Sponges are mainly marine animals that live attached to the sea floor. Fossil sponges are numerous in some parts of Illinois. They are not the flexible sponges you and I know, but instead have a hard skeleton of calcium carbonate or silica. The oldest ones are known from Cambrian rocks and are about 500 million years old.
Receptaculites, called the "sunflower coral," is common in Ordovician rocks of north-central and northwestern Illinois. Although here placed with the sponges, paleontologists are increasingly convinced that Receptaculites belongs with the algae.
Another, called Hindia, is found in Silurian rocks exposed in quarries in the Chicago region. Hindia looks like a small round ball, but, when broken, is seen to be made of thousands of radiating rods of calcium carbonate.