Turtles and tortoises belong to the taxonomic order Testudinata. The Testudinata is the only surviving order of the subclass Anapsida. The Anapsida are characterized by a primitive skull with no temporal openings. There are two suborders of Testudinata:
World wide there are some 12 different families embracing about 250 species. Seven families with about forty eight species are reported for the United States. All North American species are hidden-necked. Texas is one of the best turtle states as there are representatives of all seven families, listed here with examples of several local species:
Turtles are the oldest living reptiles. All reptiles have descended from the cotylosaurs (the 'stem' reptiles) appearing in the fossil record some 250 million years ago. Turtles (chelonians) apparently evolved directly from the Amniota (vertebrates with amniotic eggs). In common with other reptiles, turtles lay shelled eggs. The innovation of the shelled egg has allowed the turtle to be successful away from water, an adaptation not shared by amphibians which generally must lay their unprotected eggs in water. Tus
It is now thought that the shell developed from bony plates that were attached to the skin, later becoming attached to the ribs to form the shell. Many dinosaurs had bony plates attached to the skin.
Turtles were clearly established by the beginning of the Cretaceous period approximately 140 million years ago.
Turtles have been successful in a wide range of environments, from the oceans to the deserts. All turtles are poikilothermic (commonly called cold blooded), that is they regulate their body temperature by behavior. Turtles are often seen basking in the sun in order to warm themselves. In the Houston area, turtles often estivate during the hottest summer days
The symbol of the turtle is its shell. The shell consists of two parts - the carapace (the upper shell) and the plastron (the lower shell). The carapace and plastron are joined by two connecting bone bridges, one along each side of the turtle. The carapace is formed from the bone plates attached to the ribs of the turtle and the turtles backbone lies along the middle of the carapace. Like all vertebrates, the turtle has a backbone with attached ribs. As a result, most of the length of the turtle's backbone is rigid, with only the tail and neck retaining flexibility. The shell serves to protect the turtle, and is no doubt one reason that the order Testudinata has been so successful.
The shell is composed of two distinct layers, the flattened bones inside that are attached to the ribs and (usually) the external covering of horny plates called scutes. Softshell turtles and leatherback sea turtles have substituted a thick, smooth, leathery skin for the scutes but the rigid understructure remains.
The shell has been the focus of several adaptations that turtles have experimented with. Box turtles have developed flexible hinges at the bridges and across the plastron that allow them to completely withdraw and protect the soft body parts (limbs, head, and tail). Being largely terrestrial, the box turtles and tortoises have evolved rounded, spacious carapaces. Water turtles on the other hand have evolved flattened carapaces that increase the turtle's mobility and speed in the water. Softshell turtles have abandoned the protective scutes on the outer surface of the shell but have gained increased agility in the water. Softshells can overtake fast prey and conversely can avoid fast predators due to their increased speed in the water. Some water turtles have all but abandoned the plastron (for example, snapping turtles and musk turtles)
On the outside surface of the shell are the horny shields or scutes. The carapace has at least four types of scutes:
The scutes are often used in identifying specimens, for example the alligator snapping turtle has an additional partial row of scutes between the marginals and the costals that the common snapping turtle lacks.
The plastron has six types of scutes, which, from front to rear are:
These scutes are usually present in pairs and the pairs are joined along a central seam. The length of the segments of the central seam can be used to help indentify a particular species. Comparing relative lengths of these seams results in what is called the platral formula (see Turtles of the World, Ernst and Barbour). For example, the common mud turtle, Kinosternon subrubrum, has a plastral formula given as:
A major division of turtle species is based on the structure of their necks. Side-necked turtles are found in Africa, South America, and Australia. These turtles withdraw their heads by bending their necks in a horizontal plane. Hidden-necked turtles, such as all of the turtles of North America, withdraw their heads by folding the neck in a vertical plane.
Turtles have no teeth, but their jaws hold sharp, horny, lips that cut with a scissors action.
Turtles have a well developed ear but no external ear opening, and appear to be keenly sensitive to vibrations.
The turtle's shell prevents it from using chest movements to move air in and out of its lungs. Turtles have a sling across the rear part of the shell which is used to actively breath. The sling has attached muscles that can actively fill and empty the lungs.
Turtles have many adaptations that allow it to survive with very little oxygen. When most vertebrates hold their breath the signal that forces them to resume breathing is not insufficient oxygen but an accumulation of carbon dioxide which in turn decreases the blood pH. Turtles can tolerate a much higher level of carbon dioxide than most vertebrates because of a blood chemistry that buffers changes in the pH level. During vigorous exercise vertebrates will have a build up of lactic acid in the muscles. Eventually this build up will force the animal to reduce the level of exertion. Turtles can tolerate a much higher level of lactic acid than most vertebrates.
Turtles often draw in their heads and limbs for defense. When fully withdrawn, the limbs take up most of the lung volume and leave little room for respiration. They must essentially hold their breath when the limbs are withdrawn. Turtles can more completely empty their lungs than most vertebrates so that there is little initial carbon dioxide when they first draw in the limbs. In addition, turtles can store oxygen not only in the blood but also in the muscular myoglobin.
When a turtle dives, its heart rate slows, reducing its oxygen requirements. Most aquatic turtles can also obtain oxygen directly from the water. This ability is enhanced in cold tempertures since the turtle's oxygen requirements are reduced and the amount of oxygen in the water is increased. Softshell turtles have a vascularized region in the throat that acts like a gill. Because of the forgoing adaptations, aquatic turtles can hibernate under water and can stay active under water for prolonged periods.
The leatherback sea turtle (the most massive living reptile) is one of the world's premier divers, reaching depths in excess of 4000 feet.
The plastron of the male of many species is concave, facilitating the mating embrace wherein the male mounts the female from above and wraps his tail under hers for copulation. The female's plastron in most species is flat or even convex, providing more internal space for the developing eggs. An additional common sexual dimorphism is the longer tail of the mail, required in order to reach under the female's tail from above. The anal opening, through which copulation occurs, is further down the tail in the male, the female's anal opening frequently being underneath (covered by) the rear edge of the plastron. In several aquatic species the male will have longer front toenails that are used in courtship behavior.
The only reliable method of age determination in wild turtles is the mark and release method. No other known method of age determination has been shown to be accurate, especially for older specimens.
One method that can be used with some success by amateurs is to estimate the age by scute growth zones. The scutes provide an externally visible record of the growth of an individual. Scute growth responds to seasonal changes in the environment, most importantly the annual hibernation-active cycle.
Each scute develops from the skin that covers the shell. The line between each scute indicates an indentation in the epidermis into the dermis below. New scutes form beneath the scute(s) of the preceding growth cycle and the new scute lifts the old scute or scutes off of the living epidermis (skin).
Turtles with hard scutes display two patterns of scute growth. Some species shed the older scute each year, however the indentation often persists in the new scute and age can still be estimated. The indentations becomes less evident with age. Other species retain the older scutes, losing them only to abrasion after many years. In shedding species, the new scutes are uniformly thick while in non-shedding species, the new scutes are thicker at the edges and thinned in the portion lying beneath the retained older scutes.
Both types of growth patterns respond to the annual hibernation-active cycle as well as minor growth rate variations such as injury or a temporary lack of food. These minor influences result in indentations in a scute but not the formation of a new scute.
In a non-shedding species the layers of scutes are counted resulting directly in an age estimate. Care must be taken to ignore minor growth marks and to estimate if any older scutes have been lost. Clearly this technique is more reliable on younger turtles, perhaps to 5 to 7 years of age. If a population is being studied, the scute layers should be counted on the same scute for each individual, typically the fourth vertebral scute or the fourth abdominal scute. The same technique is used for shedding species, with less reliability!