Invertebrates: Molluscs—Ammonites, Nautiloids, Gastropods and Bivalves

There are tens of thousands of fossil molluscs in the collections of the University of Otago Geology Museum. Most of these fossils are from sites in Otago and Southland, but also other important localities around the South Island. They range in age from the Devonian to the present day, with large numbers from the Permian in Western Southland, Triassic and Jurassic rocks of the Hokonui Hills and the Catlins, a smaller number of Cretaceous age, including from sites around Ōtepoti Dunedin, and many collections from rocks of Paleocene, Eocene, Oligocene, Miocene and Pliocene age around Southland, Otago and South Canterbury. There are also numerous examples of species still living in the seas around Aotearoa New Zealand.

Chitons, limpets, cockles, mussels, oysters, snails, squid and octopus are molluscs – animals with an ancestor that had a muscular ‘foot’ used for locomotion and a body wall that produced a shell. From the ancestral mollusc body plan has arisen a great amount of biodiversity. For example, in the cephalopod group of molluscs the muscular foot has divided to become a set of arms, and the shell was either lost (as in octopus), was brought inside the body (as in squid, cuttlefish and belemnoids), or grew outwards into a many-chambered structure that is coiled in most species. In the oceans of the Late Cretaceous there were two lineages of cephalopods in which the animal lived in the largest chamber of a multiple-chambered shell. These two lineages were the ammonoids and the nautiloids. Nautiloids generally have less elaborate shells than ammonoids and have survived until the modern day. In contrast, ammonoids tend to have more ornamented shells and became extinct during the same mass extinction event that claimed all non-avian dinosaurs. In addition to being an iconic shape, ammonoid fossils can be useful indicators for the age of a sedimentary horizon.