The primary predators were conical shelled nautiloid cephalopods - relatives of modern coiled nautiloids, squid, and octopi. R eefs, both ancient and modern, are delicate, intricate ecosystems with an amazing diversity of life. They are formed by the shells of organisms bound together into mounds which grow upward from the sea floor. Reef mounds may reach hundreds of meters in height. Framework organisms grow upward and binding organisms cement shells to the framework, giving the reef a firm structure.
Many animals provide shells to the reef mass.
The surface of the reef is very irregular, providing numerous habitats for organisms. Through geologic time reefs have been the most diverse communities in the oceans. More different kinds of organisms live in reefs than in any other marine setting. T he kinds of animals inhabiting reefs have changed many times during earth history. For example, the predators in the Silurian reef were conical nautiloids.