What is a fossil?

Professor Cam Nelson, on site at a fossil-rich limestone outcrop, defines the term ‘fossil’. This particular limestone outcrop is full of giant oyster fossils, with some 30 cm in size. Cam describes the likely conditions that these oysters once lived in using the present conditions in Foveaux Strait, a rich oyster fishery, as an example. 

Transcript

PROFESSOR CAM NELSON
A fossil is the preserved remains of ancient life, and that life of course can be animals, can be plants, but in the majority of limestones, we’re talking about animal remains particularly invertebrate remains, and the invertebrate remains are typically the shells of the organism –either their internal skeleton or their external skeleton. The soft tissues of the animal, of course, are long gone in the case of fossils. 

The limestone that we’re looking at here is particularly interesting because it’s crammed full of these giant oysters. The oysters can be anything up to 30 cm in size, more typically 15 cm in size, but their large size is extraordinary. And they occur in the Orahiri limestone whose age is around about 25, 28 million years ago. And of course, that’s an important feature about fossils more generally. Fossils can give you the age of rocks – they don’t have to be limestones – you can find scattered fossils in sandstones and mudstones, but in the case of limestones and in the case of these ones, the characteristic fossils that we find indicate a late Oligocene age.

We’re looking at a bed here which is metres of stacked giant oysters. But these things are not reefs. These giant oysters lived one against each other – they weren’t cemented to any substrate. It’s a very interesting question as to why do we find them here. One of the key things probably is food. They need lots of food, and we believe that these oysters accumulated in a tidal-flushed seaway. The best example in New Zealand today would be Foveaux Strait where what do we find on the bottom of Foveaux Strait today but oysters – smaller, much smaller than these, but they form oyster banks, mounds of these moderate-sized oysters. Here we’ve got mounds of giant-sized oysters, so we think these are the Oligocene equivalents of what today we really see in Foveaux Strait. And in Foveaux Strait, they’re living at water depths of 25–50 metres below sea level, so we infer that kind of water depth here. These limestones and oysters probably accumulated at water depths 30, 40, 50 metres below the sea surface. 

These are the same oysters that we’re seeing in outcrop except we’ve broken off specimens from elsewhere outside of the reserve. You see the layered nature of the oyster shells and the creamer colour around them. Slightly more rusty or fawn is the matrix limestone which surrounds the oysters as they lived on the seafloor – presumably this material has been washed in and around the oysters.

Acknowledgements:
Associate Professor Kathleen Campbell, University of Auckland
Waitomo Caves Discovery Centre