Hexagonaria percarinatum

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.