Skull and cervical vertebrae of a cryptodiran turtle from the Emydidae family

The Cryptodira (') are a

suborder Order () is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy. It is classified between family and class. In biological classification, the order is a taxonomic rank used in the classification of organisms and recognized ...

Testudines Turtles are reptiles of the order (biology), order Testudines, characterized by a special turtle shell, shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Crypt ...

that includes most living

tortoise Tortoises ( ) are reptiles of the family Testudinidae of the order Testudines (Latin for "tortoise"). Like other turtles, tortoises have a shell to protect from predation and other threats. The shell in tortoises is generally hard, and like o ...

s and

turtle Turtles are reptiles of the order (biology), order Testudines, characterized by a special turtle shell, shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Crypt ...

s. Cryptodira is commonly called the "Hidden-Neck Turtles" or the "Inside-Neck Turtles". Cryptodira differ from

Pleurodira The Pleurodira are one of the two living suborders of turtles, the other being the Cryptodira. The division between these two suborders represents a very deep evolutionary divide between two very different types of turtles. The physical differ ...

(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 The Chelydridae is a Family (biology), family of turtles that has seven extinct and two extant genera. The extant genera are the snapping turtles, ''Chelydra'' and ''Macrochelys''. Both are Endemic (ecology), endemic to the Western Hemisphere. Th ...

tortoises Tortoises ( ) are reptiles of the family Testudinidae of the order Testudines (Latin for "tortoise"). Like other turtles, tortoises have a turtle shell, shell to protect from predation and other threats. The shell in tortoises is generally hard ...

, 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. These motions are largely due to the morphology and arrangement of

cervical vertebrae In tetrapods, cervical vertebrae (: vertebra) are the vertebrae of the neck, immediately below the skull. Truncal vertebrae (divided into thoracic and lumbar vertebrae in mammals) lie caudal (toward the tail) of cervical vertebrae. In saurop ...

. In all recent turtles, the cervical column consists of nine joints and eight vertebrae. 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. 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 ''Chelodina'', collectively known as snake-necked turtles, is a large and diverse genus of long-necked Chelidae, chelid turtles with a complicated Biological nomenclature, nomenclatural history. Although in the past, ''Macrochelodina'' and ''Macr ...

'' (pleurodire) versus that of '' Apalone'' (cryptodire), found an absence of the longissimus and

iliocostalis Iliocostalis muscle is the muscle immediately lateral to the longissimus that is the nearest to the furrow that separates the epaxial muscles from the hypaxial. It lies very deep to the fleshy portion of the serratus posterior muscle. It late ...

systems and reduced epaxial musculature. 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.

Systematics and evolution

Cryptodires

evolved Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...

from pleurodires during the early

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...

period, originating from

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a considerably smaller portion in the Northern Hemisphere. It can also be described as the southern Subregion#Americas, subregion o ...

and

Southeast Asia Southeast Asia is the geographical United Nations geoscheme for Asia#South-eastern Asia, southeastern region of Asia, consisting of the regions that are situated south of China, east of the Indian subcontinent, and northwest of the Mainland Au ...

. This split corresponds with the breakup of the supercontinent

Pangaea Pangaea or Pangea ( ) was a supercontinent that existed during the late Paleozoic and early Mesozoic eras. It assembled from the earlier continental units of Gondwana, Euramerica and Siberia during the Carboniferous period approximately 335 mi ...

. 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 The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

Eocene The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...

Europe Europe is a continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, the Mediterranean Sea to the south, and Asia to the east ...

, and produced a family of marine species, the Bothremydidae. The Cryptodira suborder has four living superfamilies, the

Chelonioidea Sea turtles (superfamily Chelonioidea), sometimes called marine turtles, are reptiles of the order Testudines and of the suborder Cryptodira. The seven existing species of sea turtles are the flatback, green, hawksbill, leatherback, loggerhe ...

(sea turtles), Testudinoidea (tortoises and pond turtles), Kinosternoidea (Central American river turtle and mud turtles) and Trionychoidea (soft-shell turtles and relatives).

Chelydridae The Chelydridae is a family of turtles that has seven extinct and two extant genera. The extant genera are the snapping turtles, ''Chelydra'' and '' Macrochelys''. Both are endemic to the Western Hemisphere. The extinct genera are '' Acherontemys ...

(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 Trionychia is a superfamily of turtles which encompasses the species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typic ...

. These two clades likely diverged in the middle of the Jurassic. Two circumscriptions of the Cryptodira are commonly found. One is used here; it includes a number of primitive

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

lineages known only from

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...

s, 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

, thus Cryptodira. As per the system used here, the Cryptodira can be classified as: * †'' Hangaiemys'' * † Bashuchelyidae *Pan-Cryptodira ** Family † Macrobaenidae? Sukhanov 1964 **Family † Xinjiangchelyidae? Nesov, 1990 ** Family † Sinemydidae? Yeh, 1963 ** Clade Pandurocryptodira *** Clade Americhelydia **** Clade Chelydroidea ***** Family

(snapping turtles) ***** Superfamily Kinosternoidea ****** Family Dermatemydidae (river turtles) ****** Family

Kinosternidae The Kinosternidae are a family of mostly small turtles that includes the mud turtles and musk turtles. The family contains 25 species within four genera, but taxonomic reclassification is an ongoing process, so many sources vary on the exact num ...

(mud turtles) **** Clade Panchelonioidea ***** Clade † Angolachelonia? ****** Clade † Thalassochelydia? ******* Family † Eurysternidae? ******* Family † Plesiochelyidae? ******* Family † Thalassemydidae? ******* †'' Owadowia''? ****** Family † Sandownidae? (may also belong in Thalassochelydia) ***** Family †

Protostegidae Protostegidae is a family of extinct sea turtle, marine turtles that lived during the Cretaceous period. The family includes some of the largest sea turtles that ever existed. The largest ''Archelon'' had a head long. Like most sea turtles, the ...

? (may also belong in Thalassochelydia) ***** Family † Toxochelyidae ***** Family †

Ctenochelyidae Ctenochelyidae is an extinct family of sea turtles which lived in North America during the Santonian-Maastrichtian stages of the Late Cretaceous. Classification The family is defined as a monophyletic clade arising from the most recent ancestors ...

***** Superfamily

(sea turtles) ****** Family

Cheloniidae Cheloniidae is a family of typically large marine turtles that are characterised by their common traits such as, having a flat streamlined wide and rounded shell and almost paddle-like flippers for their forelimbs. They are the only sea turtles ...

(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 A pond is a small, still, land-based body of water formed by pooling inside a depression (geology), depression, either naturally or artificiality, artificially. A pond is smaller than a lake and there are no official criteria distinguishing ...

box A box (plural: boxes) is a container with rigid sides used for the storage or transportation of its contents. Most boxes have flat, parallel, rectangular sides (typically rectangular prisms). Boxes can be very small (like a matchbox) or v ...

and water turtles) ***** Clade Testuguria ****** Family Geoemydidae ( Asian river turtles, Asian leaf turtles, Asian box turtles and

roofed turtle ''Kachuga'' (roofed turtles) is an obsolete genus formerly used for several species of Asian turtles, now placed in ''Batagur'' and ''Pangshura'': * ''Batagur dhongoka'' – three-striped roofed turtle * ''Batagur kachuga'' – red-crowned roofed ...

s) ****** Family

Testudinidae Tortoises ( ) are reptiles of the family Testudinidae of the order Testudines (Latin for "tortoise"). Like other turtles, tortoises have a shell to protect from predation and other threats. The shell in tortoises is generally hard, and like ot ...

(tortoises) ** Clade Pantrionychia *** Clade † Adocusia **** Family † Adocidae **** Family † Nanhsiungchelyidae *** Superfamily

**** Family Carettochelyidae (pignose turtles) **** Family

Trionychidae Trionychidae is a family of turtles, commonly known as softshell turtles or simply softshells. The family was described by Leopold Fitzinger in 1826. Softshells include some of the world's largest freshwater turtles, though many can adapt to li ...

(softshell turtles) File:Manchuroche.JPG, '' Ordosemys liaoxiensis'' was a member of Sinemydidae. File:Kinixys belliana nogueyi 1 by diotime.jpg, '' Kinixys belliana'' from the

File:Lissemys punctata1.jpg, The Indian flapshell turtle (''Lissemys punctata'') from the

is a highly advanced eucryptodire.

Distribution

(softshell turtles) are found from North America, Africa, South and East Asia to New Guinea. *

(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

External links

* {{Taxonbar, from=Q499621 Extant Early Jurassic first appearances Pliensbachian first appearances Taxa named by Edward Drinker Cope

Neck retraction

Systematics and evolution

Distribution

References

Further reading

External links