Adapted from Dinosaurs: The Very Latest Information and Hands-On Activities From the Museum of the Rockies, by Liza Charlesworth and Bonnie Sachatello-Sawyer. A Scholastic Professional Book.
Dinosaurs, as a group of animals, lived on Earth from 245 to 65 million years ago, during the Mesozoic Era. They survived for an amazing 180 million years. Why did these animals thrive? Paleontologists aren't sure, but it is clear that over time dinosaurs evolved and adapted to a variety of ecosystems — living alongside insects, lizards, frogs, crocodiles, flying reptiles, birds, and mammals.
The "Mesozoic Era" (245 to 65 million years ago) is so long that it's divided into three parts: The Triassic, Jurassic, and Cretaceous Periods. During this era, the world went through a series of slow but radical changes.
At the beginning of the "Triassic Period" 245 to 208 million years ago), all of the continents were massed together, forming one supercontinent called "Pangaea." It was during the Triassic Period that dinosaurs first appeared, evolving from their crocodile-like relatives. The oldest-known dinosaurs were fairly small, but their ability to walk with their legs underneath their bodies gave them the advantage of speed over their relatives. By the end of the late Triassic Period, Pangaea began to break up and there were many groups roaming the Earth, including "Plateosaurus," "Procompsognathus," and "Herrerasaurus."
During the "Jurassic Period" (208 to 135 million years ago) the continents continued to break up (Africa began to separate from North America; India began splitting away from Africa and Antarctica). As the continents parted, dinosaurs were forced to adapt to new climates and environments. In the Jurassic Period, huge sauropods, such as "Brachiosaurus" and Apatosaurus" appeared and thrived, as did armored dinosaurs like "Stegosaurus." Birds and mammals also evolved.
By the beginning of the "Cretaceous Period" (135 to 65 million years ago) most of the continents had separated. Lands once joined were now far apart and many were covered by shallow seas. There was even a sea across the middle of North America! During the Cretaceous Period, dinosaurs diversified rapidly — especially the herbivores — as the first flowering plants appeared on Earth. While some of the species adapted to live in the chilly regions of Alaska, others grew accustomed to the desert environments of Central Asia. "Tyrannosaurus rex" and "Triceratops" appeared toward the end of this period and were among the very last dinosaurs alive.
The Dinosaurs' Changing World
During the Triassic Period, all the continents were joined. The Earth had heavy vegetation near costs, lakes, and rivers, but desert in its interior.
During the Jurassic Period, the continents gradually broke apart. The world was warm, moist, and full of green plants.
During the Cretaceous Period, most of the continents had separated. The seasons became more pronounced, and flowering plants were more abundant.