Other than fish, they are the most accomplished swimmers in the sea. The shell is divided into internal gas-filled chambers by septa . The animal lives in the last chamber, the body chamber , and moves forward each time a new septum is secreted. A single tube passes through each septum and connects the chambers. Each septum meets the inner wall of the shell along a suture line .
Ammonites are amongst the most abundant and well known of fossils.
They have a beautiful coiled shell, often strikingly ornamented with external ribbing. Individual ammonite species survived for relatively brief intervals, often a million years or less. Their rapid evolution , abundance , and widespread distribution makes them one of the most valuable index fossils , especially for Triassic, Jurassic, and Cretaceous periods of the Mesozoic . Most ammonites are preserved as internal molds and suture lines show up clearly. Microfossils A fossil too small to be studied without the aid of a microscope.