600 million years ago: Expanding Habitats. 540 million years ago: The Co-evolution of Planets and Insects 550-540 million years ago: The Evolution of Vertebrates 260-250 million years ago: The Largest Mass extinction of the Phanerozoic Eon. Collision with a Dark nebula In the early Paleozoic era, the salt concentration in the ocean had decreased to about twice the current level, and the habitat of organisms spread into the ocean. After vertebrates were born, they evolved over time from fish into amphibians, reptiles, and then dinosaurs and mammals. And, the co-evolution of plants and insects also progressed on land. At the end of the Paleozoic era, Paleozoic animals became extinct because of the collision between the dark nebula and the solar system.