Reptiles have acquired several advances over amphibians that have allowed them to move successfully into terrestrial habitats. Their skin, for example, is more heavily cornified and is protected with surface scales that are impervious to water. To help conserve water the kidney produces a concentrated urine, and the volume has been reduced. Since reptiles have internal fertilization, water isn't even needed for mating.
The eggs have undergone extensive modification compared to those of amphibians and are termed cleidoic eggs. Reptiles have three extraembryonic membranes the chorion, amnion and allantois. The amnion is a fluid-filled sac that encloses the embryo, providing it with its own private pond. The chorion and allantois are vascularized membranes that lie against the shell. The allantois develops as an outgrowth of the hindgut. It helps with respiration and serves as a Johnny-on-the-spot for the developing embryo. The chorion is the outermost extraembryonic membrane and is mainly concerned with respiration. These membranes are surrounded by albumen, and then a shell. The egg shell is porous and either leathery or limy. This adaptation allows respiration through the shell without losing too much water. On the down side, most reptile eggs become waterlogged if exposed to water for too long. Females usually lay their eggs, but some lizards and snakes retain them for internal development.
To keep up with their greater activity there are changes in reptiles' pelvic and pectoral girdles to make them stronger than those of amphibians. Some even have four-chambered hearts with separate pulmonary and systemic circulations. Respiration is by lungs, although cloacal respiration can act as an auxiliary system in some aquatic turtles. Many reptiles adjust their body temperature by behavioral mechanisms, basking in the sun to warm up in the morning and then seeking shelter in the heat of the day. Physiological mechanisms sometimes aid in this behavioral thermoregulation: Chromatophores can open to darken the skin so that it absorbs more heat, and blood flow to the skin can be regulated.
The first reptiles were amphibian-like beasts called cotylosaurs appeared during the Carboniferous period (about 350 million years before present). Turtles are most similar to the cotylosaurs (although the latter did not have shells) and are placed in the same subclass. A turtle’s body is covered on the dorsal surface by a bony shell with horny plates (to which the vertebrae are fused making it impossible for turtles to leave their shells as they do in cartoons). On the ventral surface a plastroncovers the belly. Inspect a turtle skull Unlike the other reptiles, turtles have replaced their teeth with a horny plate. Alligators, on the other hand, have jaws that house teeth that are all alike, albeit of differing size (homodont dentition). The conical thecodont teeth are set in sockets called
alveoli in the premaxillae, and dentary (lower jaw). The alligator skull has a large number of cranial bones and in this respect is more primitive than that of the frog (even though reptiles evolved after the amphibians).
A reptile's scales are very different in structure from that of fish. The outer layer of skin is the thick stratum corneum (thick layers of dead, keratin-filled cornified cells). These cells are organized into horny scales covering the entire outer surface. The scales are important in protecting the animal from abrasion and drying out. The plates making up the shell of turtles are composed of a similar material. Hinges between the scales permit flexible movements. The lower epidermis, like that of amphibians, is the stratum germinativum (it produces the upper cell layers). Unlike the amphibians, there are few skin glands.
Relationships among the major groups of living reptiles. (A) The classical phylogeny based on morphology and the fossil record (1, 2). (B) Maximum likelihood phylogeny of combined sequences from 11 nuclear proteins (1943 amino acids). Scale bar indicates amino acid substitutions per site. (C) Consensus phylogeny of combined sequences from four nuclear protein-coding genes for which sequences of tuatara are available (785 amino acids). For the molecular trees, confidence values (%) supporting the nodes are separated by slash marks and based on the following four methods: interior-branch test, and bootstrap analyses of neighbor-joining, maximum likelihood, and maximum parsimony, respectively. In (B) it was determined that myoglobin had the greatest effect in lowering confidence values; removal of that gene did not change significance of turtle-crocodilian node but raised support for bird-turtle-crocodilian node to 94/90/94/88.