TAXONOMY

Chelonia have a dramatic ability to flex their cervical vertebrae and are classified into two suborders according to their mode of retracting their head into their shell (Fig. 3.1).

The Pleurodira or sideneck turtles have three main points of neck flexure so the neck forms a horizontal “s” shape. They are more primitive than the Cryptodira, being unable to retract their head inside their shell, instead placing it sideways. These are mainly aquatic or semi- aquatic turtles and are only found in the southern continents.

The Cryptodira or hidden-neck turtles have two points of neck flexure that form a vertical “s” bend, allowing the head to be completely withdrawn inside the shell. This has enabled them to be more successful and includes the majority of chelonian species (11 families). Some species, such as the Snapper turtle and sea turtles, have lost the ability to hide their head in their shell (Bellairs 1969a; Hoffstetter & Gasc 1970; King 1996).

Terminology

Chelonia refers to the order of shelled reptiles. In North America the word “turtle” is the common term for any reptile with a shell, although the term tortoise is used for terrestrial species like the Testudinidae. In other parts of the world different terms may be used according to habitat. Tortoises are land based, terrapins are freshwater semi-aquatic, and the word turtle is reserved solely for marine and aquatic species (Table 3.1).