Turtles are unmistakable. Their shells set them apart from all other animals, and render them perhaps the most anatomically interesting of vertebrates. The shell is not an exoskeleton, as some people mistakenly assert. The shell is a modified ribcage and part of the vertebral column. It cannot be "taken off" (as cartoons would lead us to believe) anymore than you could "take off" your spine and ribs. Because of the shell, the pectoral and pelvic girdles are uniquely located within the ribcage. The limb bones are also modified to accommodate to the shell.
All turtles are placed within the order Testudines (or Chelonia). Reptiles are traditionally classified according to the pattern of fenestration in the temporal region of the skull, and turtles are placed in the subclass Anapsida because they lack fenestration. Some workers suggest that this lack of fenestration is a secondary condition and that turtles should be placed in the Diapsida, but the diapsid vs. anapsid controversy is far from settled.
The earliest known turtles are from the Upper Triassic, and they are nearly indistinguishable from modern turtles anatomically. In these early fossils (most of the genus Proganochelys), the marginal teeth have already been lost and the presence of a keratin beak is suggested by the condition of the mandibles. The elements of the shell are present, only in greater numbers than exist in current species. Three important differences between Proganochelys and modern turtles are the presence of the palatal teeth (lost in modern species), the inability to retract the head within the shell, and the lack of a trochlear pulley in the jaw closing anatomy.
Modern turtles are placed into one of two suborders within the Testudines -- Peurodira (side-necked) and Cryptodira (hidden neck). The namesake difference between the two is the method of head retraction. In pleurodires, the neck vertebrae flex laterally, allowing the neck to bend and pull the head in sideways. In cryptodires, the neck vertebrae flex vertically, allowing the head to be drawn straight back within the shell. The Pelomedusidae and Chelidae are the only extant families of pleurodires. The Carettochelyidae, Cheloniidae, Chelydridae, Dermatemydidae, Dermochelyidae, Emydidae, Kinosternidae, Testudinidae, and Trionychidae are all cryptodires, although the ability to retract the head has been lost in the sea turtles (Cheloniidae and Dermochelyidae).
Carroll, R.L. 1988. Vertebrate Paleontology and Evolution. W.H. Freeman & Co., New York.
Ernst, C.H., and Barbour, R.W. 1989. Turtles of the World. Smithsonian Inst. Press, Washington, D.C.
Pough, F.H., Andrews, R.M., Cadle, J.E., Crump, M.L., Savitzky, A.H., and Wells, K.D. 2000. Herpetology, 2nd ed. Prentice Hall, Upper Saddle River, NJ.
Keith Pecor (author).
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature
having the capacity to move from one place to another.