Ammonoids

Introduction to Nautiloids
and Ammonoids

Deep in the waters off of the coastal reefs of Australia and Indonesia lives an uncommon cephalopod (part of a class of animals that today is comprised of mainly octopus and squid) which Charles Darwin once referred to as a “living fossil”. It is certainly not the rule today to observe externally shelled cephalopods, but if the fossil record is an accurate representation, the oceans once teemed with creatures very similar to the Pearly Nautilus.
The shell of a Nautilus has a function greater than mere protection. Behind the main living chamber lie many small, walled off sections that were once where the soft, tentacled parts resided when the animal was

in an earlier stage of development. As a nautilus grows, it moves forward in its spiral and secretes dividing walls, or septa, to create small chambers (camera). Then the slow task begins of emptying the seawater filled camera through the siphuncle, a tube that extends from the main body of the nautilus through the rest of the shell. Once the liquid is removed from the camera, these chambers can be used in the same way as the ballast tanks in a submarine, to control the animal’s buoyancy. The development of this system of gas filled chambers (the phragmacone) must have made nautiloids one of the first shelled creatures to rise from the bottom of the ocean in the late Cambrian, almost 500 million years ago.