Fossil coral Halysites louisvillensis, photographed by James St. John

The phylum Cnideria includes animals such as sea anemones, corals, and jellyfish.

Corals live on the floor of tropical oceans and are found as early as the Ordovician Period in the fossil record, more than 500 million years ago. The earliest forms were different from modern corals and became extinct 225 million years ago. Inside its hard external skeleton, the body is divided radially into chambers.

In contrast with modern corals that live in colonies of hundreds of individuals, some fossil corals were solitary. Called “cup corals”, they looked like cushions, horns, or tubes with a depression in the top where the animal lived. Colonial fossil corals are branching or closely packed. Fossil corals are commonly found in limestone, shale, and sandstone across Illinois.