During the month of December, the ground is usually covered with layers of snow here in Southwestern Michigan (USA) and the big lake looks like a frozen tundra. Fossil exploring is usually put on hold until 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. The moving sands reveal hidden treasures one must partake while it lasts.
Below, are two recent discoveries of extinct corals found on Oval Beach located 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 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.
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 where they are commonly found. 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.
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.
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.
So, if you haven’t already guessed which order the Petoskey and Charlevoix Stones belong, here’s the answer. The Petoskey belongs to the order of rugose corals with its hexagonal patterns and the Charlevoix is from the order of tabulate corals with its honeycomb patterns.