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Hexagonaria percarinatum

When did it live?

Devonian Period (416-369.2 million years ago)

Where did it live? Michigan
Who are its relatives? Corals
A little known fact Hexagonaria means ‘six-sided’

Hexagonaria percarinatum is a species of fossil coral commonly found in Michigan. Its scientific name, which means “six-sided,” refers to the shape of the starlike features covering its surface, called corallites. Its common name refers to the city of Petoskey where the fossils are commonly found, as well as to the Ottawa chief, Petosegay, after whom the city is named.

 

Hexagonaria lived during the Devonian period (416–369.2 million years ago) in the areas we now know as the American midwest, as well as some locations in Canada, England, Germany, and Asia.  The particular species which yields Petoskey stones lived during the Middle Devonian, and is found only in the Gravel Point Formation of Michigan. During the last ice age, the glaciers that created the Great Lakes carved H. percarinatum fossils out of the bedrock and deposited them on the lakeshore around Michigan’s Lower Peninsula.  Erosion by the glaciers and the lakewater wore the rocks into the rounded shapes we find Petoskey stones in today. Collectors often polish them into the smooth, glossy forms you can find in gift shops.

Polished Petoskey Stones; Image from GeologyInc

Due to plate tectonics, or the understanding of how Earth’s continents have shifted over time, Michigan was in a very different place during the Devonian than it is today.  It was near the equator, and formed a basin that was filled with a shallow, tropical sea. Hexagonaria built reefs in this sea, alongside many other creatures, including other corals, clam-like creatures called brachiopods, shelled relatives of octopus and squid, sea lilies, trilobites, coral-like creatures called bryozoans, sponges, and armor-plated fish.  Hexagonaria probably lived very similarly to modern corals, with each corallite containing a single coral animal, called a polyp, that captured food with stinging cells on its tentacles. Like modern corals, the polyps may have had a symbiotic relationship with algae living in their tissues. The algae would have used sunlight to produce sugars, which they would share with the polyp in exchange for shelter from predators.

Hexagonaria belongs to a group of corals called rugose corals. Rugose corals lived through most of the Paleozoic Era, before going extinct during the mass extinction at the end of the Permian period. Their evolutionary relationship with modern stony corals, which belong to the group Scleractinia, is uncertain.  Scleractinian corals first appear in the fossil record during the Triassic period, several million years after rugose corals went extinct. It’s possible that modern corals are direct descendants of rugose corals, or they may be descended from distant cousins. More research is needed to settle the true relationship.

There are several ways to distinguish Hexagonaria from other similar Michigan fossils. Since it is a rugose coral, each starlike corallite contains a central pillar, called a columella, with radiating features called septae that look like the spokes of a bicycle wheel. Tabulate corals and coral look-alikes called bryozoans do not share these features. The corallites of tabulate corals tend to be only a few millimeters across, while the corallites of rugose corals tend to be larger. Hexagonaria is a colonial coral, so it has many corallites, and all of them maintain close physical contact. This distinguishes it from some other rugose There are several ways to distinguish Hexagonaria from other similar Michigan fossils. Since it is a rugose coral, each starlike corallite contains a central pillar, called a columella, with radiating features called septae that look like the spokes of a bicycle wheel. Tabulate corals and coral look-alikes called bryozoans do not share these features. The corallites of tabulate corals tend to be only a few millimeters across, while the corallites of rugose corals tend to be larger. Hexagonaria is a colonial coral, so it has many corallites, and all of them maintain close physical contact. This distinguishes it from some other rugose corals, in which the corallites are spaced apart or even solitary, as in horn corals.

Learn more about Hexagonaria and rugose corals:

 

Spanish Translation

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The information presented in this module comes from UMMP Student Seamus Callaghan, reviewed by Jen Bauer, UMMP Invertebrate Collection Manager