Many rugose horn corals occur local to the Pennsylvanian rocks of Armstrong County, Pennsylvania. Stereostylus is a common solitary horn coral in Pennsylvania. The subclass Rugosa Milne Edwards & Haime 1850 went extinct at the end of the Permian during the most significant known mass extinction.

The specimens below show perhaps the same species. The top piece of polished limestone is a cut cross-section. The bottom part is an eroded end sticking out of stone. The shell material eroded past the matrix, showing an interesting pattern.

Horn Coral section on a polished piece of limestone.
A polished piece of Brush Creek limestone with an embedded horn coral. The septa and mesenteries are visible as lines that radiate from the center. The outside shape and texture are due to the theca, or the coral outer wall.
Rugosan coral
A second large horn coral with partially dissolved calcium carbonate walls. After some dissolution, the infilled matrix—the steinkern—sticks out a little. A purer calcium carbonate is less acid-resistant than the rest of the matrix. Thus, it dissolves away quicker.
A horn coral from the Pine Creek limestone of Armstrong County
CG-0451—A rugose horn coral from the Pine Creek limestone of Armstrong County, Pennsylvania. This specimen—coated in ammonium chloride to increase contrast—shows the structure of the theca, which is the outside wall of a horn coral. Scale bar = 5 mm.
Horn Coral
CG-0452—A rugose horn coral from the Pine Creek limestone of Armstrong County, Pennsylvania. Scale bar = 5 mm.

Pennsylvanian corals that are not Horn Corals

The majority of corals recovered are solitary horn corals. These have a horn shape and can be referred to as rugose corals due to their wavy shape. Yet, solitary corals were not the only ones around. Corals that formed colonies existed but were harder to find. The fossil below is Michelinia sp.

CG-0334—Michelinia sp., a colonial coral. Horn corals are widespread, while these colonial varieties can be rare to recover.

Further Reading on Horn Corals

Further Reading about Michelinia