodira]]
   |sublabel1='' (Side-necked turtles) ''
   |1={{clade
      |1={{clade <!--extra dummy clade-->
         |label1=[[Chelidae]]
         |1=[[File:Erpétologie générale, ou, Histoire naturelle complète des reptiles (Chelus fimbriata).jpg|70px]]
         }}
      |2={{clade
         |label1=[[Pelomedusidae]]
         |1=[[File:Erpétologie générale, ou, Histoire naturelle complète des reptiles (Pelomedusa subrufa).jpg|70px]]
         |label2=[[Podocnemididae]]
         |2=[[File:Erpétologie générale, ou, Histoire naturelle complète des reptiles (Podocnemis expansa).jpg|70px]]
         }}
      }}
   |label2=[[Cryptodira]]
   |sublabel2='' (Hidden-necked turtles) ''
   |2={{clade
      |1={{clade
         |label1=[[Trionychia]]
         |sublabel1='' (Softshell turtles) ''
         |1={{clade
            |label1=[[Carettochelyidae]] 
            |sublabel1='' (Pig-nosed turtle) ''
            |1=[[File:Pig-nosed turtle (Carettochelys insculpta) (cropped).jpg|65px]]
            |label2=[[Trionychidae]]
            |2=[[File:Erpétologie générale, ou, Histoire naturelle complète des reptiles (Lissemys punctata).jpg|65px]]
            }}
         }}
      |2={{clade <!--extra dummy clade-->
         |label1=<!--Durocryptodira--Crawford 2015-->
         |sublabel1='' (Hardshell turtles) ''
         |1={{clade
            |1={{clade
               |label1=[[Testudinoidea]]
               |1={{clade
                  |1={{clade
                     |label1=[[Emydidae]]
                     |sublabel1='' (Terrapins) ''
                     |1=[[File:Emydoidea blandingiiHolbrookV1P03A flipped.jpg|70px]]
                     |label2=[[Platysternidae]] 
                     |2=[[File:Erpétologie générale, ou, Histoire naturelle complète des reptiles (Platysternon megacephalum).jpg|70px]]
                     }}
                  |2={{clade
                     <!--|sublabel1=''Pond, river, & wood turtles''-->
                     |label1=[[Geoemydidae]] 
                     |1=[[File:Erpétologie générale, ou, Histoire naturelle complète des reptiles (Morenia ocellata).jpg|70px]]
                     |label2=[[Testudinidae]]
                     |sublabel2='' (Tortoises) ''
                     |2=[[File:Erpétologie générale, ou, Histoire naturelle complète des reptiles (Centrochelys sulcata).jpg|70px]]
                     }}
                  }}
               }}
            |2={{clade
               |label1=[[Chelonioidea]]
               |sublabel1='' (Sea turtles) ''
               |1={{clade
                  |label1=[[Cheloniidae]]
                  |1=&nbsp;[[File:Erpétologie générale, ou, Histoire naturelle complète des reptiles (Chelonia mydas).jpg|90px]]
                  |label2=[[Dermochelyidae]]
                  |sublabel2='' (Leatherback) ''
                  |2=&nbsp;[[File:Erpétologie générale, ou, Histoire naturelle complète des reptiles (Dermochelys coriacea).jpg|90px]]
                  }}
               |label2=[[Chelydroidea]]<!--Crawford 2015-->
               |2={{clade
                  |1={{clade <!--extra dummy clade-->
                     |label1=[[Chelydridae]]
                     |1=[[File:Erpétologie générale, ou, Histoire naturelle complète des reptiles (Chelydra serpentina).jpg|70px]]
                     |sublabel1='' (Snapping turtles) ''
                     }}
                  |label2x=[[Kinosternoidea]] <!-- Reptile DB now uses Chelydroidea -->
                  |2={{clade
                     |label1=[[Dermatemydidae]]
                     |1=[[File:ChloremysAbnormisFord white background.jpg|70px]]
                     |label2=[[Kinosternidae]]
                     |2=[[File:Erpétologie générale, ou, Histoire naturelle complète des reptiles (Sternotherus odoratus).jpg|70px]]
                     }}
                  }}
               }}
            }}
         }}
      }}
   }}
}}

=== Differences between the two suborders ===

{{Multiple image
|header = Neck retraction
|total_width = 410
|perrow = 2
|image1 = T.h. hermanni con speroni 5.JPG
|caption1 = [[Cryptodira]] retract their necks backward.
|alt1 = Photograph of a cryptodiran with its head pulled back straight into its shell
|image2 = Pelomedusa subrufa (cropped).JPG
|caption2 = [[Pleurodira]] retract their necks sideways.
|alt2 = Photograph of a pleurodiran with its head and neck folded toward the side
|image3 = Turtle neck retraction.svg
|caption3 = The different mechanisms of neck retraction in the two suborders of turtles
|alt3=Diagrams of the top-down bending of the neck of cryptodirans, and the left-right bending of the neck in pleurodirans
}}

Turtles are divided into two living suborders: Cryptodira and Pleurodira.<ref name="Joyce 2021">{{Cite journal |last1=Joyce |first1=Walter G. |last2=Anquetin |first2=Jérémy |last3=Cadena |first3=Edwin-Alberto |last4=Claude |first4=Julien |last5=Danilov |first5=Igor G. |last6=Evers |first6=Serjoscha W. |last7=Ferreira |first7=Gabriel S. |last8=Gentry |first8=Andrew D. |last9=Georgalis |first9=Georgios L. |last10=Lyson |first10=Tyler R. |last11=Pérez-García |first11=Adán |date=February 9, 2021 |title=A Nomenclature for Fossil and Living Turtles Using Phylogenetically Defined Clade Names |journal=[[Swiss Journal of Palaeontology]] |volume=140 |issue=1 |page=5 |doi=10.1186/s13358-020-00211-x |issn=1664-2384 |display-authors=3 |doi-access=free |bibcode=2021SwJP..140....5J |hdl=11336/155192 |hdl-access=free }}</ref> The two groups differ in the way the neck is retracted for protection. Pleurodirans retract their neck to the side and in front of the shoulder girdles, whereas cryptodirans retract their neck backward into their shell. These motions are enabled by the morphology and arrangement of neck vertebrae.<ref name="Werneburg 2015">{{cite journal |last1=Werneburg |first1=Ingmar |last2=Wilson  |first2=Laura A. B. |last3=Parr |first3=William C. H. |last4=Joyce |first4=Walter G. |date=March 1, 2015 |title=Evolution of Neck Vertebral Shape and Neck Retraction at the Transition to Modern Turtles: an Integrated Geometric Morphometric Approach |journal=Systematic Biology |volume=64 |issue=2 |pages=187–204 |doi=10.1093/sysbio/syu072 |pmid=25305281 |issn=1063-5157 |doi-access=free|hdl=1959.4/unsworks_54973 |hdl-access=free }}</ref><ref name="Herrel 2008">{{cite book |last1=Herrel |first1=Anthony |last2=Van Damme |first2=Johan |last3=Aerts |first3=Peter |chapter=Cervical Anatomy and Function in Turtles |editor-last=Wyneken |editor-first=Jeanette |editor-last2=Bels |editor-first2=V. L. |editor-last3=Godfrey |editor-first3=Matthew H. |title=Biology of Turtles |year=2008 |publisher=CRC Press |isbn=978-0-8493-3339-2 |oclc=144570900 |pages=163–186}}</ref> Sea turtles (which belong to Cryptodira) have mostly lost the ability to retract their heads.{{sfn|Orenstein|2012|p=110}}

The adductor muscles in the lower jaw create a pulley-like system in both subgroups. However, the bones that the muscles articulate with differ. In Pleurodira, the pulley is formed with the [[pterygoid bone]]s of the [[palate]], but in Cryptodira the pulley is formed with the otic capsule. Both systems help to vertically redirect the [[Anatomical terms of motion#Abduction and adduction|adductor]] muscles and maintain a powerful bite.<ref name="Ferreira Lautenschlager Evers Pfaff 2020">{{cite journal |last1=Ferreira |first1=Gabriel S. |last2=Lautenschlager |first2=Stephan |last3=Evers |first3=Serjoscha W. |last4=Pfaff |first4=Cathrin |last5=Kriwet |first5=Jürgen |last6=Raselli |first6=Irena |last7=Werneburg |first7=Ingmar |title=Feeding Biomechanics Suggests Progressive Correlation of Skull Architecture and Neck Evolution in Turtles |journal=Scientific Reports |volume=10 |issue=1 |date=March 26, 2020 |issn=2045-2322 |doi=10.1038/s41598-020-62179-5 |pmid=32218478 |pmc=7099039 |bibcode=2020NatSR..10.5505F |display-authors=3 |at=article 5505 |doi-access=free }}</ref>

A further difference between the suborders is the attachment of the pelvis. In Cryptodira, the pelvis is free, linked to the shell only by ligaments. In Pleurodira, the pelvis is [[Suture (anatomy)|sutured]], joined with bony connections, to the carapace and to the plastron, creating a pair of large columns of bone at the back end of the turtle, linking the two parts of the shell.<ref name="Wise Stayton 2017">{{cite journal |last1=Wise |first1=Taylor B. |last2=Stayton |first2=C. Tristan |title=Side-necked Versus Hidden-necked: Comparison of Shell Morphology Between Pleurodiran and Cryptodiran Turtles |journal=Herpetologica |date=2017 |volume=73 |issue=1 |pages=18–29 |doi=10.1655/HERPETOLOGICA-D-15-00038 |jstor=26534349 |s2cid=90226667 }}</ref>