Favosite and Rugose Corals Named after two Michigan Cities

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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. 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 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.

rugose-coral-morphology-diagram

 

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.

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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 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.

 

 

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Petoskey Stone

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Side View of Petoskey Stone

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Details of Petoskey Stone Corallites

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Petoskey Stone, “Hexagonaria, percarinata” Rendering/Drawing

 

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.

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Charlevoix Stone

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Side view of Charlevoix Stone

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Charlevoix Stone “Favosite” Rendering/Drawing

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.

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Petoskey and Charlevoix are far north of Saugatuck’s Oval Beach

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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.

 

What is a Petoskey Stone?

Petoskey Stone

It’s a fossil coral. Why is it called a Petoskey Stone?  Because many of their kind are found abundantly in and around the Lake Michigan shores near the northern city of Petoskey, Michigan (USA)!

They are also called “lucky stones” so it’s really great to find one!

How could the remains of a coral  which thrived in tropical warm waters possibly find its way to Michigan?  Because 350 million years ago during the Devonian Time Period much of North America was covered by warm shallow seas.  Later, the corals were buried under deep layers of sediment.  Millions of years after that, when the great glaciers retreated, they scraped and dug into those forgotten layers of earth.  The glaciers deposited them where we can now enjoy the good fortune of discovering their mysteries.

Petoskey Stones were mass coral colonies of Hexagonaria, percarinata. Each hexagon (very visible in the stone) held a single animal which opened a mouth exposing its tentacles in order to take in food. The tentacles were also used to sting any organism or other corallite that came too close. Calcite, silica and other minerals replaced the original corallite exoskeleton over many millions of years.

Details of Petoskey Stone   Corallites

Details of Petoskey Stone Corallites

CLASSIFICATION

Common Name: Petoskey Stone or Lucky Stone

Scientific Name: Hexagonaria, percarinata

Kingdom: Animalia

Phylum: Cnidardia (means to sting)

Class: Anthozoa (ie coral, sea pens, sea anemones)

Subclass: Zoantharia (true corals)

Order: Rugosa (means wrinkled wall)

Family: Hexagonaria (means six sides)

Species: percarinata

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Petoskey Stone “Hexagonaria, percarinata”