Ammonoids and nautiloids belong to the order of molluscs. They are a type of cephalopod, (an animal like an octopus, a squid or a cuttlefish) which lives in a single body chamber, at the very top of a chambered shell. As the animal grows older it adds to its shell and the body chambers become larger to accommodate the animal. The most recent additional chamber to the shell is at the top and the oldest part of the shell is at the base, or in the centre of the spiral in coiled ammonoids)
Ammonites and nautiloids have some fundamental differences in shell structure. In particular the siphuncle (a tissue used to adjust the animals buoyancy) is located in different places. It runs through the centre of the chamber in nautiloids and around the outside wall of the chambers in ammonoids. Ammonoids also have complicated sutre structures -- these are the patterns that are formed when the edges of the internal chambers meet the shell walls. There is no definitive explanation for these convolute patterns. One suggestion is that the sutre lines mark the mantle (the base) of the animal within the shell. There is no way of checking this for certain as ammonites became extinct during the cretaceous. The K-T mass extinction event, which ended the era might have been the final blow, but ammonites had become increasingly rare during the whole of the period and had evolved strange shapes to fill niche environments.
Nautiloids are still present in the modern oceans. They may be less related to the extinct ammonoids than octopuses and squids, but still offer exciting clues to the behaviour of the fascinating, ubiquitous and long dead ammonoids.