Favosite and Rugose Corals Named After Two Michigan Cities


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 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.


Southwestern Michigan 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.




Petoskey Stone


Side View of Petoskey Stone

Petoskey 2

Details of Petoskey Stone Corallites


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.


Charlevoix Stone


Side view of Charlevoix Stone


Favosite Drawing Rendering  “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 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.


Recent Find (Halysite Coral)

Golden Beach grass dune

Oval Beach Saugatuck, Michigan  Winter 2012


I was super excited to discover some fossils on Oval Beach in Saugatuck, Michigan (USA) that are highly unusual to find in winter. The sample below is an extinct tabulate coral, a reef building colony type coral called halysite. But we often find these saltwater varieties from the Paleozoic Era on our fresh water Lake Michigan shores. To explain this phenomenon I’ve written an article  entitled “I Found a Fossil on the Beach and Wondered” at hubpages.com where I’m also the Fossillady. A link is provided in the box to the right.

This halysite coral is easy to distinguish due to the chain link appearance coining it with the common name “chain coral”. It was lying under layers of sand that aren’t usually exposed during the winter months. But the mild weather with little snow has allowed winter winds to push sand off the under layers. It’s also due to low water levels from a long dry spell during summer 2012, there’s more beach to explore.

The extinct halysite corals displayed small tubes from which resided the jelly-like coral animals called polyps.  The coral polyps contained stinging cells for protection and also grasped plankton food that passed by in the ocean currents. As the halysite corals grew, they built up walls of tube-like chambers called theca which steadily multiplied adding more links to the chain. In their hay-day as they continued to multiply, they built large limestone reef structures on the seabed. They thrived mostly during the Silurian period up to 425 million years ago!


Kingdom: Animal

Phylum: Cnidaria (means stinging animal)

Class: Anthozoa (means flower animal)

Order:  Tabulata (possess inner horizontal dividing walls from growth patterns)

Family: Halisitidae (means chain coral)

Genus: Halysite  Species:unknown

Halysite Coral

Halysite Coral Fossil


Halysite Colony “Chain Coral” Rendering

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 where their coral polyps poked out from their calcium carbonate substrate. Preserved in sedimentary claystone, the fossil sample below was found in a field in Allegan County of Southwestern Michigan, USA. It’s more commonly found in locations where you find the Michigan Petoskey Stones and Charlevoix Favosite Stones situated in the Traverse Group Geological Formations in the far northern sections of the state.

Favosite Coral

Favosite Coral Fossil

Like all coral, Favosite corals thrived in warm, shallow sunlit seas. They were a colony type forming colorful quilt work reefs and 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!  


Favosite Drawing Rendering


Common Name: Honeycomb Coral        Scientific Name: Favosite, cervicona 

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.


Favosite Coral Reveals Horizontal "Tabulate" Growth Layers

Favosite Coral Revealing Inner Growth Layers




Interesting Horn Corals


Grewingkia, canadensis (horn coral)

Grewingkia, canadensis (horn coral) VIEW LARGE


 The horn corals have long gone by the wayside, but in their hay day they must have added  a certain beauty to the diorama of the Paleozoic seafloor. Some varieties dominated the scene reaching meters in length.  At night they flung out their long tentacles in order to sweep up  unsuspecting organisms passing by.  They attached themselves to the sea floor with the narrowed ends of their exoskeletons, whereas their wide-opened top ends encased the tentacles; hence the reference to the shape of a horn.

fossils 129

Heliophyllum Horn Coral

* Horn corals were extremely abundant during the Paleozoic time slot and most were individual varieties with a few colony variety exceptions.

Prehistoric Horn Corals

Prehistoric Horn Corals




Scientific Name: Grewingkia, canadensis                Heliophyllum

Common Name: Horn Coral                                       Same

Kingdom: Animalia                                                       Same

Phylum: Cnardia (means to sting)                             Same

Class: Anthozoa (means flower animal)                   Same

Order: Rugosa (means wrinkled wall)                      Same  

Suborder: Stauriida                                                      Same

Family: Streptelasmatidae                                     Zaphrentidea

Genus:  Grewingkia                                                   Heliophyllum

Species: canadensis                                                    Unknown

As a very general rule, rugose coral have stronger radial septa (septum) or vertical growth walls that radiate out from the center (like bicycle spokes).  Rugose corals differ from other corals by this pattern by which they add septa through their growth.  Named for their wrinkly outer skin they possessed less developed horizontal partitions, but stronger vertical ones.


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


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


Petoskey Stone “Hexagonaria, percarinata”