Reptile

Reptiles, in their traditional rather than cladistic definition, constitute a group of ectothermic tetrapods characterised by shelled amniotic eggs and a predominantly terrestrial mode of life. This group includes turtles, crocodilians, lizards, snakes, and the tuatara. Although modern evolutionary biology redefines their boundaries, the traditional concept of Reptilia remains widely used in general zoology, ecology, and herpetology.
Reptiles share a suite of morphological and physiological traits enabling survival in diverse terrestrial and aquatic environments. Around 12,000 living species had been formally described by 2023, representing a wide array of ecological niches and evolutionary histories. Their study is closely linked with that of amphibians, forming the combined discipline of herpetology.

Traditional Characteristics and Diversity

Traditional reptiles are defined by several hallmark features: ectothermy, reliance on external heat sources; keratinised skin, which reduces water loss; and amniotic eggs, which allow reproduction away from water. Unlike amphibians, reptiles lack an aquatic larval stage, and their young are essentially miniature adults at hatching or birth.
Modern reptiles comprise four principal orders:

• Testudines: turtles and tortoises, distinguished by their bony shells.
• Crocodilia: crocodiles, alligators, caimans, and gharials, representing semi-aquatic predators with robust skulls and powerful tails.
• Squamata: the largest order, encompassing lizards, snakes, and amphisbaenians.
• Rhynchocephalia: represented today only by the tuatara of New Zealand.

Reptiles vary greatly in body size and morphology. The tiny gecko Sphaerodactylus ariasae may reach only a few centimetres in length, whereas the saltwater crocodile Crocodylus porosus can exceed several metres and is among the largest living reptiles. Many species are oviparous, but a notable number of squamates exhibit viviparity, sometimes accompanied by placental structures that nourish developing embryos internally.

Evolutionary Origins and Major Lineages

The earliest reptile-like amniotes arose during the Carboniferous period, evolving from reptiliomorph tetrapods that increasingly adapted to dry land. One of the earliest recognised true reptiles, Hylonomus, lived during the Late Carboniferous in what is now Nova Scotia and resembled a small lizard.
During the Permian period, the two major evolutionary branches of traditional reptiles separated:

• Archosauromorpha, giving rise to crocodilians, dinosaurs, birds, and their extinct relatives.
• Lepidosauromorpha, the lineage that includes modern lizards, snakes, and the tuatara.

Reptiles once dominated terrestrial ecosystems with a diversity of groups now extinct, such as pterosaurs, plesiosaurs, ichthyosaurs, and non-avian dinosaurs. Many of these lineages disappeared during mass extinction events, particularly the Cretaceous–Palaeogene event, which drastically reshaped vertebrate life.

Reproduction and Development

As amniotes, reptiles produce eggs surrounded by protective membranes that facilitate gas exchange and prevent desiccation, enabling reproduction away from water. Most species lay eggs in sheltered environments, though their incubation strategies vary widely.
Viviparity evolved multiple times, particularly in squamates. In these forms, embryos develop within the mother, often supported by placental analogues. This adaptation is particularly common in colder climates, where internal gestation offers thermal advantages.

Early Taxonomy and Medieval Perspectives

During the Middle Ages in Europe, the category of “reptile” was loosely defined and included a wide range of egg-laying and crawling creatures, along with mythical animals. Works such as Vincent of Beauvais’s Mirror of Nature grouped reptiles with amphibians, worms, and fantastical beings.
By the 18th century, systematic taxonomy began to formalise these groupings. Carl Linnaeus classified reptiles and amphibians together in the class Amphibia in the 10th edition of Systema Naturae. The terms “reptile” and “amphibian” were used interchangeably in many languages, with “reptile” (from Latin repere, to creep) especially favoured in France. Josephus Nicolaus Laurenti later refined the term Reptilia, though he too included species now regarded as amphibians. This broad grouping laid the foundation for the discipline of herpetology.

Nineteenth-Century Developments

In the early 19th century, zoologists recognised that reptiles and amphibians represented distinct lineages. Pierre-André Latreille established Batracia for amphibians in 1825. Thomas Henry Huxley, working with Richard Owen, expanded the definition of Reptilia to include many extinct fossil reptiles, such as dinosaurs and synapsids like Dicynodon. Huxley also introduced the terms Sauropsida and Ichthyopsida to distinguish major vertebrate divisions.
Ernst Haeckel later emphasised reproductive adaptations, notably the amniotic egg, as a key distinction among vertebrates. His concepts influenced Edwin Stephen Goodrich, who in 1916 formalised the division between Sauropsida (reptiles and birds) and Theropsida (mammals and their extinct relatives). Goodrich also proposed that both derived from an ancestral group he termed Protosauria, which encompassed early reptile-like tetrapods.
D. M. S. Watson in 1956 revised this system, arguing that Goodrich’s ancestral group had already split into sauropsids and theropsids early in amniote evolution. He further refined Sauropsida to include numerous extinct groups, such as procolophonians, eosuchians, ichthyosaurs, turtles, squamates, sphenodontians, crocodilians, and archosaurs.

Skull Structure and Historical Classification

A major 19th- and 20th-century classification system divided reptiles into subclasses based on the number of temporal fenestrae, openings in the skull behind the eyes:

• Anapsida: no fenestrae, including early reptiles and traditionally turtles.
• Synapsida: one lower fenestra, comprising pelycosaurs and therapsids.
• Euryapsida/Parapsida: one upper fenestra, seen in various marine reptiles such as plesiosaurs and ichthyosaurs.
• Diapsida: two fenestrae, a group including lizards, snakes, crocodilians, dinosaurs, and pterosaurs.

This system, widely used throughout the 20th century, has largely been abandoned due to advances in molecular biology and phylogenetics. In particular, the anapsid skull condition, once considered a defining trait of turtles, is now understood to be secondarily derived, with turtles placed firmly within Diapsida, closely related to archosaurs.

Phylogenetic Redefinition in Modern Biology

Modern cladistics regards the traditional Class Reptilia as paraphyletic, because it excludes mammals and birds, although both evolved from within the larger reptilian radiation. Birds, as members of Dinosauria, share a more recent common ancestor with crocodilians than with other traditional reptiles, leading many evolutionary biologists to include birds within a more expansive Reptilia.
Alternatively, some authors prioritise the clade Sauropsida, which encompasses all amniotes more closely related to modern reptiles than to mammals. Both perspectives reflect the principle that taxonomic groups should ideally represent complete evolutionary lineages.