The reproduction of the earliest amphibians was similar to that of fish. Females laid eggs in the water and males fertilized them externally. These eggs lacked a protective shell and dried out easily outside aquatic environments.
However, as some groups of amphibians began spending more time on land, one of the most important evolutionary innovations appeared: the amniotic egg. The embryo developed inside protective membranes and was surrounded by amniotic fluid, which protected it from dehydration and physical shock. At the same time, the yolk provided the nutrients necessary for its development.
This evolutionary development allowed organisms to reproduce away from water and opened the way for the complete colonization of land. All reptiles, birds, and mammals belong to the amniotes.
The first organisms to successfully exploit the amniotic egg evolved into the earliest reptiles. Gradually, they developed stronger lungs, tougher skin, better hearing, and a more advanced nervous system compared to amphibians. Their limbs became better adapted for movement on land, allowing them to move farther away from humid coastal environments.
The earliest known reptile is considered to be Hylonomus, which lived about 310 million years ago. It measured approximately 20 centimeters in length including the tail, possessed small sharp teeth, and fed mainly on insects and other small arthropods. Its appearance resembled that of small modern lizards.
The spread of reptiles into different environments led to a great diversity of forms. Although they initially dominated life on land, some groups later adapted once again to aquatic environments in order to exploit the rich food resources of the seas.
During the Mesozoic Era, from about 250 to 66 million years ago, numerous groups of reptiles appeared and dominated the land, seas, and skies. Among the most important groups were the archosaurs, from which crocodiles, pterosaurs, and dinosaurs evolved.
A defining characteristic of reptiles is their skin, which is covered with keratin — a material that forms scales or plates and protects the body from water loss and harsh environmental conditions. In some species, such as snakes, scales also assist in movement.
Most reptiles are ectothermic (“cold-blooded”), meaning that their body temperature depends largely on the surrounding environment. As a result, they consume less energy than warm-blooded animals such as mammals and birds.
The teeth of many reptiles are adapted mainly for catching and holding prey rather than chewing. For this reason, they often swallow their food in large pieces or whole.
Today, the best-known groups of reptiles are turtles, lizards, snakes, and crocodiles.
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