Opalised belemnite rostrum

Belemnites were squid-like molluscs that became extinct at the end of the Mesozoic Era (66 million years ago).

In both belemnites and squid, the molluscan shell is internal, though in modern squid it is reduced to a simple, chitinous ‘pen’. In contrast, the belemnite shell was complex. It consisted of three parts, which combined to keep the body rigid, but also evolved to perform other functions. Just behind the animal’s head, the pro-ostracum extended over its internal organs. The animal could control its buoyancy by pumping gases into or out of chambers in the middle section of the shell, the phragmocone. At the distal end of the body was the bullet-shaped rostrum. Made of relatively heavy calcite, it acted as a counterbalance to the head and arms, thus improving stability while swimming. The rostrum is the most commonly preserved part of the animal.

In this specimen from Coober Pedy in South Australia, the original calcite has been replaced with precious opal during the fossilisation process. Australia is the only source of precious opalised animal fossils in the world.