Hexagonaria was a genus of coral

Coral

Corals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...

. Fossils are found in rock formations dating to the Devonian

Devonian

The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...

 period, about 350 million years ago. Specimens of Hexagonaria can be found in most of the rock formations of the Traverse Group in Michigan

Michigan

Michigan is a U.S. state located in the Great Lakes Region of the United States of America. The name Michigan is the French form of the Ojibwa word mishigamaa, meaning "large water" or "large lake"....

. Fossils of this genus form Petoskey stone

Petoskey stone

A Petoskey stone is a rock and a fossil, often pebble-shaped, that is composed of a fossilized coral, Hexagonaria percarinata. The stones were formed as a result of glaciation, in which sheets of ice plucked stones from the bedrock, grinding off their rough edges and depositing them in Michigan's...

s, the state stones of Michigan

Michigan

Michigan is a U.S. state located in the Great Lakes Region of the United States of America. The name Michigan is the French form of the Ojibwa word mishigamaa, meaning "large water" or "large lake"....

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Hexagonaria is a common constituent of the coral reefs exposed in Devonian Fossil Gorge below the Coralville Lake

Coralville Lake

Coralville Lake is a reservoir formed by Coralville Dam, a dam on the Iowa River just upstream from the city of Coralville, Iowa. Construction started in 1949, was delayed by the Korean War, and finally finished in 1958...

 spillway. and in many exposures of the Coralville Formation in the vicinity of Coralville, Iowa

Coralville, Iowa

Coralville is a city in Johnson County, Iowa, United States. It is a suburb of Iowa City and part of the Iowa City Metropolitan Statistical Area...

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