Over 8,700 reptile species are known today, including turtles, crocodilians, and snakes. These tetrapod animals have evolved a wide suite of adaptive features, enabling them to live in many types of habitats.
In traditional taxonomy, reptiles were organisms that could not regulate their body temperature (ectothermic) and possessed scales. However, phylogenetic analyses, which group organisms based on how similar they are GENETICALLY, show that birds, crocodiles, and lepidosaurs are more closely related than they are to lizards.
Class Reptilia
Class Reptilia is the group of ectothermic vertebrates that includes all living turtles, snakes, lizards and crocodiles as well as many extinct species. They are all members of the Animal Kingdom’s phylum Chordata, and have some similarities to amphibians, but are dry-skinned (not slimy) and covered in scales or a shell, and their limbs end in claws rather than toes. They are also unable to generate their own body heat, and instead must regulate their core temperature by basking in the sun or hiding under cover.
They lay encased eggs with hard or leathery shells and reproduce by internal fertilisation. Some are ovoviviparous, keeping the eggs inside their bodies until they hatch, while others are viviparous, giving birth to live offspring. They are typically diurnal, and have better visual depth perception than amphibians and mammals, although they do not have external ears.
Most modern reptiles are herbivorous, but a few, such as the green iguana of Central and South America, and the chuckwalla of North Africa and southwestern United States, are carnivorous or omnivorous. They feed on insects, mollusks, birds, fish and other reptiles. They are typically active during the warmer months of the year, and can be found soaking up the sunshine on warm rocks or in the water to raise their body temperatures. In the colder months they may hibernate.
Sub-class Chelonia
Chelonia is a sub-class of reptiles that contains tortoises and sea turtles. There are around 360 living species, most of them freshwater turtles and the majority of these belong to the genus “Testudines”.
The green sea turtle (Chelonia mydas), also known as the black sea turtle, is a large, marine, warm-water, omnivorous sea turtle in the family Cheloniidae. It is the only species in the genus Chelonia and is named for the green fatty deposits found under its carapace.
This genus was described by Linnaeus in 1758. The name comes from the Greek words chelone which means tortoise and mydas which means water.
Despite having passed through several major extinction filters over the course of their long history, the Chelonians have continued to thrive in this world. They are the ultimate survivors and are one of the most adaptable organisms.
They have evolved a unique adaptation that allows them to float in the water by filling their air sacs with air from the surface. This allows them to move around even if their body is in the water and it also protects them from shark attacks. It is this unique trait that has enabled them to survive so many times throughout the centuries. In an early idea for the 2009 Easter Special which would later become Planet of the Dead, the Chelonians were to make their debut as part of a war with other aliens.
Sub-class Archosauria
The sub-class Archosauria includes all dinosaurs and pterosaurs (flying reptiles) along with their close relatives. They are called archosaurs because of the two openings in their skulls that resemble the “roof” of a crocodile. This group radiated in the Triassic period shortly after the end-Permian mass extinction and included bizarre hippo-size herbivores like the rhynchosaurs, evil-looking terrestrial predators such as the erythrosuchids and proterosuchids and the true crocodile ancestors of Euparkeria. Several important new features evolved in these groups such as the fifth toe in their feet (homologous with your pinky toe) which was reduced, the palate (roof of the mouth) bore extra teeth and a special ankle joint that developed differently in Pseudosuchia (the crocodilian lineage) and Ornithosuchia (birds).
Living birds and crocodylians (and extinct non-avian dinosaurs and pterosaurs) comprise this crown clade, Archosaurus. Other dinosaurs and a few other lepidosaurs lie further down the archosaur branch and are placed in a morphologically distinct sub-class Archosauromorpha. Older morphological definitions of archosaurs (including the crown-group) relied on such diagnostic features as antorbital fenestrae in the skull, serrated teeth and an upright stance.
Sub-class Lepidosauria
Lepidosauria is the sub-class that contains lizards, snakes and amphisbaenians. This group is monophyletic (as a single clade) and sister to Archosauria, which includes birds and crocodilia. The tuatara is also a member of this group, although it is not closely related to the other lepidosauriforms.
The tuatara is the only living representative of this group, and it lives in New Zealand. It is a superficially lizard-like reptile with a very long neck and special features that enable it to hunt and kill prey that is much larger than itself. It has a strong sense of smell, specialized organs to detect chemicals and even the ability to ingest air through its skin.
Like all reptiles, tuataras are amniotes and lay eggs. Their eggs have a leathery outer shell that protects them until they are fertilized. The fetuses then develop inside the female. The embryos are then deposited in the water where they can grow into a fully-grown adult.
The tuatara is an example of a basal lepidosauriform, which is a very diverse group that contains many types of lizards and snakes. Its members have large keratinous scales, a flexible skull with a single occipital condyle and an absence of a post-parietal or tabular bone; and a fused astragalus and calcaneum. This clade is close to the Early Triassic kuehneosaurid genus Marmoretta and the Late Triassic sphenodontid genus Rhynchocephalia, but its exact affinity with these groups has not been established.