Cryptodira Temporal range: Late Jurassic–Present PreꞒ Ꞓ O S D C P T J K Pg N

Aldabra giant tortoise (Aldabrachelys gigantea)
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Order:	Testudines
Suborder:	Cryptodira Cope, 1868^[1]
Subgroups
See text
Synonyms^[1]^[2]
Cryptoderes Duméril and Bibron, 1834 Cryptodera Lichtenstein, 1856 Cryptodira Cope, 1868 Cryptodiramorpha Lee, 1995 Pancryptodira Joyce, Parham, and Gauthier, 2004

The Cryptodira (Greek: hidden neck) are a suborder of Testudines that includes most living tortoises and turtles. Cryptodira differ from Pleurodira (side-necked turtles) in that they lower their necks and pull the heads straight back into the shells, instead of folding their necks sideways along the body under the shells' marginals. They include among their species freshwater turtles, snapping turtles, tortoises, softshell turtles, and sea turtles.

Neck retraction

The Cryptodira are characterized by retraction of the head in the vertical plane, which permits for primarily vertical movements and restricted lateral movements outside of the shell.^[3] These motions are largely due to the morphology and arrangement of cervical vertebrae. In all recent turtles, the cervical column consists of nine joints and eight vertebrae.^[4] Compared to the narrow vertebrae and the closely positioned zygapophyses of the pleurodires, the cryptodires’ vertebrae take on the opposite shape. Their cervical vertebrae are more distended, and their zygapophyses (processes that interlock adjacent vertebrae) are much more widely spaced—features allowing for a condition called ginglymoidy, and ultimately, their “hidden” neck retraction. Ginglymoidy refers to the double articulation where articulation between the sixth and seventh vertebrae and the seventh and eighth vertebrae allows for bending of the neck into an S shape. Formation of this S shape occurs in one plane that enables retraction into the shell.^[5]

Cryptodiran neck retraction is also dependent on associated cervical musculature for its characteristic motions. A study that focused solely on the mechanism of neck retraction in Chelodina (pleurodire) versus that of Apalone (cryptodire), found an absence of the longissimus and iliocostalis systems and reduced epaxial musculature.^[4] Absence of longissimus musculature, which primarily functions in moving the neck via ipsilateral flexion and contralateral rotation, contributes to the backwards retraction of the neck into the shell. Lack of this muscular system also results in poorly developed transverse processes (the lateral processes of a vertebra), forcing them to be developed in a more cranial direction. The iliocostalis system, used for lateral flexion and extension of the vertebral column, is commonly absent in all turtles. With the presence of a shell, these muscular movements are no longer possible. Epaxial musculature that functions in alternated forms of stepping and walking is minimized in turtles, due to their restricted stride lengths and heavily weighted shells.^{[citation needed]}

Systematics and evolution

Cryptodires evolved from pleurodires during the early Jurassic period, originating from South America and Southeast Asia.^[6] By the end of the Jurassic, cryptodires had almost completely replaced pleurodires in the lakes and rivers, while beginning to develop land-based species. Meanwhile, pleurodires became the dominant freshwater testudines in the Cretaceous to Eocene of Europe,^[7] and produced a family of marine species, the Bothremydidae.

The Cryptodira suborder has four living superfamilies, the Chelonioidea (sea turtles), Testudinoidea (tortoises and pond turtles), Kinosternoidea (Central American river turtle and mud turtles) and Trionychoidea (soft-shell turtles and relatives). Chelydridae (snapping turtles) form a sister group to Kinosternoidea. The former three subfamilies (and Chelydridae) are classified in the clade Durocryptodira, while the latter is classified in the clade Trionychia. These two clades likely diverged in the middle of the Jurassic.^[6]^[8]

Two circumscriptions of the Cryptodira are commonly found. One is used here; it includes a number of primitive extinct lineages known only from fossils, as well as the Eucryptodira. These are, in turn, made up from some very basal groups, and the Centrocryptodira contain the prehistoric relatives of the living cryptodires, as well as the latter, which are collectively called Polycryptodira or Durocryptodira.

The alternate concept restricts the use of the term "Cryptodira" to the crown clade (i.e. Polycryptodira). The Cryptodira as understood here are called Cryptodiramorpha in this view. A recent study placed Plesiochelyidae as an Angolachelonia and outside Testudines, thus Cryptodira.^[9]

As per the system used here, the Cryptodira can be classified as:^[8]^[10]

*Manchurochelys liaoxiensis* was a member of Sinemydidae.

†Hangaiemys
†Bashuchelyidae
Pan-Cryptodira
- Family †Macrobaenidae? Sukhanov 1964
- Family †Xinjiangchelyidae? Nesov, 1990
- Family †Sinemydidae? Yeh, 1963
- Clade Pandurocryptodira
  - Clade Americhelydia
    - Clade Chelydroidea
      - Family Chelydridae (snapping turtles)
      - Superfamily Kinosternoidea
        Family Dermatemydidae (river turtles)
        
        Family Kinosternidae (mud turtles)
    - Clade Panchelonioidea
      - Clade †Angolachelonia?
        Clade †Thalassochelydia?
        Family †Eurysternidae?
        
        Family †Plesiochelyidae?
        
        Family †Thalassemydidae?
        
        †Owadowia?
        
        Family †Sandownidae? (may also belong in Thalassochelydia)
      - Family †Protostegidae? (may also belong in Thalassochelydia)
      - Family †Toxochelyidae
      - Family †Ctenochelyidae
      - Superfamily Chelonioidea (sea turtles)
        Family Cheloniidae (green sea turtles and relatives)
        
        Family Dermochelyidae (leatherback sea turtles)
  - Clade Pantestudinoidea
    - Family †Lindholmemydidae?
    - Superfamily Testudinoidea
      - Family †Haichemydidae
      - Family †Sinochelyidae
      - Clade Emysternia
        Family Platysternidae (big-headed turtle)
        
        Family Emydidae (pond, box and water turtles)
      - Clade Testuguria
        Family Geoemydidae (Asian river turtles, Asian leaf turtles, Asian box turtles and roofed turtles)
        
        Family Testudinidae (tortoises)
- Clade Pantrionychia
  - Clade †Adocusia
    - Family †Adocidae
    - Family †Nanhsiungchelyidae
  - Superfamily Trionychia
    - Family Carettochelyidae (pignose turtles)
    - Family Trionychidae (softshell turtles)

Distribution

Trionychidae (softshell turtles) are found from North America, Africa, South and East Asia to New Guinea.
Kinosternidae (mud and musk turtles) are found from eastern North America to the Amazon drainage of South America.
Dermatemydidae (Mesoamerican river turtles) are found in the Caribbean-Gulf drainage of Mesoamerica.
Emydidae (cooters, sliders, American box turtles, and Allies) are found from Europe to Ural Mountains and North America southward to Eastern Brazil.

References

Baur, George (June 1890). "On the Classification of the Testudinata". The American Naturalist. 24 (282): 530–536. doi:10.1086/275138. JSTOR 2450882.
Rhodin, Anders G.J.; van Dijk, Peter Paul; Inverson, John B.; Shaffer, H. Bradley; Roger, Bour (2011-12-31). "Turtles of the world, 2011 update: Annotated checklist of taxonomy, synonymy, distribution and conservation status" (PDF). Chelonian Research Monographs. 5. Archived from the original (PDF) on 2012-01-31.
Rhodin, Anders G.J.; van Dijk, Peter Paul; Parham, James F. (2008-12-08). "Turtles of the world, 2008 checklist" (PDF). Chelonian Research Monographs. 5. Archived from the original (PDF) on 2012-04-23. Retrieved 2012-05-22.

Wikispecies has information related to Cryptodira.

Testudines

Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Clade: Diapsida
Order: Testudines

Genus

Chelonioidea
(Sea turtles)

Cheloniidae	†Allopleuron Caretta †Carolinochelys Chelonia †Eochelone Eretmochelys †Gigantatypus †Glarichelys †Itilochelys Lepidochelys †Mexichelys †Miocaretta Natator †Pacifichelys †Syllomus †Tasbacka
Dermochelyidae	†Arabemys †Corsochelys †Cosmochelys Dermochelys †Eosphargis †Mesodermochelys †Psephophorus
	†Euclastes †Peritresius †Procolpochelys †Protosphargis †Puppigerus

Kinosternoidea

Dermatemydidae	Dermatemys
Kinosternidae	Claudius †Hoplochelys Kinosternon Staurotypus Sternotherus

Testudinoidea

Emydidae	†Acherontemys Chrysemys Clemmys Deirochelys Emys Actinemys Emydoidea Glyptemys Graptemys Malaclemys Pseudemys Terrapene Trachemys †Wilburemys
Geoemydidae	Batagur †Banhxeochelys Cuora Cyclemys Geoclemys Geoemyda Hardella Heosemys Leucocephalon Malayemys Mauremys Melanochelys Morenia Notochelys Orlitia Pangshura Rhinoclemmys Sacalia Siebenrockiella Vijayachelys
Platysternidae	Platysternon
Testudinidae	Aldabrachelys Astrochelys Centrochelys Chelonoidis Chersina Cylindraspis †Cymatholcus †Floridemys Geochelone Gopherus †Hadrianus †Hesperotestudo Homopus Indotestudo Kinixys Malacochersus Manouria †Megalochelys †Oligopherus Psammobates Pyxis †Solitudo Stigmochelys †Stylemys Testudo

Trionychia

Carettochelyidae	†Allaeochelys †Anosteira Carettochelys
Trionychidae	Amyda Apalone †Axestemys Chitra Cyclanorbis Cycloderma †Drazinderetes Dogania †Gilmoremys †Hutchemys †Khunnuchelys Lissemys Nilssonia Palea †Palaeoamyda Pelochelys Pelodiscus Rafetus Trionyx
	†Basilochelys †Sinaspideretes

Chelydridae	Chelydra †Chelydrops †Chelydropsis †Emarginachelys †Macrocephalochelys Macrochelys †Planiplastron †Protochelydra
†Nanhsiungchelyidae	†Anomalochelys †Basilemys †Jiangxichelys
†Protostegidae	†Alienochelys †Archelon †Atlantochelys †Bouliachelys †Calcarichelys †Cratochelone †Desmatochelys †Iserosaurus †Notochelone †Ocepechelon †Pneumatoarthrus †Protostega †Rhinochelys †Santanachelys †Terlinguachelys
	†Adocus †Argillochelys †Bashuchelys †Ctenochelys †Prionochelys †Toxochelys

Pleurodira

†Araripemydidae	†Araripemys
†Bothremydidae	†Araiochelys †Arenila †Azabbaremys †Bothremys †Cearachelys †Chedighaii †Chupacabrachelys †Eotaphrosphys †Foxemys †Galianemys †Ilatardia †Inaechelys †Itapecuruemys †Jainemys †Kinkonychelys †Kurmademys †Labrostochelys †Nigeremys †Phosphatochelys †Polysternon †Puentemys †Rosasia †Rhothonemys †Sankuchemys †Taphrosphys †Ummulisani †Zolhafah
Chelidae	Acanthochelys Chelodina Chelus Elseya Elusor Emydura Hydromedusa †Lomalatachelys Mesoclemmys Myuchelys Phrynops Platemys †Prochelidella Pseudemydura Rheodytes Rhinemys †Yaminuechelys
Pelomedusidae	Pelomedusa Pelusios
Podocnemididae	†Albertwoodemys †Bauruemys †Brontochelys †Caninemys †Carbonemys †Cerrejonemys †Cordichelys Erymnochelys †Lapparentemys †Latentemys Peltocephalus Podocnemis †Stupendemys
†Sahonachelyidae	†Sahonachelys †Sokatra

	†Caribemys †Caririemys †Tacuarembemys

Phylogenetic arrangement of turtles based on Turtles of the World 2017 Update: Annotated Checklist and Atlas of Taxonomy, Synonymy, Distribution, and Conservation Status. † = extinct.

See also List of Testudines families

Taxon identifiers
Cryptodira	Wikidata: Q499621 Wikispecies: Cryptodira BioLib: 16332 CoL: 87BW5 EoL: 46559463 Fauna Europaea: 15550 Fauna Europaea (new): a657826b-f777-4743-8b09-2a7d3deae97b iNaturalist: 474372 ITIS: 948943 NBN: NBNSYS0000180887 NCBI: 8464 Paleobiology Database: 37622 WoRMS: 148741

Neck retraction

Systematics and evolution

Distribution

References

Further reading