Hexagonaria percarinata (or H. percarinata, for short) is the coral present in Petoskey stone. It was a rugose coral, a variety of coral with a rugose, or “wrinkled,” exterior wall texture. While Hexagonaria percarinata was colonial, forming tightly packed groups of individuals, many rugose corals were solitary, with each polyp growing as an individual tube-like structure. These often resembled a curved, pointed horn, hence their nickname, “horn corals.” All rugose corals are now extinct, but their fossils are among the most common invertebrates found today, especially in Michigan’s limestone formations.
This long tube-like horn coral perfectly illustrates the “wrinkled” nature of rugose corals.
Pointed tip of a horn Coral
Water-worn horn coral with no horn-shape present, but the patterning is distinctive
Corallite of Hexagonaria percarinata
H. percarinata was also a stony coral, producing large, hard dome-like structures composed of hard calcite. These domes often began small, sometimes with just dozens of corallites, but could grow to enormous sizes and contain tens of thousands or even millions of individuals, producing immense coral reefs on a scale unlike anything that exists today. The corallites themselves are taller than they are wide and contain vertical segments, called septa, that are supported by a central rod-like structure called the columella.
