Reptiles are a diverse class of ectothermic vertebrates in the clade Sauropsida, characterized by dry, scaly skin that minimizes water loss, amniotic eggs with leathery shells that enable terrestrial reproduction, and lungs as the primary respiratory organs.[1] They include over 12,500 described species, primarily distributed across four major orders: Squamata (lizards and snakes, comprising about 96% of species), Testudines (turtles and tortoises), Crocodilia (crocodiles, alligators, caimans, and gharials), and Rhynchocephalia (tuataras).[2] While traditionally defined as excluding birds, modern cladistic taxonomy often includes Aves (birds) within Reptilia as descendants of theropod dinosaurs, though common usage refers to non-avian reptiles.[3]Reptiles first appeared during the Carboniferous period approximately 320 million years ago, evolving from amphibian-like ancestors as amniotes capable of fully terrestrial life.[4] This adaptation allowed them to dominate Mesozoic ecosystems, with iconic groups like dinosaurs representing a major radiation before the Cretaceous-Paleogene extinction event 66 million years ago, which wiped out non-avian dinosaurs but spared many modern lineages.[5] Today, reptiles inhabit every continent except Antarctica, thriving in diverse environments from deserts and rainforests to oceans and freshwater systems, with some species like sea turtles being fully aquatic.[6]As ectotherms, reptiles regulate body temperature primarily through behavioral means, such as basking in sunlight or seeking shade, which influences their activity patterns and distributions.[7] Their scales, made of keratin, provide protection and prevent desiccation, while many species exhibit remarkable adaptations like venom in snakes or powerful jaws in crocodilians.[8] Reproduction is typically oviparous, with eggs laid in nests, though some give live birth (viviparity) or retain eggs internally (ovoviviparity).[9] Despite their resilience, many reptile species face threats from habitat loss, climate change, and human activities, with 21.1% assessed as threatened with extinction (as of the 2022 global assessment).[10]
Definition and Classification
Defining Characteristics
Reptiles, as a clade of sauropsid amniotes, are defined by several key biological traits that distinguish them from other vertebrates, including their reliance on external heat sources for metabolic regulation and adaptations for terrestrial life.[11] Over 12,000 extant species exist as of November 2025, encompassing primarily terrestrial forms alongside aquatic species like sea turtles and aerial ones such as flying lizards.[12]Reptiles are ectotherms, meaning their body temperature is primarily determined by environmental conditions rather than internal metabolic heat production. To maintain optimal physiological function, they employ behavioral thermoregulation, such as basking in sunlight to raise body temperature or burrowing in cooler substrates to avoid overheating. These strategies allow reptiles to exploit diverse habitats while conserving energy compared to endothermic vertebrates.[13][14][15]A hallmark of reptilian reproduction is the amniotic egg, which features a leathery or calcified shell enclosing the embryo and four extraembryonic membranes: the chorion for gas exchange and protection, the amnion forming a fluid-filled sac to cushion the embryo, the allantois for waste storage and respiration, and the yolk sac providing nutrients. This structure enables development on land without desiccation or dependence on aquatic environments, marking a pivotal adaptation for terrestrial independence.[1][16]The skin of reptiles consists of keratinized scales, scutes, or plates derived from the epidermis, forming a tough, overlapping barrier that minimizes water loss and offers mechanical protection against predators and abrasion. The α-keratin in this epidermal layer provides waterproofing, preventing dehydration in arid conditions while allowing flexibility for movement.