reptiles

reptiles Reptiles, together with birds and mammals, constitute a single monophyletic group (an ancestor and all its descendants) that is called the Amniota. Amniotes are characterized by the presence of an amniote egg, that is an egg with a hard outer covering that could be laid on land and that contained a membrane, the amnion, that surrounded the embryo in a fluid-filled sac. This feature distinguishes them from the amphibians, which have to lay their eggs in moist conditions and whose young typically hatch out at an immature or larval stage. The organisms that we generally consider to be reptiles, the lizards and snakes, crocodiles, and turtles, together with fossil groups such as the dinosaurs, form a paraphyletic group (an ancestor and some but not all of its descendents), since the birds and the mammals are not included (Fig. 1).

The development of an amniote egg appears to have occurred in one group of small anthracosaurs (labyrinthodont amphibians) during Carboniferous time. This was a major step forward, for it freed tetrapods from their dependence on water and allowed them to colonize the land. The earliest reptiles are known from Carboniferous deposits in Nova Scotia, where they occur in the hollow stumps of fossil trees which apparently acted as traps for these small animals. Genera such as Hylonomus were small and superficially lizard-like in appearance. Although no fossil eggs have been found with them they are known to be reptiles because of the presence of a suite of advanced features of the skeleton. These features indicate that the animals in question were now fully terrestrial, a step that could have been taken only after the development of the amniote egg. The developments include a change in skull shape from broad and flat to tall and narrow, which brought the eyes more to the side of the head, and changes to the jaw musculature that improved the mechanical efficiency for maximizing force at all stages as the jaws were closed. In the limbs, the main bones elongated and became more slender, while some of the bones of the wrist and ankle fused to provide additional strength. In addition, the attachment between the pelvis and the vertebral column enlarged to provide additional strength in response to an increase in the amount of force generated by the hind limbs. It is from this group of primitive reptiles known as the captorhinidans that the major lineages of reptiles developed in the Late Carboniferous.

The major reptile groups are classified according to the pattern of openings in the skull roof behind the orbits. These openings developed to give increased area for the attachment and bulging of jaw muscles. In the captorhinidans and their relatives the turtles, no openings are present behind the orbits and they are referred to as being anapsid. Anapsids were important as an ancestral group for the reptiles but their only modern representatives are the turtles, which have a history going back to the Late Triassic. Well-preserved skeletons of this age from Germany and the western USA show that most of the turtle characteristics had already been developed by that stage. In the Triassic Proganochelys, teeth were already absent from the jaws and the body was protected by a heavy shell. During the Late Triassic to Late Jurassic the two modern groups of turtles were developed. The pleurodires, in which the neck bends sideways when the head is pulled into the shell, were widely distributed in the Cretaceous but are now restricted to the southern hemisphere. The cryptodires, in which the neck bends vertically, have become important since the Cretaceous, evolving along many lines of adaptive radiation, and they now have a worldwide distribution.

The synapsids are forms in which only one opening is developed behind the orbit. These are represented initially by the mammal-like reptiles, a group that was diverse and important in the Permian and into the Triassic, but was then eclipsed by the dinosaurs. Ultimately this line produced the mammals, which came into their own at the end of the Cretaceous after the demise of the dinosaurs.

The major reptilian group is the diapsids, in which there are two openings behind the orbit. They include the marine plesiosaurs and ichthyosaurs and also the two major groups of archosaurs (‘ruling reptiles’) and the lepidosaurs, which include snakes and lizards. The major archosaur group was the dinosaurs, which dominated the terrestrial scene during the Mesozoic. In addition, their relatives the pterosaurs and the crocodiles demonstrate the diversity of this group. The crocodiles originated in the Triassic and diversified during the Mesozoic, developing from lightly built terrestrial carnivores to the aquatic predators that we know today. Some of the later Mesozoic forms were indeed formidable; Deinosuchus from the Cretaceous of Texas was about 15 m long and probably preyed on dinosaurs.

Lizards developed at the same time as the dinosaurs, but because of the great difference in size there was probably little competition between them. The main modification in the lizard skeleton is the loss of the lower edge of the lower of the two openings behind the orbit; this gave greater flexibility to the jaw. Although lizards are generally small animals the modern komodo dragon is up to 3.5 m long and is an effective predator on mammals. During the Cretaceous the mosasaurs, a group of large marine lizards, developed. These were 9 to 12 m in length, had paddle-like feet, and propelled themselves with a laterally flattened tail. Snakes developed from limbless lizards during the Early Cretaceous and have developed specializations of the skull to allow them to ingest large prey and to inject poison in some cases. Like all the members of the Reptilia, they are a vigorous and diversified group occupying many different habitats.

David K. Elliott

Bibliography

Carroll, R. L. (1988) Vertebrate paleontology and evolution. W. H. Freeman and Co., New York.
Colbert, E. H. and and Morales, M. (1991) Evolution of the vertebrates. Wiley-Liss, Inc., New York.