Reptilia

Reptilia is a class of amniote vertebrates that includes turtles (Testudines), lizards and snakes (Squamata), and crocodilians (Crocodylia). The tuataras (Sphenodontia) are closely related and are sometimes treated as a separate lineage within Reptilia. Reptiles are characterized by dry, scaly skin that minimizes water loss, and by amniotic eggs or birth on land. They are primarily ectothermic, relying on environmental heat and behavioral strategies such as basking to regulate body temperature.

Most reptiles respire with lungs. Reproduction is largely oviparous (egg-laying), but several lizard and snake species are viviparous or ovoviviparous. Heart anatomy varies: lepidosaurs (lizards and snakes) and turtles typically have three chambers, while crocodilians possess a four-chambered heart, reflecting different evolutionary paths.

In taxonomy, traditional classifications placed reptiles in a single class that sometimes included birds. In modern cladistics, Reptilia is often defined to exclude birds and is part of a larger reptile–bird lineage known as Sauropsida. The group comprises roughly 11,000 to 12,000 living species and occupies a wide range of ecological roles, from herbivores to apex predators.

Fossil reptiles first appear in the late Carboniferous, with extensive diversification during the Mesozoic Era. Today, reptiles inhabit diverse habitats worldwide, including deserts, forests, freshwater systems, and oceans, and they play integral roles in many ecosystems.