Pleurodira

Side-necked turtles
Eastern long neck tortoise - (Chelodina longicollis)
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Sauropsida
Order: Testudines
Suborder: Pleurodira
Cope, 1865

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.

Contents

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 With the head withdrawn, some of the neck always remains exposed.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 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

[1]

Evolution

Pleurodira and Cryptodira appear to be sister clades.[1] The genera Chelodina and Chelus diverged 83 million years ago.

References

  1. ^ Wang L, Zhou X, Nie L, Xia X, Liu L, Jiang Y, Huang Z, Jing W. The complete mitochondrial genome sequences of Chelodina rugosa and Chelus fimbriata (Pleurodira: Chelidae): implications of a common absence of initiation sites (O(L)) in pleurodiran turtles. Mol Biol Rep.