Reptiles are ectothermic animals with scales, and there are over 8,200 species, making them more diverse than mammals. They are polyphyletic, and birds, which descended from dinosaurs, are not considered reptiles. Reptiles and synapsids split from each other a long time ago, and there are four living orders of reptiles: Crocodilia, Sphenodontia, Squamata, and Testudines. The relationship between groups of reptiles can be confusing.
Reptiles, also known as Class Reptilia, Class Sauropsida, or simply sauropsids, are ectothermic (“cold-blooded” – although this term is now out of fashion) animals covered in scales. There are over 8,200 species of reptiles, about 50% more than their relatives, the mammals. Reptile is one of the few taxonomic classifications among tetrapods that are polyphyletic, meaning that the group deviates from the convention of including all descendants of a common ancestor. Birds are actually descendants of ancient reptiles, dinosaurs, but they are not considered reptiles. If birds were included in Reptilia, the group would be monophyletic.
Reptiles are one of two groups of amniotes, the other being the synapsids, represented today by mammals. Amniotes feed their embryos using a series of complex membranes. Amphibians, i.e. non-amniotes, have simple eggs that must be laid in water.
Reptiles and mammalian ancestors, synapsids, split from each other a long time ago. The earliest known reptile, Hylonomus, and the earliest known synapsid, Archaeothyris, both lived during the Late Carboniferous Period, about 315 million years ago. Both superficially resembled small lizards, but gave rise to radically different descendants. Since then synapsids and sauropsids have alternated in dominating the Earth, with synapsids ruling from the Carboniferous until the Permian-Triassic extinction event about 251 million years ago. From then on, the dinosaurs began to evolve and the balance tipped again in favor of the reptile. That’s why this period of time was called the Age of Reptiles. After the Cretaceous-Tertiary extinction 65 million years ago, all non-avian dinosaurs were wiped out and synapsids, in the form of mammals, regained the upper hand.
There are four living orders of reptiles: Crocodilia (crocodiles, alligators, caimans and gharials, 23 species), Sphenodontia (tuatara from New Zealand, 2 species), Squamata (lizards, snakes and amphisbenids (“worm-lizards”), about 7,900 species) and Testudines (turtles and tortoises, about 300 species). The tuatara is considered a living relic and is a popular subject of study among the phylogenetic and taxonomic communities.
The relationship between groups of reptiles can be confusing. Testudines either detached very early or are more closely related to the others – we don’t know. Turtles don’t have holes in their skulls, which is similar to some early reptiles, but they may have lost their holes in their skulls more recently. Besides the Testudines, there are two main groups: the archosaurs, which include crocodilians and dinosaurs (including birds, which are descended from dinosaurs), and the lepidosaurs, which includes everything else.
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