Evolution Of Dinosaurs

Beginning about 235 million years ago, dinosaurs, a group of extinct reptiles, dominated life on land for roughly 150 million years. They evolved from thecodonts, an extinct group of crocodilelike reptiles. Dinosaurs include a wide variety of reptiles that were adapted to very different environments. One factor that affected dinosaur evolution was the movement of the continents. Early in the Mesozoic era, Pangaea started to break apart. The climates of the separate landmasses changed. Some species of dinosaurs could not adapt and became extinct, while new kinds flourished.

figure 41-1

When dinosaurs first appeared, all of Earth's landmasses were joined in a single supercontinent called Pangaea, shown above.

figure 41-1

When dinosaurs first appeared, all of Earth's landmasses were joined in a single supercontinent called Pangaea, shown above.

Mass extinction

Cretaceous period 146-66 mya

Cretaceous period 146-66 mya

Jurassic period 200-146 mya

Jurassic period 200-146 mya

Mass extinction

Triassic period 251-200 mya

Triassic period 251-200 mya

Mass extinction

Permian period 299-251 mya

Carboniferous period 359-299 mya

(mya = million years ago)

figure 41-2

The evolution of reptiles has been marked by three mass extinction events. For more information about the geologic time scale, see the Appendix at the back of this textbook.

Triassic Dinosaurs

The oldest known dinosaur fossils are in rocks from the early Triassic period, about 235 million years ago. By the end of the Triassic, small, carnivorous dinosaurs, were very common and had largely replaced the thecodonts.

There are at least three reasons why dinosaurs were so successful. First, legs positioned directly under the body provided good support for the dinosaur's body weight, enabling dinosaurs to be faster and more agile runners than the thecodonts. Second, dinosaurs were well adapted to the dry conditions found in Pangaea during the late Triassic period. Finally, at the end of the Triassic period, another mass extinction wiped out about 80% of living species, including the thecodonts and the last of the large amphibians. Recent research suggests that asteroid impacts in what are now Canada, France, the Ukraine, and Minnesota occurred almost simultaneously and contributed to this mass extinction event. This event marked the end of the Triassic period, outlined in Figure 41-2. Reduction in competition for resources may have allowed dinosaurs to flourish.

Jurassic and Cretaceous Dinosaurs

The Jurassic period, graphed in Figure 41-2, is considered the golden age of dinosaurs because of the variety and abundance of dinosaurs that lived during this time. They included the largest land animals of all time, the sauropods (SAWR-uh-PAHDZ) shown in Figure 41-3a.

By the late Jurassic period, a new type of dinosaur had evolved. The carnivorous theropods (THER-uh-PAHDZ) shown in Figure 41-3c stood on two powerful legs and had short arms. Their large heads were equipped with sharp teeth, and each foot had sickle-shaped claws used for ripping open prey. Theropods preyed on the large herbivorous dinosaurs and were the dominant terrestrial predators until the end of the Cretaceous (kruh-TAY-shus) period.

figure 41-3

Sauropods (a) were herbivores. Some herbivores, like triceratops (b), had armor to defend themselves against carnivorous (meat-eating) dinosaurs like the theropods (c).

figure 41-3

Sauropods (a) were herbivores. Some herbivores, like triceratops (b), had armor to defend themselves against carnivorous (meat-eating) dinosaurs like the theropods (c).

W (c)

Dinosaur Diversity

Dinosaurs were not limited to terrestrial habitats. Some Mesozoic reptiles called pterosaurs (TER-uh-SAWRS), shown in Figure 41-4a, evolved the ability to fly. Several other groups of reptiles, including the ichthyosaurs (IK-thee-uh-SAWRS) and ple-siosaurs, lived in the oceans. Ichthyosaurs, illustrated in Figure 41-4b, were sleek aquatic reptiles that resembled modern bottlenose dolphins. Plesiosaurs had long, flexible necks and compact bodies.

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