Which is a difference between the Paleozoic and Mesozoic era?

Explanation:

Animals lived in the water during the Mesozoic and on land in the Paleozoic. Animal complexity increased during the Paleozoic while the first flowering plants appeared in the Mesozoic era. ...

The Paleozoic era, not the Mesozoic era, had the first dinosaurs. The first mammals emerged in the Paleozoic era, not the Mesozoic era. The Mesozoic era, not the Paleozoic era, had the first animals with shells.

what's the

several tissues working together form an organ, and several organs together make up an organisms.

the group of cell that carries out special jobs are tissue.