Museum Notebook: A Fossil is found of a Giant Crab

Tumidocarcinus giganteus, giant fossil crab. Photo / supplied

In 1990, a local Whanganui resident captured a giant crab in the Ahu Ahu Valley, inland from Whanganui.

That's a curious creature to find so many kilometres from the coast. It was, however, not a potential family feast.

It was a large fossil embedded in a spherical boulder, known in geological terms as a concretion. A concretions is a hard rock that forms around an object such as a fossil, protecting it from damage.

Concretions can often be found weathering out of soft mudstone. If a concretion is cut open very carefully, it may reveal an interesting fossil, well preserved within the boulder. Because mudstone is very soft, it can be generally be cleaned off the fossil using water and a stiff brush.

This particular fossil crab was alive approximately 15 million years ago, during the middle of the Miocene period, when the Ahu Ahu Valley, along with the rest of the Whanganui region, was under the sea.

It is an example of the extinct species Tumidocarcinus giganteus, a deep-water crab that lived along the seabed in warmer waters than we enjoy today, on the Whanganui coast.

During the middle of the Miocene period, which lasted from 24 million years ago to 5 million years ago, temperatures are estimated to have been four to five degrees warmer over most of the planet than they are today, and the sea level was correspondingly much higher.

Large numbers of Tumidocarcinus giganteus fossils have been recovered from the soft papa rock that is characteristic of the hills between Taranaki and Whanganui.

Papa is formed from thick muddy sediments accumulating in the ocean around the western coast of the North Island.

The numbers of these crabs found indicates that they were a reasonably common species in New Zealand seas during the Miocene.

An interesting feature of the Tumidocarcinus giganteus is that the right pincer is usually much larger than the left. On males, the right claw could grow up to twice the size of the left claw. It was probably used for fighting and perhaps for attracting female crabs, as well as feeding.

By discovering fossils, such as this giant crab a very long way from the ocean, we can get a much clearer picture of what the land-masses we now inhabit might be like if the earth's climate became similar to the middle Miocene again.

It is challenging for us to imagine what the planet might be like if temperatures throughout the world continue to rise at the current rate.

It is clear, however, that seas will be significantly higher, and much of the New Zealand land mass, especially coastal regions, will probably be underwater.

The Whanganui region probably won't be so great for humans, but giant crabs and other enormous sea creatures might be plentiful again.

Margie Beautrais is the Educator at Whanganui Regional Museum