Fossil of the internal nautilus cephalopod. A highly decorative Internal Gomphoceras mineralised shell and last largest living chamber. The living chamber where the Nautilus would reside and being the largest chamber. The extinct marine cephalopods soft body, its head and numerous tentacles would have reached out of the largest last chamber and possibly the aperture had cover which would have been made of calcite, a hinged lid over the opening, referred here to as the mouth aperture, if the Nautilus felt in danger the aperture would close.
The cephalopod added chambers as it grew, each progressively larger than the preceding one. The largest chamber cavity is easily viewed as the wide end of the fossilised shell in our illustrations, now the foot end cut for display, which is most apt, as the name cephalopod derived from the Greek, pod being a foot and (head extrapolate to cephalo) cephalopod. Gomphoceras is thought to be part of the nautiloidea subclass of cephalopods as described by Sowerby in 1839 and later Agassiz, 1847.
Interestingly sightings of the modern extant Nautilus Pompilius have been made off the Great Barrier Reef at depths between 200 to 400 meters, showing the pressures these Cephalopods can attain. Nautiloids are part of the cephalopod family which includes the ammonoidea group of cephalopods.
The Ammonites extinction is dated around 65 million years. Ammonites are important index fossils, it is often possible to link the sediment layer in which they are found to specific geological time periods. The largest ammonite to date is around 2.5 meters in diameter from Germany. Whereas the nautiloids persist through to today in the Indian ocean.
Phylum: Mollusca, Class: Cephalopoda, Subclass: Nautiloidea, Order: Oncocerida, Family: undetermined, Genus: Gomphoceras described by Sowerby 1839.