A few fossils interpreted as jellyfish have been found, but the corals have the external skeleton necessary for common fossilization. Corals have a hard, calcareous skeleton, and may be solitary or colonial. We tend to think of corals as been colonial with small individuals, but many Paleozoic species are fairly large and solitary such as the horn coral in the picture on the left. Colonial corals are composed of many polyps living together. The skeletal parts formed by polyps are called corallites.
Tabulate corals lack septae.
The picture on the left is a rugose coral. The rugose corals are all extinct and lived between the Cambrian and the Permian. They lack septae. Tabulate corals occur in the fossil record between the Ordovician and the Permian. Most are colonial, but a few are solitary.