The Paleozoic Era

Artist rendition of a dimetrodon of the Permian Era
Rendition of a dimetrodon of the Permian Era

Geologically speaking, it was the breakup of the supercontinent Pannotia that led to the Cambrian Explosion. The Earth's surface land mass was then spread among a number of smaller continents for the first part of the Paleozoic Era but by the end of the era, plate tectonics had pushed all the land masses together again into a supercontinent named Pangaea. At the height of that concentration of land, the Permian-Triassic Extinction Event was triggered. Geologists have set the end of the Paleozoic Era and the beginning of the Mesozoic Era at the end of that extinction event, about 252.17 million years ago.

Utah's Fossil Mountain is composed of Ordovician shales and limestones
Fossil Mountain, Utah: Composed of shales and limestones from the Ordovician Period
Upper image courtesy of Nobu Tamura, CCA 3.0 License
Lower images are in the public domain