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.

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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.

 

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What is a favosite?

A favosite is a type of extinct coral. One way to identify them is by the honeycomb patterns on their surfaces, which is where their coral polyps lived. Preserved in sedimentary claystone, the fossil sample below was found in Allegan County of Southwestern Michigan, USA in a field. It’s more commonly found in locations near where you find the Michigan Petoskey Stones and Charlevoix Stones situated in the Traverse Group Geological Formations in the far northern sections of the state.

Favosite Coral

Favosite Coral Fossil

Favosites, like most coral, thrived in warm, shallow sunlit seas. They were a colony type of coral forming colorful quilt work reefs. They fed by filtering microscopic plankton with their stinging tentacles. They were most prevalent during the Silurian and Devonian time slots, but date as far back as the Ordovician and forward to the Permian between 251-488 mya. That’s over 200 million years of living on earth . . . amazing!  

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Favosite Rendering

CLASSIFICATION

Common Name: Honeycomb Coral        Scientific Name: Favosite, cervicona (above)

Kingdom: Animal

Phylum: Cnidaria (means to sting)

Class: Anthozoa (means flower animal)

Order:  Tabulata (possess inner horizontal dividing walls)

Family: Favositidae (honeycomb pattern on exoskeleton)

Genus: Favosite Species: Alpenensis

Favosite Coral Preserved in Gray Shale

Favosite Coral Preserved in Gray Shale Revealing Honeycomb Patterns

The tabulae (horizontal internal layers) place the favosite corals in the order of tabulata with internal chambers that built outward and upwards as the organism grew.

The walls between corallites were pierced by pores known as mural pores which allowed transfer of nutrients between polyps.

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Favosite Coral Reveals Horizontal "Tabulate" Growth Layers

Favosite Coral Revealing Inner Growth Layers