While both the Mesozoic and Paleozoic Eras ended with mass extinctions, the Mesozoic ended at approximately the same time a massive meteor is believed to have hit the Earth. This probably caused drastic climate change, killing off most of the dinosaurs and many other species.
The Paleozoic Era saw the development of higher and complex life forms: plants, fish, reptiles, amphibians and insects. These creatures were mostly adapted to shallow water and swampy land. When Pangaea broke apart and these ecosystems began shrinking, it may have caused a massive die-off. The Mesozoic – the era when dinosaurs, mammals and seeded plants developed – had relatively stable ecosystems that changed slowly. A sudden drastic event, likely an asteroid collision or a massive volcanic explosion, caused rapid climate change to which most animals could not adapt.