Section 1

Origin and Evolution of Reptiles

• An abundance of food, a dry climate, and a mass extinction of other species led to the dominance of reptiles.The Mesozoic era is often called the age of reptiles.

• Several factors contributed to the success of dinosaurs. Legs positioned under their bodies made dinosaurs faster and more agile than other reptiles. Dinosaurs were well adapted to dry conditions. At the end of the Triassic period, a mass extinction reduced competition and allowed dinosaurs to flourish.

• Dinosaurs became extinct about 65 million years ago. Evidence suggests that environmental conditions over

Vocabulary dinosaur (p. 819) asteroid impact hypothesis (p. 821)

amniotic egg (p. 823) amnion (p. 823) yolk sac (p. 823)

many years endangered dinosaur survival, and either a single asteroid impact or multiple asteroid impacts might have triggered a mass extinction event.

• There are four modern orders of reptiles: Chelonia (turtles and tortoises), Crocodilia (alligators and crocodiles), Squamata (lizards and snakes), and Rhynchocephalia (tuataras).

• Three characteristics that contribute to the success of reptiles on land are the amniotic egg, watertight skin, and efficient respiration and excretion.

allantois (p. 823) chorion (p. 823) albumen (p. 823)

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