Ammonoids

Ammonoid fossil from the Lower Jurassic of England. Source: Museum Victoria.

These extinct marine animals belonged to the class of molluscs called cephalopods, and so were related to the octopus, squid and pearly nautilus. Ammonoids possessed ink sacs, had jaws like an octopus, and like other cephalopods were probably active predators as well as scavengers on the remains of dead sea creatures.

The ammonoid shell, which in most forms was tightly coiled in a single plane, was divided into chambers. The animal was able to grow by adding chambers to the shell, but always lived in the outermost chamber, hiding behind a pair of opercula (or doors) which sealed the entrance when danger threatened. The inner chambers were used to aid buoyancy by injecting or withdrawing liquid. Ammonoid shells were sufficiently strong to be able to survive the water pressure down to depths of about 100 metres.

Ammonoids are among the most commonly found and well-known of all fossils. They first appeared during the Devonian Period (approximately 410 million years ago). They almost died out at the end of the Permian Period (251 million years ago), and again at the end of the Triassic Period (205 million years ago). They finally became extinct at the end of the Cretaceous Period (65 million years ago), perhaps as a result of a lowering of ocean levels.

These fossils are especially well known from the Mesozoic Era (251 to 65 million years ago)-that is the Triassic, Jurassic and Cretaceous Periods. Ammonoids are very useful as a means of dating rocks from these three periods, as the various species quickly evolved and replaced each other (around every million years or so) during this time. A few rare early species have been found in Victoria at Buchan.