At one time during the Paleozoic era

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During the Paleozoic era, two-thirds of North America was covered by water, with shallow seas and many marine animals living in those waters. This era also saw the development of fish, amphibians, insects, and reptiles. However, mammals did not appear until much later, and prehistoric man did not arrive in North America until the end of the Paleolithic period, long after the Paleozoic era had ended. While there were mountain ranges during the Paleozoic era, they did not cover all of North America.

At One Time During the Paleozoic Era