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A fossil bone found in New Zealand has been identified as coming from one of the biggest dinosaurs to roam the planet.

The tailbone from a titanosaurid, a four-legged plant-eater with massive neck and tail, was discovered in a remote stream in inland Hawkes Bay.

The solitary bone allows only rough estimates of its owner's size, but the biggest titanosaurid specimens found would have weighed more than 100 tonnes and were 45 metres long.

One of New Zealand's most successful fossil hunters, Dr Joan Wiffen, found the bone in a stream west of Mohaka in northern Hawkes Bay in 1999.

Thorough research and peer analysis ensued, leading to the discovery being published now.

GNS Science palaeontologist Dr Hamish Campbell told the Herald yesterday that the discovery was the first evidence the mighty titanosaurids lived in New Zealand.

The new find was "big news" for New Zealand's heritage, and was dated at 80 million years old - three million years after New Zealand split from Gondwanaland, he said.

New Zealand 80 million years ago was low lying, lushly forested and devoid of any significant mountains, Dr Campbell said.

The dinosaur would have been a "plant vacuum cleaner" he said, living in the fringes of bush and shorelines.

The tuatara would have been the titanosaurid's contemporary, as would the mammals living in New Zealand at the time.

New Zealand would have been about half the size of present-day Australia at the time, before it gradually sank into the sea.

Although not many dinosaur fossils had been found in New Zealand, many more were certainly hiding under the surface, he said.

The key for budding fossil hunters was finding the right rocks, he said.

The sandstone cliffs around Auckland were not only far too young - at about 16 million years old - they also came from between 1 and 2km under the ocean before being thrust up to form Auckland's landmass.

The best rocks were sedimentary, aged between 65 million and 85 million years old, and had come from terrestrial setting.

The closest of those to Auckland were at Port Waikato and included significant plant matter, probably from a river mouth.

The East Coast and Hawkes Bay were also good regions for hunting dinosaurs, he said.

* A large titanosaurid would have weighed more than 100 tonnes.
* A double-decker bus weighs about 8 tonnes.
* The elephants at Auckland Zoo weigh about 4 tonnes.
* A large titanosaurid would measure about 45 metres in length.
* That's almost one quarter the way up to the Sky Tower's Orbit restaurant, which is 190m up.