So, big news! We are moving into the Paleozoic era and specifically, the Ordovician period. Laurentia is still straddling the equator as shown in the picture and a large percentage is submerged under shallow tropical seas. In fact, most of the continent is below sea level. Earth experiences vast changes in the global sea level, sometimes dropping so that wide scale erosion occurs but generally rising generating epic flooding.
Although land is barren still, life is bubbling in the seas! A rapid burst of evolutionary activity swarmed the waters. Most species were trilobite and brachiopods followed by sea lilies, moss animals, mollusks, nautiloids, stony corals and jawless fishes clad in bony armor. Large limestone deposits form, the contribution of countless shells from marine creatures that live and then die, drifting to the bottom to accumulate and harden. Fossils that can be found from this time period intact or ground down forming fine carbonate ( lime) mud. This compacted to form limestone- CaCo3. Sometimes the calcium was replaced by another element, magnesium forming the mineral dolomite, and changing the limestone into a sedimentary rock called dolostone. Both of these carbonates are quarried to this day in the UP of Michigan.
Towards the end of this period, large continental glaciers formed once again dropping the sea level world-wide due to global cooling. The abundant sea life experienced massive die offs. This was the 1st of the five great mass extinction events on Earth. The fossils are left to tell their story.
Thanks to the USGS and Michigan Geography and Geology by R. Schaetzl, J.Darden and D. Brandt