Reptiles are the animals in class Reptilia that have dry scaly skin and lay soft-shelled eggs on land. They are all amniotes that have paired limbs, lungs for respiration, and internal fertilization.
They are classified by a combination of traits, including skull morphology and the positioning of temporal fenestrae. The surviving members of the order Crocodilia, which include modern alligators and crocodiles, are distinguished by their flattened snouts and tough skin.
Crocodilian
Crocodilian reptiles, the order Crocodylia, are semiaquatic carnivores. Their bodies are covered with non-overlapping scales, and they lay shelled eggs. They are cold-blooded, meaning they cannot control their internal temperatures and rely on external sources to regulate their body temperature.
Their eyes are positioned high on each side of the head, turrets that provide 270deg widescreen coverage and 25deg of binocular overlap directly ahead to accurately judge distances. They have color vision by day, and at night they can see by the light reflected off of a layer behind the retina called the tapetum lucidum. They also have highly developed hearing.
During hunting, crocodiles use their tails to herd fish into shallow water so they can be scooped up with a sweep of open jaws. In addition, their tails can push the body vertically out of the water, ideal for catching prey flying over the surface or hanging from low branches.
Like all reptiles, crocodiles can breathe when submerged by exposing their nostrils. Inhaled air enters sinuses separated from the mouth by a bony secondary palate, and any chemicals in the air can be detected by sensory epithelial cells in their skin. They can also move on land in a low walk and a high run, and some smaller species can gallop. Crocodilian reptiles are the world’s deadliest predators, responsible for hundreds of human deaths each year in Africa and Southeast Asia.
Chelonian
Turtles and tortoises belong to a class called Reptilia, or more commonly known as reptiles. They are cold-blooded animals that have scales and a three chambered heart. They also have lungs rather than gills for respiration and can move on land or water. This group is considered one of the oldest living groups and appeared before snakes and crocodiles.
The classification of Chelonian reptiles has changed over time. In the past, turtles were placed in a separate order called Testudinata, but now they are classified under Reptilia along with snakes and crocodiles. The new classification is based on the skull and other features.
These reptiles are characterized by the presence of suborbital fenestrae (paired openings in the skull that are located ventral to the orbital region). They also have a short trachea with complete cartilaginous rings and a partial hard palate. These features help distinguish chelonians from the rest of the vertebrates.
These reptiles are also distinguished from other vertebrates by their ability to use the dive reflex. This allows them to breathe without having to surface and helps buffer lactic acid accumulation in the body. They also have the highest level of bicarbonate in their bodies, which is important for anaerobic breathing. They do this by breathing through the mouth, with air entering through external nares and passing through the partial hard palate to the pharynx.
Archosauria
The archosauria are the so-called “ruling reptiles” of the Mesozoic. This monophyletic group includes crocodilians, birds and extinct dinosaurs and pterosaurs. It is a subclass of the diapsid reptiles and contains the orders Crocodilia, Ornithischia, Saurischia, Pterosauria, and Thecodontia. Its name derives from the Greek words arkho, to rule, and saura, lizard.
The earliest true archosaurs evolved in the late Permian and early Triassic periods. They shared a number of distinct features with synapsids, including a fifth toe on the foot (homologous to your pinky) and a specialized ankle joint. Their skulls also featured a pair of holes, or fenestrae, in front of the eyes that are sometimes called antorbital fenestrae, and were more upright in their stance.
Later, the archosauria separated into two major lineages. The one that became the lepidosauria – containing modern snakes, lizards and tuataras, their direct ancestors, and the extinct sea reptiles such as Champsosauridae and Euparkeria – and the other, which today contains only crocodilians, birds, dinosaurs and pterosaurs, was further differentiated by a lowered snout, serrated teeth and the calcaneal tuber, a bony process on the heel bone that provides attachment points for some of the lower leg flexor muscles.
Some lepidosaurids were ovoviviparous, laying eggs that were fertilized inside the mother, while others were viviparous and gave birth to live young. The earliest known oviparous lepidosaur was Dinocephalosaurus, an ancestor of the tyrannosaur dinosaurs.
Lepidosauria
Reptiles (Class Reptilia) are a diverse group of vertebrates that includes turtles (Testudines), snakes and lizards (Lepidosauria), and the dinosaurs. Like other amniotes, reptiles are ectotherms, animals that get their heat from the environment. They have paired limbs with 5 toes, a centralized nervous system, lungs instead of gills for respiration, and a 3 or 4 chambered heart. Their eggs are covered with a leathery or calcium-based shell that is partially or completely lost in some species that give birth to live young.
These features are what make reptiles different from mammals, which also belong to Class Mammalia. Traditionally, phylogenetic (cladistic) systematics of reptiles has been based on the morphological distinctness of groups and not on their ancestry. However, modern developmental palaeobiology is now able to generate evolutionary histories of organogenesis that can be used for ancestral state reconstruction. Such histories are highly consistent with both morphological and molecular hypotheses of squamate phylogeny.
Lepidosauria has a global distribution and currently comprises >9000 species that fill a variety of ecological niches. However, the fossil record for this clade extends back only to the Late Triassic, and some of the more derived taxa such as Brachyrhinodon and Diphydontosaurus appear to be nested within a less derived rhynchocephalian clade (Rhynchocephalia) [5-10]. Thus, the earliest history of Lepidosauria remains unknown. However, cladistic analyses consistently nest stout-toothed taxa such as Sphenodon punctatus, the New Zealand tuatara, amongst rhynchocephalian genera such as Rhynchocephalosaurus, suggesting that a rapid diversification was the norm for this group.