Southwest Michigan Oval Beach old pier pilings and rocks reveal low water levels in 2012
During the month of December, the ground is usually covered with layers of snow here in Southwest Michigan (USA) and the big lake looks like a frozen tundra. Fossil exploring is usually put on hold till Spring, but 2012 so far has shown to be a mild winter. As a result, the sand along the shores of Lake Michigan is more exposed to strong winds which has actually assisted fossil finding ventures. The moving sands reveal hidden treasures one must partake while it lasts.
Below, are two recent discoveries of extinct corals found on Oval Beach in Michigan’s southwestern tourist town of Saugatuck. Although closely related and most common during the Silurian and Devonian time periods, the two corals derive from different orders by virtue of their skeletal growth patterns (tabulate and rugose) respectfully. Colonial building favosites are the most common form belonging to the tabulate group while the individual horn corals are probably the most well known among the rugose group. But don’t be fooled, the rugose order also included some colonial varieties.
Follow below as I define some of the characteristics of each order and try to guess for yourself which orders these two fossils fall under. Answer at the very end.
The “tabulae” from the order tabulate refers to the horizontal internal partitions visible from a side view of a good specimen. Observe the diagram below of a favosite fossil.
Rugose corals (named for their wrinkly outer skin) had less developed horizontal partitions, but stronger vertical ones. Most rugose corals have septae radiating from the center (like bicycle spokes) when observed in a cross section.
Many tabulate corals can be associated with smaller honeycomb patterns of their corallite structures which housed the jelly-like polyp creatures. Whereas the rugose corals are often associated with larger more hexagonal patterns of their corallites.
Southwester Michigan location of Saugatuck’s Oval Beach
The first Michigan fossil I found on Oval Beach in southwestern Michigan is often referred to as the Petoskey Stone “Hexagonaria, percarinata” named after the northwestern city of Petoskey, Michigan. It’s about the size of a fist and is rather smoothed out from water and sand action, yet it’s a nice large specimen that reveals the entire coral. From the side view, you can clearly see how it was attached to the seafloor by its stem.
Side View of Petoskey Stone
Details of Petoskey Stone Corallites
The other extinct Michigan coral fossil I found on Oval Beach is nicknamed the Charlevoix Stone. It’s smaller in total size, but is especially distinguished by its smaller honeycomb like corallite patterns.
Side view of Charlevoix Stone
The two Michigan cities of Petoskey and Charlevoix lie in close proximity to each other in the NW region of the lower peninsula. Yet, both cousin fossils can be found on the far southern shores of Lake Michigan’s coastline where I found my fossils south of Grand Rapids seen on the map.
Petoskey and Charlevoix are far north of Saugatuck’s Oval Beach
Petoskey and Charlevoix in close vicinity
So, if you haven’t already distinguish which order the Petoskey and Charlevoix Stones belong, I will tell you. The Petoskey belongs to the order of rugose and the Charlevoix is a favosite, (not yet determined which species) from the order of tabulate.