Mesozoic Era

Dinosaurs were the dominant life forms during the Mesozoic Era.
Tyrannosaurus Rex Skeleton. David Monniaux

Following both the Precambrian Time and the Paleozoic Era on the Geologic Time Scale came the Mesozoic Era. The Mesozoic Era is sometimes called the "age of the dinosaurs" because dinosaurs were the dominant animals for much of the era.

After the Permian Extinction wiped out over 95% of ocean dwelling species and 70% of land species, the new Mesozoic Era began about 250 million years ago. The first period of the era was called the Triassic Period.

The first big change was seen in the types of plants that dominated the land. Most of the species of plants that survived the Permian Extinction were plants that had enclosed seeds, like gymnosperms.

Since most of the life in the oceans became extinct at the end of the Paleozoic Era, many new species emerged as dominant. New types of corals appeared, along with water dwelling reptiles. Very few types of fish remained after the mass extinction, but those that did survive flourished. On land, the amphibians and small reptiles like turtles were dominant during the early Triassic Period. By the end of the period, small dinosaurs began to emerge.

After the end of the Triassic Period, the Jurassic Period began. Most of the marine life in the Jurassic Period stayed the same as it was in the Triassic Period. There were a few more species of fish that appeared, and toward the end of the period, crocodiles came into being.

The most diversity occurred in plankton species.

Land animals during the Jurassic Period had more diversity. Dinosaurs got much bigger and the herbivorous dinosaurs ruled the Earth. At the end of the Jurassic Period, birds evolved from dinosaurs.

The climate changed to more tropical weather with a lot of rain and humidity during the Jurassic Period.

This allowed land plants to undergo a large evolution. In fact, jungles covered much of the land with many conifers in higher elevations.

The last of the periods within the Mesozoic Era was called the Cretaceous Period. The Cretaceous Period saw the rise of flowering plants on land. They were helped along by the newly formed bee species and the warm and tropical climate. Conifers were still really abundant throughout the Cretaceous Period as well.

As for marine animals during the Cretaceous Period, sharks and rays became commonplace. The echinoderms that survived the Permian Extinction, like starfish, also became abundant during the Cretaceous Period.

On land, the first small mammals started to appear during the Cretaceous Period. Marsupials evolved first, and then other mammals. More birds evolved, and reptiles got bigger. Dinosaurs were still dominant, and carnivorous dinosaurs were more prevalent.

At the end of the Cretaceous Period, and the end of the Mesozoic Era, came another mass extinction. This extinction is generally called the K-T Extinction. The "K" comes from the German abbreviation for Cretaceous, and the "T" is from the next period on the Geologic Time Scale - the Tertiary Period of the Cenozoic Era.

This extinction took out all dinosaurs, except birds, and many other forms of life on Earth.

There are different ideas as to why this mass extinction occurred. Most scientists agree it was some sort of catastrophic event that caused this extinction. Various hypotheses include massive volcanic eruptions that shot dust into the air and caused less sunlight to reach the surface of the Earth causing photosynthetic organisms like plants, and those who depended on them, to die off slowly. Some others believe a meteor hit causing the dust to block the sunlight. Since plants and animals that ate plants died off, this caused top predators like carnivorous dinosaurs to also perish.