Which is the difference between the Paleozoic and Mesozoic era

The major difference is time, Paleo means old and Meso means Medium or middle. From Paleozonic era nly fish and marine critters are alive and the rest of the life forms are extincted and from Mesozoic era about half the life forms are still around like crocodiles, fish, insects, ferns, amphibians, some trees, reptiles. Another half or thereabouts are extinct.