Halysites Chain Coral Uncovered

Golden Beach grass dune
Oval Beach Saugatuck, Michigan  (Winter 2012)

I was super excited to discover several fossils on Oval Beach in Saugatuck, Michigan, USA that are highly unusual to find in winter. The fossil below was lying under deep layers of sand, but luckily, the mild weather with minimal snowfall allowed winter winds to push sand off the under layers. Also contributing to fossil hunting were the low water levels from a long dry spell during summer 2012, which produced more beach to explore.

Halysite Coral
Halysites Chain Coral Fossil Found on Lake Michigan Beach

These fossil samples are extinct tabulate corals, reef building colony-type corals, Halysites, commonly called, Chain Corals. They are fairly easy to distinguish due to the chain-link raised marks for which they’re named.

Halysites Chain Coral Fossil Found on Lake Michigan Beach

In life, the extinct Halysites corals possessed small tubes where the jelly-like polyps resided. The coral polyps contained stinging cells for protection and also for siphoning plankton and organic matter passing by in the ocean currents. As the Chain Corals grew, they built up walls of tube-like chambers called theca which steadily multiplied while adding more links to the chain. In their heyday, they built large limestone reef structures on the seabed. They thrived especially during the Silurian period as far back as 425 million-years-ago!

Halysites Chain Coral Classification

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: Halysites  Species: unknown

Halysites Chain Coral Rendering Showing Polyps Extended

Explore more Lake Michigan fossils in another photo-essay I provided, otherwise, keep scrolling!

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What is a favosites?

Favosite Coral
(Charlevoix Stones) Favosites Honeycomb Coral Fossil

A favosites is a type of extinct coral. Favosites coral fossils most identifying feature is the honeycomb-like openings (coralites) revealing where the animal (polyps) lived. The polyp tentacles could tuck inside for safety, typically at night, or stretch out from their calcium-carbonate substrates in order to filter tiny food particles floating by in ocean currents.

Favosite Coral Preserved in Gray Shale
Favosites Honeycomb Coral Fossil Preserved in Gray Shale

The large fossil sample shown first is preserved in sedimentary claystone. It was found in a field in the city limits of Saugatuck, Michigan, part of the Ellsworth-Antrim Geological Formation (Mississippian-Devonian), Allegan County, Southwestern Michigan, USA. This particular type of honeycomb coral fossil is more commonly found in Charlevoix, Michigan, situated in the Traverse Group Geological Formations in the far northeastern region of the state. Consequently, they’re often called, Charlevoix Stones.

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

Diagram of Favosites Internal Structure

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

Favosite Coral Reveals Horizontal "Tabulate" Growth Layers
(Charlevoix Stone) Favosites Honeycomb Coral Fossil Reveals Inner Growth Layers (Found on Oval Beach, Lake Michigan)

Like all coral, favosites corals thrived in warm, shallow, sunlit seas. They were a colony type coral 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!  

Favosites Honeycomb Coral Fossil

Favosites Classification

Common Name: Honeycomb Coral        Scientific Name: Favosites 

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: Favosites Species: Alpenensis (Charlevoix Stone)

Cora Honeycomb-studio textre
Rendering Drawing of Extinct Favosites Honeycomb Coral Showing Polyps Drawn-out

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Interesting Horn Corals

Horn Coral
Horn Coral Fossils (Grewingkia, canadensis) #1

The horn corals have long gone by the wayside, but in their heyday, they must have added an extraordinary beauty to the diorama of the Paleozoic seafloor. Some varieties dominated the underwater, prehistoric scene reaching multiple-meters in height. At night, the coral animal flung out its long tentacles in order to sweep up unsuspecting tiny organisms passing by in the ocean currents.

Horn Coral Fossil (Heliophyllum) #2

Horn corals (rugose corals) attached themselves to the seafloor with the narrowed ends of their exoskeletons. As the organism grew, the top-end widened where the tentacles were encased; hence the reference to the shape of a horn.

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 Rendering Drawing

Two Horn Coral Species Classification

Scientific Name: #1 Grewingkia, canadensis             #2 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

Horn Coral Internal Structure

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


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What is a Petoskey Stone?

Polished Petoskey Stone Coral Fossil (Hexagonaria, percarinata)

Simply put, a Petoskey Stone is fossilized coral. Why is it called, a Petoskey Stone? Because so many of its kind are found abundantly in and around Lake Michigan shores, especially near the Northern Michigan city of Petoskey (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 during the era they lived, around 416 to 369 million years ago (Devonian Time Period), much of North America was more tropical and was covered under warm shallow seas. Later, the corals were buried under deep layers of sediment. Many 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.

Rough unpolished Petoskey Stone Coral Fossil (Hexagonaria, percarinata) found on Lake Michigan beaches.

Petoskey Stone fossils originate from mass coral colonies of Hexagonaria, percarinata. Each hexagonal corallite (visible in the stone) held a single animal which opened a mouth exposing tentacles that siphoned food particles floating by in ocean currents. The tentacles were also used to sting any organism or other corallites that came too close. Calcite, silica and other minerals replaced the original exoskeleton over many millions of years.

Corallites Detail of Petoskey Stone Coral Fossil (Hexagonaria, percarinata) found on Lake Michigan Beach


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 Coral (Hexagonaria, percarinata) Rendering Drawing

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