Solitary Rugose Corals - Horn corals
When: Mid-Ordovician to End Permian (488 - 250 million years ago)
Where: Paleo-oceans worldwide
What: All modern corals are colonial; meaning each structure is actually made up of the shells of masses of tiny animals. Some members of the group Rugosa were instead solitary, growing orders of magnitude larger than any single living coral. Corals are cnidarians, a group that also includes jellyfish. The soft body of a single coral looks a lot like a jellyfish with only its tentacles visible outside of the shell. These tentacles have stinging cells which paralyze any prey animal that gets too close; and then it is guided towards the centrally located mouth.
These large corals did not cement themselves to the substrate, but instead the tip of the ‘horn’ was shallowly buried in ocean floor sediments. This would lead to corals falling over on occasion due to strong currents; the animal would then build its shell towards the ‘new up’, which is why some rugose coral fossils are bent rather than straight shelled. These corals could grow to roughly 3 feet long (~1 meter), but most had shells only about 3-4 inches (8-10cm) long. These solitary corals were one of the groups lost in the end Permian mass extinction, the largest extinction event in Earth’s history.