So we have arrived in our time trip to the Devonian period ( 416-359 Ma). This is known as the Age of Fishes because at no other point in time were there so many groups of fishes present in Earth’s oceans. Armored Placoderms and Ostracoderms, Spiny Acanthodians, early sharks and rays and the birth of the modern bony fish in 2 groups: ray finned ( modern fish) and lobe fish ( coelocanths) swam the seas. Lobe fish gave rise to the 1st modern vertebrates – amphibians.
Corals flourished as the primary marine invertebrates in the shallow Michigan basin. Particularly striking but now extinct genera known as Hexagonaria and Prismatophyllum fossilized into limestones known as Petoskey stones. Water and waves erode the shoreline breaking coral heads from their little catacombs. Polished, they appear on the shores of northern Michigan lakes and rough, if you have the eye in the vineyards. Each hexagon of the Petoskey stone is a coral chamber and the “rays” are the internal walls of the individual chamber. Michigan’s state stone was born.
Flora exploded on land and the 1st forests were established. Primitive trees can be found in shale from the Devonian near Alpena. But all would change again. This period, like so many of the other periods in the Paleozoic, was one of drastic sea level changes. In the early part of the Devonian, most of Michigan was dry. After several million years, sea levels rose again. Mass extinction, the 2nd great extinction on Earth, occurred at the end of the Devonian period eliminating many species of fauna. The cause is uncertain but a slight shift in global temperature, cooling this time is hypothesized.
Thanks to Michigan Geography and Geology by R. Schaetzl, J. Darden and D. Brandt and to the USGS http://geomaps.wr.usgs.gov/parks/pltec/sc390ma.html#reconstruct.