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Reptiles are a group of four-legged vertebrates (also known as tetrapods) that diverged from ancestral amphibians approximately 340 million years ago. There two characteristics that early reptiles developed that set them apart from their amphibian ancestors and that enabled them to colonize land habitats to a greater extent than amphibians. These characteristics are scales and amniotic eggs (eggs with an internal fluid membrane).
Reptiles are one of the six basic animal groups. Other basic animal groups include amphibians, birds, fish, invertebrates, and mammals.
CrocodiliansPhoto © LS Luecke / Shutterstock.
Crocodilians are a group of large reptiles that includes alligators, crocodiles, gharials, and caimans. Crocodilians are formidable predators with powerful jaws, a muscular tail, large protective scales, streamlined body, and eyes and nostrils that are positioned on top of their head. Crocodilians first appeared about 84 million years ago during the Late Cretaceous and are the closest living relatives of the birds. Crocodilians have changed little in the past 200 million years. There are about 23 species of crocodilians alive today.
The key characteristics of crocodilians include:
- elongated, structurally reinforced skull
- wide gape
- powerful jaw muscles
- teeth set in sockets
- complete secondary palate
- adults provide extensive parental care to young
SquamatesPhoto © Danita Delimont / Getty Images.
Squamates are the most diverse of all the reptile groups, with approximately 7,400 living species. Squamates include lizards, snakes, and worm-lizards. Squamates first appeared in the fossil record during the mid-Jurassic and probably existed before that time. The fossil record for squamates is rather sparse. Modern squamates arose about 160 million years ago, during the late Jurassic Period. The earliest lizard fossils are between 185 and 165 million years old.
The key characteristics of squamates include:
- most diverse group of reptiles
- exceptional skull mobility
TuataraPhoto © Mint Images Frans Lanting / Getty Images.
Tuatara is a group of reptiles that are lizard-like in appearance but they differ from the squamates in that their skull is not jointed. Tuatara was once widespread but today only two species of tuatara remain. Their range is now restricted to just a few islands in New Zealand. The first tuatara appeared during the Mesozoic Era, about 220 million years ago, about the same time the first dinosaurs appeared. The closest living relatives of the tuatara are the squamates.
The key characteristics of tuataras include:
- slow growth and low reproductive rates
- reach sexual maturity at 10 to 20 years of age
- diapsid skull with two temporal openings
- prominent parietal eye on top of the head
TurtlesPhoto © M Swiet Productions / Getty Images.
Turtles are among the most ancient of the reptiles alive today and have changed little since they first appeared some 200 million years ago. They have a protective shell that encloses their body and provides protection and camouflage. Turtles inhabit terrestrial, freshwater, and marine habitats and are found both in tropical and temperate regions. The first turtles appeared more than 220 million years ago during the Late Triassic Period. Since that time, turtles have changed little and it is quite possible that modern turtles closely resemble those that roamed the Earth during the time of the dinosaurs.
The key characteristics of turtles include:
- keratinized plates in place of teeth
- body enclosed in a shell that consists of carapace and plastron
- a keen sense of smell, good color vision, poor hearing
- bury eggs in the ground