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A. Paired respiratory muscles of Testudo graeca; B. Pulmonary pressure changes of Testudo graeca, in correlation with changes in body cavity volume during expiration and inspiration. (Illustration by Jacqueline Mahannah)
forward during a rainfall to catch water dripping from the shell.
Large openings, or fontanelles, between the pleural bones in the carapace have evolved in several genera. The leatherback seaturtle (Dermochelys coriacea) shows the greatest divergence from the characteristic turtle shell; tiny platelets embedded in the leathery carapace are all that remain of the bony shell. A reduced carapace decreases the physiological costs associated with building and maintaining a heavy shell (such as sequestering minerals to a substrate where they are not readily accessible for some physiological processes), as well as the energy cost for locomotion in terrestrial species, and provides greater buoyancy in the aquatic forms.
The turtle skull is also unique among living vertebrates for the absence of temporal fenestra. Formerly believed to be a true anapsid (i.e., without such openings), many researchers now believe that the turtle evolved from diapsid ancestors. In this scenario, the two pairs of temporal openings were secondarily lost, giving an anapsid-like appearance. Vestiges of the temporal openings can be seen in the slightly arched posterior margin of some turtle skulls. These large openings in the back of the skull allow the muscles of the jaw to expand beyond the confines of the adductor chamber.
Dietary preferences range from completely herbivorous to totally carnivorous; however, many turtles consume a mixture of plant and animal matter. In some species there is a dietary shift from the carnivorous diet of hatchlings to the mostly herbivorous adults. Although modern turtles lack teeth, there are many modifications of the maxillary, premaxillary, and dentary bones for feeding. A pronounced beak made of ker atin may be used to hold and tear food. The palate of herbivorous turtles contains a series of ridges that assist in the maceration of plant matter. Macrocephaly, characterized by an enlarged head (as in many female map turtles), often develops in mollusk-feeding species. The broad crushing surfaces and powerful musculature allow them to exploit an abundant food item that may be unavailable to turtles that cannot extract this meal from the mollusk's protective shell.
The limbs of most aquatic turtles terminate in five independent digits; however, most terrestrial turtles and tortoises have reduced phalanges. The limbs of freshwater turtles are flattened laterally, and the digits are generally webbed. The sturdy limbs of tortoises are round in cross section. In some highly aquatic species, such as seaturtles and pig-nose turtles, the limbs are paddlelike, and digits are reduced to just a few claws.
The texture of turtle skin ranges from virtually scaleless and smooth in highly aquatic softshell species, to the coarse scaly texture of terrestrial tortoises. Keratin scales of various shapes and sizes are found on the head and limbs of most species. The large, thickened scales of tortoises are adapted to their dry environments. Epidermal appendages such as chin barbels, warts, and the fringe of matamatas provide cryptic camouflage that may assist in prey acquisition and/or provide protection.
The major organs of the turtle circulatory system are similar to those of other reptiles. Although heart rate is largely dependent upon temperature, the three-chambered heart beats slowly, especially in tortoises. The paired lungs are dorsal to the visceral organs and cannot be expanded by the action of
1. A straight necked turtle, the eastern box turtle (Terrapene carolina); 2. A sideneck turtle, the New Guinea snapping turtle (Elseya novaeguineae). Straight necked and sideneck turtles protect their heads in their shells differently, based on their different anatomy. (Illustration by Barbara Duperron)
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