The Pleurodira are one of the two living

suborder Order ( la, ordo) 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 ...

s of

turtles 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 ...

, the other being the

Cryptodira The Cryptodira ('' el, 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 ...

. The division between these two suborders represents a very deep evolutionary divide between two very different types of turtles. The physical differences between them, although anatomical and largely internal, are nonetheless significant, and the zoogeographic implications of them are substantial. The Pleurodira are known more commonly as the side-necked turtles and the name Pleurodira quite literally translates to side neck, whereas the Cryptodira are known as hidden-necked turtles. The Pleurodira turtles are currently restricted to freshwater habitats in the Southern Hemisphere, largely to Australia, South America, and Africa. Within the Pleurodira, three living families are represented:

Chelidae Chelidae is one of three living families of the turtle suborder Pleurodira, and are commonly called Austro-South American side-neck turtles. The family is distributed in Australia, New Guinea, parts of Indonesia, and throughout most of South Ame ...

, also known as the Austro-South American side-necked turtles, the Pelomedusidae, also known as the African mud terrapins, and the

Podocnemididae Podocnemididae is a family of pleurodire (side-necked) turtles, once widely distributed. Most of its 20 genera and 30 species are now extinct. Seven of its eight surviving species are native to South America: the genus '' Peltocephalus'', with o ...

, also known as the American side-neck river turtles. However, they were cosmopolitan clade during 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 ...

and most of the

Cenozoic The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configu ...

, and even occurred in marine environments around the world.

Definition and description

The Pleurodira are identified by the method with which they withdraw their heads into their shells. In these turtles, the neck is bent in the horizontal plane, drawing the head into a space in front of one of the front legs. A larger overhang of the carapace helps to protect the neck, which remains partially exposed after retraction. This differs from the method employed by a

cryptodira The Cryptodira ('' el, 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 ...

n, which tucks its head and neck between its forelegs, within the shell. The different methods of bending the neck require completely different anatomies of the cervical

vertebrae The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristi ...

. All extant turtles studied so far have eight vertebrae in the neck. In the Pleurodira, these vertebrae are narrow in cross-section and spool-shaped with biconvex centra on one or more of the cervicals. These centra act as a double joint, allowing a large degree of sideways movement and providing a means of folding the neck onto itself in the lateral plane. Conversely, in the Cryptodira, the neck bones are wide and flat. The biconvex centra in some of the cryptodiran cervicals allow the neck to fold onto itself in the vertical plane. Pleurodirans also differ from cryptodirans in the emarginations of their skulls. Skull emargination provides room and anchorage for the jaw muscles. The connection points and the position of the emarginations relate to different bones of the skull. Another difference is in the arrangement of the bones of the shell and the scutes overlaying them. Pleurodiran turtles have 13 scutes on the plastron of the shell, whereas cryptodiran turtles have only 12. The extra scute is called the intergular and is at the front of the plastron between the gular scutes. Pelomedusid turtles also possess mesoplastra, further differentiating this group.

Suction feeding

One of the three extant families in this suborder is the family

, which have a specially adapted strategy for catching prey. While the majority of the family Chelidae are omnivores there are 17 species that are carnivores; '' Chelus fimbriatus'' and species of the ''

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 ...

'' genus. This special strategy is referred to as a gape-suck mechanism. The turtle first opens its mouth little by little. Then, when the turtle is within striking range of the prey, it will open its mouth completely sucking in water at such a rate that the current into its mouth is too strong for prey to escape and it engulfs the prey within 0.004 seconds. This strategy also circumvents issues to quick capture of underwater prey, such as resistance to rapid movement in water, pressure waves due to a rapid strike, and rapid water intake when feeding.

Taxonomy

After Ferreria, et al. 2018. *

Panpleurodira 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 ...

**†

Dortokidae Dortokidae is an extinct family of freshwater pan- pleurodiran turtles, known from the Cretaceous and Paleocene of Europe. Only four species have been named, but indeterminate fossils show that they were abundant across western and eastern-central ...

(

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

) (Early Cretaceous-Paleocene) **†

Platychelyidae Platychelyidae is an extinct family of pan-pleurodiran turtles, known from the Late Jurassic and Early Cretaceous of Europe, South America, North America, and the Caribbean. It represents the oldest known clade of stem-pleurodires. All known memb ...

(extinct) (Late Jurassic-Early Cretaceous) **Pleurodira ***

***Pan-Pelomedusoides ****† Araripemydidae (extinct) (Early Cretaceous) ****† Euraxemydidae (extinct) ****Pelomedusoides ***** Pelomedusidae *****†

Peiropemydidae Podocnemididae is a family of pleurodire (side-necked) turtles, once widely distributed. Most of its 20 genera and 30 species are now extinct. Seven of its eight surviving species are native to South America: the genus ''Peltocephalus'', with ...

(extinct) *****†'' Atolchelys'' (extinct) (Early Cretaceous) *****†

Sahonachelyidae Sahonachelyidae is an extinct family of pelomedusoid turtles from the Late Cretaceous ( Maastrichtian) Maevarano Formation The Maevarano Formation is a Late Cretaceous sedimentary rock formation found in the Mahajanga Province of northweste ...

(extinct) (Late Cretaceous) *****

***** †

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 ...

(extinct) (Early Cretaceous-Miocene)

References

{{Taxonbar , from=Q621851 Taxa named by Edward Drinker Cope Tetrapod suborders