The horn corals have been extinct for millions of years, 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 off the seafloor. At night, the coral animal flung out its long tentacles in order to sweep up unsuspecting tiny organisms passing by in the ocean currents.

Two horn coral fossils featured:

• Grewingkia canadensis horn coral lived primarily during the Ordovician period approximately 485-443 million years ago. The largest could reach up to 5 inches long. They are the most common species found in North American deposits, particularly in the Cincinnati Arch region of Kentucky, Indiana, and Ohio. 
• Heliophyllum halli, horn coral lived primarily during the Devonian period 419–359 Mya. The largest could reach up to 6 inches long. They are commonly found in North America (especially New York) but also in South America (Colombia).

Horn Corals (rugose order of corals, meaning wrinkled wall) were famously known as solitary corals with a few colony variety exceptions. Horn Corals attached themselves to the seafloor with the narrow ends of their exoskeletons. As the organism grew, their upper body widened where the tentacles were encased; hence the reference to the shape of a horn. Horn corals flourished during the Paleozoic time slot from about the Mid-Ordovician Period around 250 million years ago throught to the end of the great Permian mass extinction about 250 million years ago.

Two Horn Coral Species Classification

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 . Rugose corals differ from other corals due to the pattern by which they add septa (like bicycle spokes) throughout their growth spurts. Named for their wrinkly outer skin, they possessed less developed horizontal partitions, but stronger vertical partitions.

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