Ammonites were a group of extinct marine molluscs in the subclass Ammonoidea of the class Cephalopoda. They had a coiled external shell similar to that of the modern nautilus. Ammonites were born with tiny shells and, as they grew, they built new chambers onto it. They would move their entire body into a new chamber and seal off their old and now too-small living quarters with walls known as septa. Ammonites are excellent index fossils, and linking the rock layer in which a particular species or genus is found to specific geologic time periods is often possible. Ammonites probably fed on small plankton, or vegetation growing on the sea floor, and may also have eaten slow-moving animals that lived on the sea bottom, such as foraminifera, ostracods, small crustaceans, young brachiopods, corals, and bryozoa, as well as drifting, slow-swimming, or dead sea creatures. Ammonites were quite diverse and evolved rapidly.