The reptiles are a group of vertebrate animals. Reptiles are tetrapods, and also are amniotes (animals whose embryos are surrounded by an amniotic membrane). Today they are represented by four orders:
Reptiles are found on all continents except for Antarctica, although their main distribution comprises the tropics and subtropics. Reptiles don't have a constant body temperature. Modern species of reptiles do not generate sufficient body heat to mantain a constant body temperature, and instead rely on gathering and losing heat from the environment to regulate their internal temperature, such as by moving between sun and shade (see Leatherback_Sea_Turtle for an exception to this). In their natural habitats, most species are adept at this, and can maintain their core body temperature within a fairly narrow range. While this lack of internal heat imposes costs, in terms of requiring behavioral regulation, it also allows reptiles to survive on much less food than comparably sized mammals, who burn most of their food for warmth. Most reptile species are carnivorous and oviparous (egg-laying). Some species are ovoviviparous, and a few species are truly viviparous.
However, note the taxonomy issues described below; mammals and birds are all descendants of reptiles.
Classification of reptiles
Reptiles classically included all the amniotes except birds and mammals. Thus reptiles were defined as the set of animals that includes crocodiles, alligators, tuataras, lizards, snakes, and turtles, grouped together as the class Reptilia. This is still the usual definition of the term.
However, in recent years many taxonomists have begun to insist that taxa should be monophyletic, that is, groups should include all descendants of a particular form. The reptiles as defined above would be paraphyletic, since they exclude both birds and mammals, although these also developed from the original reptile. Colin Tudge writes:
- Mammals are a clade, and therefore the cladists are happy to acknowledge the traditional taxon Mammalia; and birds, too, are a clade, universally ascribed to the formal taxon Aves. Mammalia and Aves are, in fact, subclades within the grand clade of the Amniota. But the traditional class reptilia is not a clade. It is just a section of the clade Amniota: the section that is left after the Mammalia and Aves have been hived off. It cannot be defined by synamorphies, as is the proper way. It is instead defined by a combination of the features it has and the features it lacks: reptiles are the amniotes that lack fur or feathers. At best, the cladists suggest, we could say that the traditional Reptila are 'non-avian, non-mammalian amniotes'. (Tudge, p.85)
Some cladists thus redefine Reptilia as a monophyletic group, including both the classic reptiles as well as the birds and perhaps the mammals (depending on ideas about their relationships). Others abandon it as a formal taxon altogether, dividing it into several different classes. However, other biologists believe that the common characters of the standard four orders are more important than the exact relationships, or feel that redefining the Reptilia to include birds and mammals would be a confusing break with tradition. A number of biologists have adopted a compromise system, marking paraphyletic groups with an asterisk, e.g. class Reptilia*. Colin Tudge notes other uses of this compromise system:
- By the same token, the traditional class Amphibia becomes Amphibia*, because some ancient amphibian or other gave rise to all the amniotes; and the phylum Crustacea becomes Crustacea*, because it may have given rise to the insects and myriapods (centipedes and millipedes). If we believe, as some (but not all) zoologists do, that myriapods gave rise to insects, then they should be called Myriapoda*....by this convention Reptilia without an asterisk is synonymous with Amniota, and includes birds and mammals, whereas Reptilia* means non-avian, non-mammalian amniotes. (Tudge, p.85)
Evolution of the reptiles
Young American Alligator
Georgetown, South Carolina
Several thousand fossil species showing a clear smooth transition from the ancestors of reptiles to present-day reptiles exist.
The first true "reptile" or Amniotes are categorized as Anapsids, having a solid skull with holes only for nose, eyes, spinal cord, etc. Turtles are believed by some to be surviving Anapsids, as they also share this skull structure, but this point has become contentious lately, with some arguing that turtles reverted to this primitive state in order to improve their armor. Both sides have strong evidence, and the conflict has yet to be resolved.
Shortly after the first reptiles, two branches split off. One group, the Synapsida, had a pair of holes in their skulls behind the eyes, which were used to both lighten the skull and to increase the space for jaw muscles. The other group, Diapsida, possessed the same holes, along with a second pair located higher on the skull. The Synapsida eventually evolved into mammals, while Diapsida split yet again into two lineages, the lepidosaurs (which contain modern snakes, lizards and tuataras, as well as (debatably) the extinct sea reptiles of the Mesozoic) and the archosaurs (modernly represented by only crocodiles and birds, but containing pterosaurs and dinosaurs).
Reptiles have closed circulation via three-chambered heart, two atria and one ventricle, usually one pair of aortic arches, except for Crocodilians, which have a four-chambered heart. In spite of this, due to the fluid dynamics of blood flow through the heart, there is little mixing of oxygenated and deoxygenated blood in the 3-chambered heart.
All reptiles have lungs, and no species has gills. They have two external nostrils, with internal nostrils opening into the oral cavity. The lungs are typically ventilated by a combination of expansion and contraction of the ribs via axial muscles and buccal pumping. Crocodilians also rely on the hepatic piston method, in which the liver is pulled back by a muscle anchored to the pubic bone (part of the pelvis), which in turn pulls the bottom of the lungs backward, expanding them.
No hard palate, so reptiles must hold their breath while swallowing.
Reptiles do not possess a muscular diaphragm as mammals do.
Excretion via paired metanephric kidneys, uric acid main nitrogenous waste product.
Advanced nervous system, compared to amphibians. They have twelve pairs of cranial nerves.
Separate sexes, internal fertilisation.
Amniotic eggs covered with leathery or calcareous shells: Amnion, chorion, and allantois present during embryonic life. No larval stages.
- Tree of Life Website (http://tolweb.org/tree?group=Amniota&contgroup=Terrestrial_Vertebrates)
- The EMBL Reptile Database (http://www.reptile-database.org)
- Colin Tudge (2000). The Variety of Life. Oxford University Press. ISBN 0198604262.