Reptiles

Reptiles are a class of vertebrates characterized by their ability to produce cleidoic eggs (which are similar to bird eggs and were their evolutionary precursors). The development of this egg, protected by an impervious shell, was a historic step, as it allowed animals to exploit terrestrial habitats. The egg and the reptiles’ dry, horny scales differentiate all living reptiles from amphibians. Skeletal features (single bones for sound conduction in the middle ear and jaws composed of several bones) and the lack of feathers and hair differentiate reptiles from birds and mammals, respectively. It is this combination of features that characterizes reptiles. Since soft tissues—such as reproductive tracts and skin—do not fossilize well, distinguishing extinct forms from other closely related vertebrate groups is often dependent on a single characteristic and may lack precision. For example, Archaeopteryx, a primitive bird, would have been classified as a reptile had not feathers been adventitiously preserved.

Early Reptiles, Turtles, and Crocodiles

Reptiles arose from amphibians roughly 315 million years ago, during the Carboniferous period. These “stem-reptiles” gave rise to all other groups. The “cheek,” or temporal region, is very important in reptilian classification. In early forms, the region was solid (lacked openings). These forms are placed in the subclass Anapsida (without recesses or openings). Along with some of these earliest reptilian fossils are some from a distinctly different group, with a single temporal opening. These animals are placed in the subclass Synapsida. Referred to as “mammal-like reptiles,” they gave rise to mammals prior to their extinction. During the Permian period (280-215 million years ago), another reptilian group appeared. This group, characterized by two temporal recesses, is placed in the subclass Diapsida (two openings), to which most reptiles, living and extinct, belong. Another subclass, the Euryapsida, with a single opening high on the side of the skull, became extinct near the end of the Mesozoic era (approximately 175 million years ago). It included large marine (sea-dwelling) forms, such as fishlike ichthyosaurs and flat-bodied, long-necked plesiosaurs.

Anapsids include not only the stem-reptiles, but also a surviving order, the Chelonia or Testudines, composed of turtles and tortoises. Primitive turtles appear almost fully formed in the fossil record, and their relationship to the first reptiles is uncertain. Their principal feature is a shell, composed of flattened ribs fused to layers of bony tissue, usually covered by large, flat scales called scutes. The upper portion, to which vertebrae are fused, is the carapace; the underside is the plastron. They are connected by bridges. Shoulders and hips have been modified and are located within the rib cage, a unique arrangement. All lay eggs on land. The order contains almost 250 species in 75 genera and 13 families. Although it is easy to fall into the trap of thinking that “if it has a shell, it is a turtle” and “a turtle is a turtle is a turtle,” the group is quite diverse. The most common (and probably primitive) body plan is associated with marsh dwellers. Characterized by somewhat flattened shells (streamlined for locomotion through water) and webbed feet for propulsion, these surprisingly agile swimmers include the familiar sliders that drop into the water from logs or the bank when approached too closely. Though adept in water, they are no match for sea turtles; these animals have reduced, flattened shells to minimize resistance and limbs modified into paddles with which they fly through the water using movements almost identical to those of birds. Though they lay their eggs on land, they are practically helpless there. Graceful in the water, they often swim considerable distances, guided by a very effective navigational sense. Sea turtles include the largest living reptiles, the leatherbacks, which may exceed a ton in weight. Other turtles are bottom dwellers, ambush predators, or scavengers; they lie in wait or slowly crawl along the bottoms of ponds and streams. They possess webbed feet and flat, often rough shells. Algae growing on their shells serves as camouflage, hiding them from prey and predators. A final body plan characterizes land turtles, which include tortoises. A firm, generally high-domed shell minimizes surface area through which water might be lost (critical in terrestrial animals) and resists attacks by predators to which a land dweller is exposed. Some very large forms are quite long-lived; one documented record exceeds 150 years.

The term “dinosaur” is often used to describe any large, extinct reptile, including mammal-like synapsids, marine euryapsids, and diapsids such as flying reptiles and some large lizards. Used properly, however, it refers to only one group of diapsids, the Archosauria, or “ruling reptiles,” which gave rise to birds. Other close relatives are in the living order Crocodylia, which now contains only twenty-two species in eight genera and three families. These animals share some very advanced features that cause some authorities to place them in a distinct class, the Dinosauria. All have fully partitioned hearts, allowing separate circuits for oxygenated blood to be carried to the body and deoxygenated blood to the lungs. Recent evidence indicates that many dinosaurs may have possessed birdlike capabilities for temperature regulation, allowing levels of activity beyond that of other reptiles. Modern crocodilians, some exceeding 7.5 meters in length, are quite aquatic and feed principally on fish or animals ambushed as they drink. They are restricted to tropical and subtropical zones. All are egg-layers.

Lepidosauria and Squamata

All other living reptiles are in the diapsid group Lepidosauria (scaly reptiles). The tuatara, a lizardlike reptile up to sixty centimeters long, is the only surviving member of the order Rhynchocephalia, a diverse assembly that coexisted with dinosaurs. Restricted to roughly thirty small islands off the coast of New Zealand and well adapted to a cool climate, it demonstrates considerable longevity (approximately 120 years) but also has a low reproductive rate. It feeds primarily on insects and eggs and the young of sea birds or other tuataras, with which it shares burrows.

Arguably the most successful reptilian group, extinct or living, is the order Squamata. All are equipped with efficient vomeronasal organs with which they “smell” by sampling air or substrate with their tongues. They may lay eggs or give live birth. There are approximately 3,750 species of lizards, suborder Sauria or Lacertilia, in almost 400 genera and some 16 families. They are found from north of the Arctic Circle to the southern tip of South America; such range is perhaps attributable to their exceedingly efficient capacity for thermoregulation. Some lizards at below-freezing temperatures may maintain body temperatures near 20 degrees Celsius. It is difficult to characterize lizards because of their tremendous diversity. Some are legless—an adaptation for burrowing or living in dense grass. Others have the capacity for gliding. Feet may be modified for running (many run on their hind legs, in one instance so rapidly that the lizard can run on water for considerable distances) or climbing, with digits equipped with claws and/or adhesive pads. Teeth may be used for grasping, cutting, or crushing food. Tails may be prehensile (capable of grasping), may be used for balancing while climbing or running, may come equipped with spines or knobs for defense, may be capable of fat storage, and may even break off if grasped by a predator (often to regenerate rapidly). Some lizards are excellent swimmers; the marine iguana of the Galápagos Islands feeds primarily on seaweed. Two species are venomous. The smallest lizards are only a few centimeters long; the largest may exceed three meters.

Snakes

Snakes, suborder Serpentes or Ophidia, are distinct from lizards in that they lack external ears, eyelids, and limbs (at least one of which most lizards have—worm lizards lack these features and have been treated as snakes or even placed into a separate suborder). Despite these constraints, snakes are quite diverse: There are almost 2,400 species in more than 400 genera and 11 families, ranging from the Arctic Circle to the southern tip of South America. Leglessness was a primitive adaptation for burrowing, but modern snakes also swim, crawl, climb, and, in one case, even glide adeptly. The elongated body form that accompanies limblessness requires that paired internal organs be arranged longitudinally, with one often degenerating. Digestive tracts are short and straight, resulting in all snakes being carnivorous, as meat is more easily digested by snakes than plant material. Locomotion is surprisingly varied. The familiar serpentine movement works well either on land or in water, but heavy-bodied snakes often use rectilinear locomotion, pushing their bodies in straight lines by alternately raising and retracting their large belly scales. In tight quarters, snakes anchor their necks and pull their bodies forward or, alternately, push off using anchored tails (many are equipped with spines for this purpose). A few snakes, especially on loose substrates like sand, sidewind and push down to prevent sliding while lifting loops of their bodies laterally.

Snakes swallow their prey whole. Jaws, which are loosely attached to the skull and to each other, alternately slide forward and pull back on food with recurved teeth. Prey may be swallowed alive, killed by constriction, or killed with venom. Venom injection may accompany a bite or may be facilitated by special fangs in the rear or front of each jaw. In vipers, the bones to which fangs are attached rotate, so that very long fangs can be folded back when not in use. Burrowing snakes tend to be slender and small, with smooth scales and rigid heads. Aquatic snakes are usually stout, with rough scales to prevent slipping through water. Arboreal snakes (climbers) are often extremely slender. Active hunters are usually more slender than ambush predators, which eat more rarely but can consume much larger items. Some snakes have temperature-sensitive pits with which they find prey in the dark. Sizes range from a few centimeters to almost ten meters.

Studying Reptiles

Methods used to study reptiles are determined by the nature of the investigation. Historical studies rely on paleontological methods. The discovery of fossils, followed by recovery, preservation, reconstruction, and analysis, leads to an understanding of the structure and function of prehistoric animals and provides information about both how they lived and conditions in which they existed. Comparisons, especially of structures, with other fossils and with modern animals constitute much of the field of comparative anatomy and lend insights into relationships between various living and extinct forms. These studies, in turn, lead into the discipline of systematics, which attempts to reconstruct relationships and build classification schemes accordingly. The actual naming of various groups is called taxonomy. Since fossil records are typically incomplete, however, other methods must be used to fully establish the nature of relationships.

Similarities and differences between living forms may be established based on detailed anatomical studies or various biochemical techniques. In the latter case, the analysis of the molecular structure of the deoxyribonucleic acid (DNA) and proteins produced by different species (or even by different populations of the same species) allows determinations of how closely related certain forms may be. These studies also have considerable evolutionary implications, providing insights not only into methods that might have resulted in evolutionary changes among reptiles but also into the processes that were responsible for the origins of birds and mammals. One phenomenon that was first discovered in reptiles is parthenogenesis, the development of an individual from an unfertilized egg, a process that leads to all-female populations. In lizards, these often result from hybridization between two species, the offspring of which are distinctive. Analysis of mitochondrial DNA, which is passed to descendants through the egg (never the sperm), allowed determination of which hybridizing parental species was maternal.

Another field of study that uses biochemical techniques and that focuses on reptiles concerns the venoms produced by some snakes and two lizard species. Knowing the composition of venoms is important in determining their effectiveness as devices to kill prey, but it is also significant in that some substances in these venoms have been shown to possess functional traits that have important medical implications.

Reptiles have also been used widely in ecological and behavioral studies. Methods include both field and laboratory techniques. Studies in natural situations often involve extensive observations and, as such, require organisms that are easily observed. Laboratory studies, in which environmental factors are often simulated and then modified, demand subjects that are small and readily maintained in captivity. Many of these studies also have physiological implications. Studies monitoring such factors as body temperatures, food intake, and foraging strategies often rely on reptiles, especially lizards, as they are ideally suited to these observational and experimental investigations. Considerable work in reproductive physiology also relies on reptiles; their eggs are accessible, and they demonstrate developmental patterns like those in birds and mammals. This aptitude as a subject for studies has also resulted in reptiles becoming the focus of many biogeographical studies, especially on islands, where (as in deserts) they often dominate the fauna. For many of the same reasons, environmental studies of endangered species or altered habitats frequently use reptiles as models.

Reptiles and Other Animals

The study of reptiles not only increases scientists’ knowledge of this fascinating group of animals, but also has many other applications. Historically, reptiles were the first group of fully terrestrial vertebrates; they dominated the earth for many millions of years and gave rise to both birds and mammals. Thus, studies of fossil forms, with additional insights from investigations of living species, provide insights into the conditions that prevailed on the earth during prehistoric times. They also lead to theories regarding relationships between animal groups and lend understanding to the origins and nature of early birds and mammals. Studies of this type need not be restricted to arcane facts relevant only to times long past; they are also significant in understanding long-term biological processes, such as those that reflect climatic cycles and periods of mass extinction (both problems of considerable interest in the twenty-first century). Also, since many reptilian groups are sufficiently old to predate the breakup of Pangaea (the single landmass that existed historically and which has since broken up into the modern continents), or at least its subsequent parts, applications can be made to the areas of biogeography and even geology.

Physiological investigations of reptiles have been invaluable in developmental and reproductive studies and in increasing knowledge of how animals interact with their environments. Lizards, especially, have been widely studied regarding their ability to thermoregulate effectively using environmental sources of heat. These studies have numerous applications to broader investigations of homeostasis and adaptations to cold and hot environments by many animals (including man). Lizards also have been widely used as models in behavioral and ecological studies. Especially in the tropics, they are abundant, diverse, easily observed, and often remarkably well-adapted to their environment. Like birds, much of their behavior is quite stereotypical, that is, innate and consistent. As a result, the recognition of patterns is much easier than it is in secretive mammals, for example, where the problems are further magnified by frequent modifications of instinctive mechanisms by learned behaviors. This same visibility and ease of observation lend itself to ecological investigations. Lizards have been more widely utilized in niche partitioning studies than any other animal. Niche partitioning studies seek to investigate how limited resources are used by animal communities. They often center on the hypothesis that food habits are critical, but microhabitat preferences, activity cycles, and other aspects of environmental impact are also involved.

Principal Terms

Anapsida: A group of reptiles in which the temporal region of the skull lacks openings

Chelonia (testudines): A living order of reptiles composed of turtles and tortoises

Cleidoic Egg: A shelled egg equipped with internal membranes that make terrestrial reproduction possible

Crocodylia: A living order of reptiles that includes crocodiles and alligators

Diapsida: A group of reptiles in which the temporal region of the skull is characterized by two openings

Euryapsida: An extinct group of reptiles in which the temporal region of the skull is characterized by a single opening situated high on the side of the skull

Rhynchocephalia: A living order of reptiles represented by a single species, the tuatara

Squamata: A living order of reptiles composed of lizards and snakes

Synapsida: An extinct group of reptiles in which the temporal region of the skull is characterized by a single opening; this group gave rise to mammals

Venom: A toxic substance that must be injected in order to elicit damaging effects

Bibliography

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Conant, Roger. A Field Guide to Reptiles and Amphibians of Eastern and Central North America. 3rd ed. Houghton Mifflin, 1998.

Halliday, Tim R., and Kraig Adler. The Encyclopedia of Reptiles and Amphibians. Facts on File, 1998.

Hickman, Cleveland P., Larry S. Roberts, and Frances M. Hickman. Integrated Principles of Zoology. 19th ed. McGraw Hill, 2023.

Kirshner, David S. Reptiles and Amphibians. National Geographic Society, 1996.

Spellerberg, Ian F. Mysteries and Marvels of the Reptile World. Scholastic, 1995.

Stebbins, Robert C. A Field Guide to Western Reptiles and Amphibians. 2nd ed. Houghton Mifflin, 1985.

Vitt, Laurie J., Janalee P. Caldwell, and George R. Zug. Herpetology: An Introductory Biology of Reptiles and Amphibians. 4th ed. Academic Press, 2014.