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The Cryptodira ('' el, hidden neck'') are a
suborder In biological classification, the order ( la, wikt:ordo#Latin, ordo) is # a taxonomic rank used in the classification of organisms and recognized by the nomenclature codes. The well-known ranks in descending order are: life, domain (biology), do ...
of
Testudines Turtles are an order Order or ORDER or Orders may refer to: * Orderliness Orderliness is associated with other qualities such as cleanliness Cleanliness is both the abstract state of being clean and free from germs, dirt, trash, or waste, ...
that includes most living
tortoise Tortoises () are reptiles Reptiles, as most commonly defined, are the animals in the class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a ...

tortoise
s and
turtle Turtles are an order Order, ORDER or Orders may refer to: * Orderliness Orderliness is a quality that is characterized by a person’s interest in keeping their surroundings and themselves well organized, and is associated with other qu ...

turtle
s. Cryptodira differ from
Pleurodira The Pleurodira are one of the two living suborder In biological classification, the order ( la, wikt:ordo#Latin, ordo) is # a taxonomic rank used in the classification of organisms and recognized by the nomenclature codes. The well-known rank ...
(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 (biology), order of reptiles known as Testudines, characterized by a turtle shell, shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira, side-necked turtles and Cryptod ...
s, snapping turtles,
tortoises Tortoises () are reptiles Reptiles, as most commonly defined, are the animals in the class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), ...

tortoises
, softshell turtles, and
sea turtles Sea turtles (superfamily Chelonioidea), sometimes called marine turtles, are reptiles of the order Testudines Turtles are an order Order or ORDER or Orders may refer to: * Orderliness Orderliness is associated with other qualities such ...
.


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 tetrapod Tetrapods (; from Greek 'four' and 'foot') are four-limbed animals constituting the superclass Tetrapoda . It includes extant and extinct amphibians, reptiles (including dinosaurs and therefore birds), and synapsids (including ...

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 Chelidae, chelid turtles with a complicated Biological nomenclature, nomenclatural history. Although in the past, ''Macrochelodina'' and ''Macr ...
'' (pleurodire) versus that of ''
Apalone ''Apalone'' is a genus of turtles in the Family (biology), 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 sof ...
'' (cryptodire), found an absence of the
longissimus The longissimus (Latin for 'the longest one') is the muscle lateral to the semispinalis muscles The semispinalis muscles are a group of three muscle Muscle is a soft tissue found in most animals. Muscle cells contain protein Proteins ...

longissimus
and
iliocostalis Iliocostalis muscle is the muscle immediately lateral to the longissimus The longissimus (Latin for 'the longest one') is the muscle lateral to the semispinalis muscles The semispinalis muscles are a group of three muscle Muscle is a soft ...
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 In the vertebrate spinal column The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform c ...
(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 Heredity, heritable Phenotypic trait, characteristics of biological populations over successive generations. These characteristics are the Gene expression, expressions of genes that are passed on from parent to offs ...
during the
Jurassic The Jurassic ( ) is a geologic period The geologic time scale (GTS) is a system of chronological dating that classifies Geology, geological strata (stratigraphy) in time. It is used by geologists, paleontology, paleontologists, and other earth ...
period, and by the end of the Jurassic 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 A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These periods form elements of a hierarchy of division ...

Cretaceous
to
Eocene The Eocene ( ) Epoch is a geological epoch In chronology 222px, Joseph Scaliger's ''De emendatione temporum'' (1583) began the modern science of chronology Chronology (from Latin Latin (, or , ) is a classical language belonging to the ...
of
Europe Europe is a continent A continent is any of several large landmass A landmass, or land mass, is a large region In geography Geography (from Greek: , ''geographia'', literally "earth description") is a field of scienc ...

Europe
, and produced a family of marine species, the
Bothremydidae Bothremydidae is an extinct family In human society A society is a Social group, group of individuals involved in persistent Social relation, social interaction, or a large social group sharing the same spatial or social territory, ...
. The Cryptodira suborder has four living superfamilies, the Chelonioidea (sea turtles),
Testudinoidea Testudinoidea is a superfamily (taxonomy), superfamily within the suborder Cryptodira of the order Testudines. It includes the Emydidae, pond turtles (Family: Emydidae), Geoemydidae, Asian turtles (Family: Geoemydidae), the monotypic taxon, monot ...
(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. 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 (evolution), 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 Turtles are an order Order or ORDER or Orders may refer to: * Orderliness Orderliness is associated with other qualities such as cleanliness Cleanliness is both the abstract state of being clean and free from germs, dirt, trash, or waste, ...
, thus Cryptodira. As per the system used here, the Cryptodira can be classified as: * †''Hangaiemys'' * †Bashuchelys, Bashuchelyidae *Pan-Cryptodira ** Family †Macrobaenidae? Sukhanov 1964 **Family †Xinjiangchelyidae? Nesov, 1990 ** Family †Sinemydidae? Yeh, 1963 ** Clade Pandurocryptodira ***Clade Durocryptodira ****Clade Panamerichelydia *****Clade Americhelydia ****** Clade Panchelydroidea *******Clade Chelydroidea ********Clade Panchelydridae *********†''Chelydropsis'' *********Family Chelydridae (snapping turtles) ********Clade Pankinosternoidea *********†''Lutemys'' *********†''Emarginachelys'' *********†''Tullochelys'' ********* Superfamily Kinosternoidea ********** Family Dermatemydidae (river turtles) **********Clade Pankinosternidae *********** 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 Testudinoidea is a superfamily (taxonomy), superfamily within the suborder Cryptodira of the order Testudines. It includes the Emydidae, pond turtles (Family: Emydidae), Geoemydidae, Asian turtles (Family: Geoemydidae), the monotypic taxon, monot ...
****** Family †Haichemydidae ****** Family †Sinochelyidae ******Clade Emysternia ******* Family Platysternidae (big-headed turtle) ******* Family Emydidae (Emys, pond, Box turtle, 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)


References


Further reading

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