**Evolution of Reptiles**:
– Proto-reptiles originated around 312 million years ago
– Eureptiles emerged during the Permian period
– Archosauromorpha and Lepidosauromorpha lineages diverged during the Permian period
– Many extinct reptile groups due to mass extinction events
– Modern non-bird reptiles inhabit all continents except Antarctica
– Turtles considered diapsids, some placed in Archosauromorpha
– Reptiles evolved from advanced reptiliomorphs
– Oldest known reptile is Hylonomus
– Stem-reptiles adapted better after Carboniferous Rainforest Collapse
– Reptiles dominated communities in the Mesozoic era
**Classification and Taxonomy**:
– Linnaean taxonomy groups reptiles under the class Reptilia
– Modern cladistic taxonomy considers Reptilia paraphyletic
– Different classification schemes proposed over time
– Vertebrates divided based on reproductive strategies
– Cladogram illustrating reptile family tree
– Genetic and fossil data combined for classification
– Major groups include Archosauriformes and Pantestudines
**Skull Types and Phylogenetics**:
– Reptiles split into subclasses based on skull characteristics
– Anapsida, Synapsida, Euryapsida, and Diapsida skull types
– Birds closer to crocodilians than to other reptiles
– Reptilia not a clade, defined by lack of fur or feathers
– Vertebrate paleontologists adopting phylogenetic taxonomy
**Physiology and Metabolism**:
– Reptiles exhibit cold-bloodedness and rely on external heat sources
– Reptilian biochemistry requires enzymes efficient over a wide temperature range
– Optimum body temperature for reptiles varies by species
– Thermoregulation and circulation play crucial roles in reptile physiology
– Reptiles have low resting metabolism, requiring less fuel
**Reproductive and Sensory Systems**:
– Reptiles have diverse reproductive strategies, including oviparity, viviparity, and ovoviviparity
– Some reptiles exhibit temperature-dependent sex determination
– Courtship behaviors vary among reptile species
– Reptiles possess various sensory adaptations for survival
– Many reptiles rely on chemical cues for communication and foraging
Reptiles, as commonly defined, are a group of tetrapods with an ectothermic ('cold-blooded') metabolism and amniotic development. Living reptiles comprise four orders: Testudines (turtles), Crocodilia (crocodilians), Squamata (lizards and snakes), and Rhynchocephalia (the tuatara). As of May 2023, about 12,000 living species of reptiles are listed in the Reptile Database. The study of the traditional reptile orders, customarily in combination with the study of modern amphibians, is called herpetology.
| Reptiles Temporal range: Pennsylvanian–Present,
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|---|---|
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| Reptilians by saurian clade listed in top-to-bottom order: six lepidosaurs and six archelosaurs. | |
Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Sauropsida |
| Class: | Reptilia Laurenti, 1768 |
| Extant groups | |
See text for extinct groups. | |
Reptiles have been subject to several conflicting taxonomic definitions. In Linnaean taxonomy, reptiles are gathered together under the class Reptilia (/rɛpˈtɪliə/ rep-TIL-ee-ə), which corresponds to common usage. Modern cladistic taxonomy regards that group as paraphyletic, since genetic and paleontological evidence has determined that birds (class Aves), as members of Dinosauria are more closely related to living crocodilians than to other reptiles, and are thus nested among reptiles from an evolutionary perspective. Many cladistic systems therefore redefine Reptilia as a clade (monophyletic group) including birds, though the precise definition of this clade varies between authors. Others prioritize the clade Sauropsida, which typically refers to all amniotes more closely related to modern reptiles than to mammals.
The earliest known proto-reptiles originated around 312 million years ago during the Carboniferous period, having evolved from advanced reptiliomorph tetrapods which became increasingly adapted to life on dry land. The earliest known eureptile ("true reptile") was Hylonomus, a small and superficially lizard-like animal. Genetic and fossil data argues that the two largest lineages of reptiles, Archosauromorpha (crocodilians, birds, and kin) and Lepidosauromorpha (lizards, and kin), diverged during the Permian period. In addition to the living reptiles, there are many diverse groups that are now extinct, in some cases due to mass extinction events. In particular, the Cretaceous–Paleogene extinction event wiped out the pterosaurs, plesiosaurs, and all non-avian dinosaurs alongside many species of crocodyliforms, and squamates (e.g., mosasaurs). Modern non-bird reptiles inhabit all the continents except Antarctica.
Reptiles are tetrapod vertebrates, creatures that either have four limbs or, like snakes, are descended from four-limbed ancestors. Unlike amphibians, reptiles do not have an aquatic larval stage. Most reptiles are oviparous, although several species of squamates are viviparous, as were some extinct aquatic clades – the fetus develops within the mother, using a (non-mammalian) placenta rather than contained in an eggshell. As amniotes, reptile eggs are surrounded by membranes for protection and transport, which adapt them to reproduction on dry land. Many of the viviparous species feed their fetuses through various forms of placenta analogous to those of mammals, with some providing initial care for their hatchlings. Extant reptiles range in size from a tiny gecko, Sphaerodactylus ariasae, which can grow up to 17 mm (0.7 in) to the saltwater crocodile, Crocodylus porosus, which can reach over 6 m (19.7 ft) in length and weigh over 1,000 kg (2,200 lb).