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

The Cryptodira ('' el, hidden neck'') are a suborder of

Testudines Turtles are an order of reptiles known as Testudines, characterized by a special shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden necked t ...

that includes most living

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

s and

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

s. 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 differen ...

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

snapping turtles 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' ...

tortoises Tortoises () are reptiles of the family Testudinidae of the order Testudines (Latin: ''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 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 chelid turtles with a complicated nomenclatural history. Although in the past, ''Macrochelodina'' and ''Macrodiremys'' have been considered s ...

'' (pleurodire) versus that of ''

Apalone ''Apalone'' is a genus of turtles in the family Trionychidae. Species of ''Apalone'' are native to North America. Geographic range Most ''Apalone'' species are restricted to the United States, though the range of the spiny softshell, ''A. spini ...

'' (cryptodire), found an absence of the

longissimus The longissimus ( la, the longest one) is the muscle lateral to the semispinalis muscles. It is the longest subdivision of the erector spinae muscles that extends forward into the transverse processes of the posterior cervical vertebrae. Structu ...

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 latera ...

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 change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...

from pleurodires during the early

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

period, originating from

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the sout ...

and

Southeast Asia Southeast Asia, also spelled South East Asia and South-East Asia, and also known as Southeastern Asia, South-eastern Asia or SEA, is the geographical south-eastern region of Asia, consisting of the regions that are situated south of mainlan ...

. 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 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

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

Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a subcontinent of Eurasia and it is located entirel ...

, and produced a family of marine species, the

Bothremydidae Bothremydidae is an extinct family of side-necked turtles (Pleurodira) known from the Cretaceous and Cenozoic. They are closely related to Podocnemididae, and are amongst the most widely distributed pleurodire groups, with their fossils having b ...

. 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 Trionychia is a superfamily of turtles which encompasses the species that are commonly referred to as softshelled turtles as well as some others. The group contains two families, Carettochelyidae, which has only one living species, the pig-nos ...

. 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 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 preserved ...

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 In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor. ...

(i.e. Polycryptodira). The Cryptodira as understood here are called Cryptodiramorpha in this view. A recent study placed Plesiochelyidae as an

Angolachelonia Angolachelonia is a clade of extinct turtles from the Late Jurassic to Paleogene of Eurasia. The group is defined as all taxa derived from the ancestor of the type genus ''Angolachelys'' and '' Solnhofia'', a definition that could potentially e ...

and outside

, thus Cryptodira. As per the system used here, the Cryptodira can be classified as:

Distribution

*Trionychidae (Soft Shell 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. Manchuroche

* †'' Hangaiemys'' * † Bashuchelyidae *Pan-Cryptodira ** Family †

Macrobaenidae Macrobaenidae is an extinct family of turtles, known from the Early Cretaceous to Paleogene of Laurasia. Their relationships to other turtles and whether or not they form a monophlyletic group are controversial. They are typically interpreted ...

? Sukhanov 1964 **Family †

Xinjiangchelyidae Xinjiangchelyidae is an extinct family of turtles known from the Lower Jurassic to the Middle Cretaceous of Asia and western Europe. They have generally been interpreted as either being basal Cryptodira, cryptodires or placed outside of crown Tes ...

? Nesov, 1990 ** Family †

Sinemydidae Sinemydidae is an extinct family of turtles from Cretaceous to Paleocene deposits in Asia and North America. Their exact position is engimatic, they have alternatively been considered stem-group cryptodires, but also "crownward stem-turtles" alo ...

? Yeh, 1963 ** Clade Pandurocryptodira ***Clade Durocryptodira ****Clade Panamerichelydia *****Clade

Americhelydia Americhelydia is a clade of turtles that consists of sea turtles, snapping turtles, the Central American river turtle and mud turtles, supported by several lines of molecular work. Prior to these studies some morphological and developmental work ...

****** Clade Panchelydroidea *******Clade Chelydroidea ********Clade Panchelydridae *********†'' Chelydropsis'' *********Family Chelydridae (snapping turtles) ********Clade Pankinosternoidea *********†'' Lutemys'' *********†''

Emarginachelys ''Emarginachelys cretacea'' is a turtle belonging to the group Cryptodira, known from well preserved fossils from the Maastrichtian stage of the Late Cretaceous of Montana. Its exact phylogenetic position within Cryptodira is uncertain; different ...

'' *********†'' Tullochelys'' ********* Superfamily Kinosternoidea ********** Family Dermatemydidae (river turtles) **********Clade Pankinosternidae *********** 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 Panchelonioidea is a clade of marine turtles that includes the sea turtle Sea turtles (superfamily Chelonioidea), sometimes called marine turtles, are reptiles of the order Testudines and of the suborder Cryptodira. The seven existing speci ...

******* Clade †

? ******** Clade †

Thalassochelydia Thalassochelydia is a clade of extinct marine turtles from the Late Jurassic and earliest Cretaceous of Europe and South America. The group is defined as including ''Eurysternum'', ''Plesiochelys'' and ''Thalassemys'' to the exclusion of '' Pel ...

? ********* Family † Eurysternidae? ********* Family † Plesiochelyidae? ********* Family †

Thalassemydidae ''Thalassemys'' is a genus of extinct thalassochelydian turtle from the Late Jurassic of western and central Europe. While the genus was originally named by Rütimeyer in 1859 for a large carapace and other associated fragments from the late Kim ...

? ********* †'' Owadowia''? ******** Family †

Sandownidae Sandownidae is a family of extinct marine turtles from the Cretaceous and Paleogene distributed around the Atlantic Ocean and adjacent areas. The family is defined as all taxa closer to the type genus '' Sandownia'' than to '' Pelomedusa'', '' ...

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

Protostegidae Protostegidae is a family of extinct 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, they had fl ...

? (may also belong in Thalassochelydia) ******* Family † Toxochelyidae ******* Family † Ctenochelyidae ******* Superfamily Chelonioidea (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 t ...

(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 an area filled with water, either natural or artificial, that is smaller than a lake. Defining them to be less than in area, less than deep, and with less than 30% emergent vegetation helps in distinguishing their ecology from ...

box A box (plural: boxes) is a container used for the storage or transportation of its contents. Most boxes have flat, parallel, rectangular sides. Boxes can be very small (like a matchbox) or very large (like a shipping box for furniture), and can ...

and water turtles) ******Clade Testuguria ******* Family

Geoemydidae The Geoemydidae (formerly known as Bataguridae) are one of the largest and most diverse families in the order Testudines (turtles), with about 70 species. The family includes the Eurasian pond and river turtles and Neotropical wood turtles. Mem ...

( Asian river turtles,

Asian leaf turtle The Asian leaf turtle (''Cyclemys dentata'') is a species of turtle found in Southeast Asia. They are quite common in the pet trade; their carapaces resemble that of a ''Cuora amboinensis The Amboina box turtle or Southeast Asian box turtle ...

Asian box turtle Asian box turtles are turtles of the genus ''Cuora'' in the family Geoemydidae. About 12 extant species are recognized.Spinks, P. Q., et al. (2012)Species boundaries and phylogenetic relationships in the critically endangered Asian box turtle gen ...

s 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) ** Clade Pantrionychia *** Clade † Adocusia **** Family † Adocidae **** Family † Nanhsiungchelyidae *** Superfamily

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

Trionychidae The Trionychidae are a taxonomic family of a number of turtle genera, commonly known as softshell turtles. The family was erected by Leopold Fitzinger in 1826. Softshells include some of the world's largest freshwater turtles, though many can a ...

(softshell turtles)

Neck retraction

Systematics and evolution

Distribution

References

Further reading