WELLINGTON, April 18 (Xinhua) -- Giant freshwater crayfish, up to 25cm long, once roamed New Zealand's ancient lakes around 20 million years ago, three times the size of today's native species, a new study has revealed.

The discovery in New Zealand's Central Otago, which centers on the unexpected fossilization of crayfish molars, marks a significant step forward in understanding ancient freshwater ecosystems, the University of Canterbury said in a release on Thursday.

While crayfish bodies rarely fossilize due to their chemical makeup, researchers found that the molars on their jaws, made of a different material, much like mammal teeth, can survive for millions of years, according to the study published in the scientific journal Alcheringa.

"This means we can now say categorically where freshwater crayfish lived in prehistoric times. That will tell us much more about prehistoric ecosystems and how they worked," said Paul Scofield, the study's co-author and Canterbury Museum Senior Curator Natural History.

The research team identified crayfish fossils from eight jaw fragments, each just 4mm long, which were painstakingly sifted from 100kg of fossil-laden mud. The study shows at least three crayfish species once coexisted there, compared to only two present-day species found in different parts of New Zealand.

Adding to the findings, the team also reclassified mysterious "yabbie buttons" -- calcium deposits from crayfish stomachs, previously mistaken for fossil fish teeth, said co-author Trevor Worthy of Australia's Flinders University. ■