Dinosaur-era fossil discovered in Hawkes Bay streambed

A large fossilised ammonite is pictured here at the rear, compared with a smaller ammonite fossil. The area outlined in red on the smaller specimen indicates the section discovered in Hawkes Bay.

The fossil of a squid-like creature wiped out with the dinosaurs has been discovered in a Hawkes Bay streambed.

The surprise find of the ammonite fossil - found contained in a 50kg boulder in the Waiau River - has excited scientists about what other specimens may lay hidden in New Zealand's under-explored wilderness.

The uncovered ammonite, which lived in the sea during the time of the dinosaurs, had a flat spiral shell that looked something like a Paper Nautilus but was between 80cm and 90cm in diameter - large compared to most other ammonites.

The species belonged to a group of predators known as cephalopods, whose living relatives include the octopus and squid.

Its shell was coiled like a snail's and comprised a series of water-tight compartments that enabled it to float and swim in the upper layers of the sea.

Scientists say the new fossil is significant not just because of its size - most found in that part of New Zealand are just a few centimetres in diameter - but also because of its age.

It was found in strata created 85 million years ago, making it one of the youngest fossilised ammonites found here.

Although found relatively frequently elsewhere, ammonite fossils are rare in New Zealand, for reasons not yet completely understood.

"We don't have a significant record of these creatures in New Zealand so this find adds considerably to what we know about New Zealand paleontological history and about what was living here at that time," said GNS paleontologist Dr James Crampton, who found the fossil with collections manager John Simes during a "rock kicking" walk up the river.

"It may help us understand more about why ammonites were so seemingly rare here when they appear to have been so common in other places.

"Even back then, it would seem, there was something unusual about New Zealand's marine environment."

The find demonstrates just how much more there was to be discovered in New Zealand's rich but under-explored fossil record, large parts of which were tucked away in inaccessible, moss-hung and waterfall-blocked streams in our remote mountains.

The river where the fossil was found borders the Maungataniwha Native Forest, near where celebrated New Zealand paleontologist Joan Wiffen first discovered evidence of land-dinosaur fossils in New Zealand.

At the time the ammonite existed, New Zealand had already been torn away from the super-continent Gondwanaland in much the same way as California was being wrenched from the North American continent today by the San Andreas fault.

Not long afterwards, a giant asteroid hit Earth and ammonites, along with dinosaurs, became extinct.

Dr Crampton said the Waiau River bed was particularly important to paleontologists because it had carved its way through a succession of geological layers.

"Walking along the Waiau is like walking back in time," he said.

"In geological terms it takes us deeper and deeper below the earth's surface without us having to dig an inch."

It was likely that the area held a wealth of secrets that could one day unlock the answers to a wide array of questions that science still has.

"If we can wander randomly up a stream-bed and pick up a fossil of this significance, in the same way as Joan Wiffen did all those years before us, imagine what we'll unearth when we really start looking."

The GNS team will attempt to remove the entire fossil from the 50kg boulder it was contained within, with the aim of displaying it at its offices in Lower Hutt.