Reptile

• Order Crocodylia (crocodiles and alligators): 23 species
• Order Rhynchocephalia (tuataras from New Zealand): 2 species
• Order Squamata (lizards, snakes and amphisbaenians {"worm-lizards"}): approximately 7,600 species
• Order Testudines (turtles): approximately 300 species

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, amphisbaenians 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)

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

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).

Systems

Circulatory

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 three-chambered heart. Furthermore, the blood flow can be altered to either shunt deoxygenated blood to the body, or oxygenated blood to the lungs, which gives the animal greater control over its blood flow, allowing more effective thermoregulation and longer diving times for aquatic species.

Respiratory

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

Nervous

Advanced nervous system, compared to amphibians. They have twelve pairs of cranial nerves.

Sexual

Separate sexes, internal fertilisation.

Amniotic eggs covered with leathery or calcareous shells: Amnion, chorion, and allantois present during embryonic life. No larval stages.

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