The post pre-cambrian time in earth's history is the span of years that ranged from 600 to 2 million years ago. The most notable aspect of this period is that shallow seas covered much of the north American continent, seas that would encroach and then recede. These seas were different from earlier ones in that they were home to plants and animals. Each time the seas would spread across the land it would bring with it a new mixture of plant and animal types. Some of the animals developed hard shells and skeletons which today are found in the various sediment layers left behind when the seas subsided. It is thought that these skeletons and shells were developed to protect creatures from ultraviolet radiation, from the stress of waves and currents, and from predator stress that occurred in more crowded ecological niches such as near the shores. Minnesota wasn't as cold during most of this time period. These marine environments were tropical to subtropical and contained corals as evidenced by the fossils.