Pleurodira
Encyclopedia
The Pleurodira are one of the two living suborders of turtles, the other being the Cryptodira
. In many cases in the nomenclature of animals, ranks such as suborder are considered of little importance apart from nomenclatural or taxonomic reasons. However, this is not the case with the suborders of the turtles. This division represents a very deep evolutionary divide between two very different types of turtle. 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-neck turtles. The Pleurodira turtles are restricted to the Southern Hemisphere, largely to Australia, South America and Africa. Within the Pleurodira, there are two living families represented: Chelidae
, also known as the Austro-South American side-necked turtles, and the Pelomedusidae
, also known as the Afro-South American side-necked turtles.
The different methods of bending the neck require completely different anatomy of the cervical
vertebrae. All turtles have seven vertebrae in the neck, as do most higher vertebrates, including mammals. In the Pleurodira, however, these bones are narrow in cross section, spool shaped and, as such, largely similar to those of other reptiles. They allow for a large degree of sideways movement, but little up and down movement. Conversely in the Cryptodira, the neck bones are wide and flat and number of the joints can flex through some 170 degrees to allow the neck to fold onto itself. This structure allows for little sideways movement.
These turtles differ in other ways, as well; some of these are at the family level, however. All turtles must have some emargination of the skull to enable the positioning and placement of the muscles used to open and close the jaws. The Pleurodires do this differently from the Cryptodira, although using the same muscles. The connection points and the position of the emargination relates 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 only have 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 differentiaing this group.
http://paleodb.org/cgi-bin/bridge.pl
and Chelus diverged .
Cryptodira
Cryptodira is the taxonomic suborder of Testudines that includes most living tortoises and turtles. Cryptodira differ from Pleurodira 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' margins...
. In many cases in the nomenclature of animals, ranks such as suborder are considered of little importance apart from nomenclatural or taxonomic reasons. However, this is not the case with the suborders of the turtles. This division represents a very deep evolutionary divide between two very different types of turtle. 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-neck turtles. The Pleurodira turtles are restricted to the Southern Hemisphere, largely to Australia, South America and Africa. Within the Pleurodira, there are two living families represented: Chelidae
Chelidae
The Chelidae are one of the three living families of the turtle suborder Pleurodira and are commonly called the Austro-South American Side Neck turtles. The Family is distributed in Australia, New Guinea, parts of Indonesia and throughout most of South America. It is a large family of turtles with...
, also known as the Austro-South American side-necked turtles, and the Pelomedusidae
Pelomedusidae
Pelomedusidae is a family of freshwater turtles, native to eastern and southern Africa. They range in size from to in shell length, and are generally roundish in shape...
, also known as the Afro-South American side-necked turtles.
Definition and description
The Pleurodira are identified by the method they use to withdraw their heads into their shells. In these turtles, the neck is bent in the horizontal plane-drawing the head inside with it tucked into the space in front of one of the front legs. With the head withdrawn, some of the neck always remains exposed. This differs from the method employed by Cryptodira turtles, which bend the neck in the vertical plane, hence pulling the head straight back between the front legs and completely hiding the neck.The different methods of bending the neck require completely different anatomy of the cervical
Cervical
In anatomy, cervical is an adjective that has two meanings:# of or pertaining to any neck.# of or pertaining to the female cervix: i.e., the neck of the uterus.*Commonly used medical phrases involving the neck are**cervical collar...
vertebrae. All turtles have seven vertebrae in the neck, as do most higher vertebrates, including mammals. In the Pleurodira, however, these bones are narrow in cross section, spool shaped and, as such, largely similar to those of other reptiles. They allow for a large degree of sideways movement, but little up and down movement. Conversely in the Cryptodira, the neck bones are wide and flat and number of the joints can flex through some 170 degrees to allow the neck to fold onto itself. This structure allows for little sideways movement.
These turtles differ in other ways, as well; some of these are at the family level, however. All turtles must have some emargination of the skull to enable the positioning and placement of the muscles used to open and close the jaws. The Pleurodires do this differently from the Cryptodira, although using the same muscles. The connection points and the position of the emargination relates 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 only have 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 differentiaing this group.
Taxonomy
- PodocnemididaePodocnemididaePodocnemididae is a family of turtles native to Madagascar and northern South America. They are side-necked turtles , which means they do not retract their heads backwards, but hide it sideways....
- ChelidaeChelidaeThe Chelidae are one of the three living families of the turtle suborder Pleurodira and are commonly called the Austro-South American Side Neck turtles. The Family is distributed in Australia, New Guinea, parts of Indonesia and throughout most of South America. It is a large family of turtles with...
- Dortokidae (extinctExtinctionIn biology and ecology, extinction is the end of an organism or of a group of organisms , normally 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 recover may have been lost before this point...
) - Eusarkiidae (extinct)
- Propleuridae (extinct)
- BothremydidaeBothremydidaeBothremydidae is an extinct family of side-necked turtle. They belong to sub-order Pleurodira and order Testudines. Bothremydidae lived in fresh water and were amphibious. They were actively mobile omnivores.-Taxonomy:...
(extinct) - PelomedusidaePelomedusidaePelomedusidae is a family of freshwater turtles, native to eastern and southern Africa. They range in size from to in shell length, and are generally roundish in shape...
http://paleodb.org/cgi-bin/bridge.pl
Evolution
Pleurodira and Cryptodira appear to be sister clades. The genera ChelodinaChelodina
This large and diverse genus of long-necked Chelid turtles has had a complicated nomenclatural history. Although in the past the following have been considered separate genera and prior to that all the same, they are now considered sub-genera of the one genus....
and Chelus diverged .