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Which animal groups are included among living reptiles?

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A painted turtle (Chrysemys picta) with a dark shell and red and yellow markings rests on a rock near water, surrounded by green aquatic plants. — reptile Painted turtle (*Chrysemys picta*).

Living reptiles are members of the class Reptilia, which encompasses air-breathing vertebrates characterized by internal fertilization, amniotic development, and epidermal scales.1

The major groups of living reptiles include:

Turtles (order Testudines)1
Tuatara (order Rhynchocephalia)1
Lizards and snakes (order Squamata)1
Crocodiles (order Crocodylia)1

While birds (class Aves) are technically a lineage of reptiles that evolved within the Archosauria, they are traditionally treated as a separate group. Additionally, scientific evidence indicates that snakes are a specialized group that arose within the lizard lineage.12