Scientists have found a five-million-year-old tooth belonging to an ancient gigantic whale estimated to be about 18 metres long and 40,000 kilogrammes in weight. Measuring an impressive 30 centimetres long and weighing about three kilogrammes, the ancient tooth is the largest ever collected in Australia. It comes from an extinct species of sperm whale that would have hunted other whales. The finding is internationally significant, as it represents the first evidence of such a gigantic sperm whale outside the Americas, said Erich Fitzgerald, a palaeontologist at Museum Victoria in Australia. A fossil enthusiast found the tooth in February this year on the Beaumaris beach in Australia, a fossil site dating back 66 million years, The Sydney Morning Herald reported.

The fossil was donated to Museum Victoria, where it will prove a key asset to scientific research and education. Hailing from the Pliocene epoch, the fossilised tooth is estimated to be about five million years old - making it the youngest known fossil of the ancient whale-eating sperm whale.

Unlike present-day sperm whales which prey on squid and fish, the tooth of this sperm whale shows it was a fearsome predator. Such insight adds to knowledge of the evolution of marine megafauna, researchers said.

"If we only had today's deep-diving, squid-sucking sperm whales to go on, we could not predict that just 5 million years ago there were giant predatory sperm whales with immense teeth that hunted other whales," Fitzgerald said.

Closely related to a species from Peru, Livyatan melvillei, the Beaumaris sperm whale had dental dimensions exceeding that of Tyrannosaurus rex.

Fitzgerald said the tooth was incomplete - missing the tip of the crown and some of the base of the root.

"If the whale had lived longer it would have been bigger," Fitzgerald said.