Turtles and Tortoises

I

Introduction

Turtles and Tortoises, common name for reptiles that are recognized by a hard shell that encloses the internal organs of the body. They are ancient life forms: the earliest recognized fossils date from the Triassic period, about 245 million years ago; thus, turtles and tortoises were in existence before the emergence of the great dinosaur groups. Unlike dinosaurs, however, turtles and tortoises have continued to adapt and flourish.

The order to which they belong is divided into two suborders: the first comprises species that pull their heads into the shell by a telescoping movement of the neck; the second comprises freshwater species that hide their heads by bending the neck sideways. A total of about 250 species are grouped into 9 families; they are found only in the temperate or tropical zones. Most species are adapted to a freshwater or terrestrial habitat, and a small group to sea life.

In Britain the name turtle is restricted to marine species, terrapin to freshwater species, and tortoise to the terrestrial members of the order. In American usage, the name turtle is more widely applied; tortoise is used additionally for certain terrestrial species belonging to the first suborder. In Australia all freshwater species are known as tortoises.

II

Physical Characteristics

Turtles and tortoises show a wide variation in size, from the 15-cm (6-in) box turtle of North America to the giant sea-dwelling leatherback, which may attain a length of about 2.4 m (8 ft). The upper shell of the turtle, under which the head, limbs, and tail can be more or less completely withdrawn, is called the carapace. The lower shell, characteristically flat, is called the plastron. The two-part shell is connected to the vertebrae and ribs. The structure and size of the carapace and plastron vary from species to species, along with adaptive changes in behaviour and mode of life.

The shell typically consists of two layers: an inner bony layer, the sections of which are called plates; and an overlapping, horny layer, made up of so-called shields. Although it is hard, and in some species quite thick, the shell is a surprisingly sensitive structure because of the many nerves embedded in it. The horny outer layer of some shells—especially that of the hawksbill turtle—has been used in making ornamental objects; this material is popularly called tortoiseshell.

The bones of the skull in tortoises and turtles are immovably connected; the animals have no teeth, although traces of them have been detected in an embryonic soft-shelled turtle. The animals also lack breastbones. The heart, as in other reptiles except crocodilians, has three chambers, but it operates almost as if it had four because of the presence of an incomplete partition in the ventricle.

III

Behaviour

Although few species of turtles and tortoises can be considered strictly carnivorous or herbivorous, some land-dwelling species prefer vegetation, and aquatic species are mostly carnivorous. All species lay eggs, which they bury in holes. Turtles and tortoises are generally long-lived; some species live for more than 100 years.

IV

Freshwater and Wood Turtles

One family of turtles and tortoises constitutes about half the species in the order. They are characterized by their fully or partly webbed feet and their usually flat, streamlined shells; both features help in diving and underwater escape. Several genera in this family, however, have evolved to terrestrial life and show the arched carapace typical of tortoises. Among these is the American box turtle. Box turtles are further protected by a hinged lower shield that affords complete closure. Other important species of this family include the terrapins, especially the diamondback; the cooters; the map turtles; the painted turtles; and the wood and pond turtles.