The Mesozoic

The Mesozoic, or era of "middle life", started around 245 million years ago and lasted about 180 million years. During this era, the supercontinent known as Pangea, which formed in the Paleozoic, began to break up into smaller continents. It was also during the Mesozoic that the dinosaurs evolved and dominated, causing the era to be known as the age of reptiles. The Mesozoic is divided into three periods: the Triassic, Jurassic, and Cretaceous. The start of this era (at the beginning of the Triassic period) was marked by a mass extinction that changed marine life drastically. Ninety-five percent of the marine species of the Paleozoic disappeared, allowing new species to evolve in the Triassic. These new marine species were similar to those that currently exist, making life on the ocean floor similar to what it is today. On land, new species were also evolving. The synapsid reptiles (or proto-mammals) were flourishing, and they diversified into a variety of plant- and meat-eating species. But they disappeared in the late Triassic, leaving behind their mammal descendents. These early mammals were not prevalent until the Cretaceous, and even then they were small creatures that survived by hiding from predatory reptilian species. In the late Triassic, the synapsid reptiles were replaced by the ancestors of dinosaurs. By the end of the Triassic, dinosaurs had spread rapidly through the land ecosystems on Earth. Dinosaurs diversified greatly during the following geologic period, the Jurassic. It was during the Jurassic that many of the most famous dinosaur species evolved. The fossil record indicates that the first birds also evolved during this period. In the Cretaceous mammals began to diversify and flowering plants evolved. Flowering plants led to the origin of new species of pollinating insects, which greatly enlarged the diversity of Mesozoic life on land. Tectonic activity during the Cretaceous resulted in the formation of high oceanic ridges, which further separated the continents and caused sea levels to rise. At the end of the Mesozoic we see another mass extinction. Unlike the one marking the start of the era, this one was very sudden. It is believed that a combination of volcanic activity and an asteroid impact around 65 million years ago caused the extinction of the dinosaurs, and many other species that were thriving in the Mesozoic.