Dinosaurs were not the only giants in Jurassic days. Even the shellfish grew to a gigantic size. This fossil ammonite, the biggest found from the age of the dinosaurs, is over 1.5 metres in diameter.
Mrs Jean Gyles spotted the fossil in a road cutting near Kawhia Harbour in 1977. The find caused great excitement. Not only was the ammonite large and old, surprisingly for earthquake country, it was also uncrushed and almost completely intact. Not for long however - geologists had to use explosives to extract it in pieces from its rocky bed. The pieces were brought to a laboratory in Lower Hutt, where the shelly exterior was cleaned, and the pieces reassembled into the massive specimen now on display in Awesome Forces.
Some of the rocky interior of this ammonite was originally silt. Parts of the interior now consist of a pale white mineral called laumontite. Laumontite is formed when certain volcanic material is lightly squeezed and slightly warmed over millions of years. And that probably explains how this animal met its death - smothered by a shower of volcanic ash under water, then eventually covered and filled by ash and sediment.
The ammonites' common name comes from the shape of their shells which are curled like a ram's horn. Ammon was an ancient Egyptian god who had the head of a ram. Ammonites were common in the oceans of the world for some 150 million years. This one was living about 140 million years ago. That means it was already preserved as a fossil in New Zealand's rocks long before our land became separated from the supercontinent Gondwanaland.
Ammonites came in a wide variety of sizes, from tiny ones no bigger than 10 millimetres to whoppers more than 2.5 metres in diameter. The biggest ever found (in Germany) would extend nearly 11 metres if it was uncoiled!
Like a squid, the ammonite caught its food with tentacles. Its body sat inside the chamber at the shell's opening. Again like a squid, it could squirt water out through a funnel in its body, propelling itself backwards. The coiled shell was like a multi-chambered buoyancy tank. The ammonite moved gas and fluids between chambers through a narrow tube, so it could rise and sink in the water like a submarine.
The ammonite's closest relative these days is the chambered nautilus with its beautiful coiled shell. The nautilus' shell is smooth-walled - it floats in surface waters. The ammonite's shell, by contrast, was knobbed, ribbed, and folded. This shows that the ammonite could live in deep water at great pressure. The shell's strength was also a protection from predators. The ammonite could retreat inside its body chamber, and 'close the door' on its enemies with a shelly plate like that of a cat's-eye shellfish.
Such a large creature must have had a big appetite. Scientists think that this ammonite would have lived in temperate or cold parts of the ocean where there are upwellings of rich food supplies. Today's large marine creatures obtain their food in similar environments, such as the waters between New Zealand and Antarctica.
The following publications may be found on the Discovery Centre bookshelves, or in Te Aka Matua Library and Information Centre on Level 4. Photographs of Te Papa collection items may be ordered from Images, on Level 4.
Hayward, Bruce. (1989). Trilobites, Dinosaurs and Moa Bones: the story of New Zealand fossils. Auckland: Bush Press.
Stevens, Graeme; McGlone, Matt and McCulloch, Beverley. (1988). Prehistoric New Zealand. Auckland: Reed.
Giant ammonite Courtesy of the Institute of Geological and Nuclear Sciences Ltd