Approximately 250 million years ago, at the end of the Permian period, or the beginning of the Triassic period (and thus, the end of the Paleozoic era and the beginning of the Mesozoic era), there was a mass extinction. This extinction eliminated close to 90 percent of all the species present on our planet (extraordinarily close to total extinction on Earth). The extinction was not selective; it eliminated organisms in the oceans and on land, including many invertebrates, armored fish, and reptiles.

The true reasons for the extinction are unknown, although there are several theories. One is that a collision with an asteroid or comet caused dust and debris to fly into the upper atmosphere, cutting off sunlight and radically changing the global climate. Another idea is that the moving continents changed the climate, sea levels, and thus, habitats, causing some species to change and adapt, while others died out. Still another theory focuses on Siberian flood basalts, in which tons of volcanic material erupted over a huge area in Asia toward the end of the Permian period, changing the climate and certain habitats.

Whatever the scenario, species that survived did so by adapting to the ecological niches that became vacant, allowing them to further evolve. After the Permian extinction, and throughout the Mesozoic era, it was reptiles in general, and the dinosaurs specifically, that diversified the most and became the dominate species on the planet.