— Fossil bones of a mouse-sized creature that died between 16 million and 19 million years ago have been discovered on the South Island of New Zealand. It is the first hard evidence that the islands once had their own indigenous land mammals.
Today the only land mammals that live in New Zealand are animals like Australian possums which have arrived since human settlement although the country does have its own species of bats, seals and sea lions.
The find, by Trevor Worthy of Adelaide University, Australia, and colleagues, includes two jawbones, and one thigh bone, from at least two of the creatures, says team member Suzanne Hand. The amazing thing is, it is unlike any other fossil mammal found anywhere else, she says.
The shape of the fossil bones suggest a very primitive mammal that would have evolved before the mammal-line split into placental mammals and marsupials, 125 million years ago, says Hand of the University of New South Wales in Sydney, Australia. As the fossil bones are only around 16 million to 19 million years old, it appears the mammal managed to survive for at least 100 million years before going extinct.
The lack of fossils of New Zealand land mammals had long been considered a major mystery. There are plenty of fossils of land mammals in Australia that date from 125 million to 100 million years ago. During that time New Zealand and Australia were both part of the same landmass, which suggests that land mammals also lived in New Zealand, and were perhaps driven to extinction at a later date.
The new discovery suggests that land mammals did indeed roam New Zealand, but have simply proved difficult to find.
The delicious and infuriating thing about palaeontology is that you depend on blind serendipity, says team member Michael Archer also at the University of New South Wales.
"It's truly a fantastic find, says John Long, director of science at Museum Victoria, in Melbourne, Australia. The discovery of non-flying mammal fossils in the Miocene of New Zealand is extremely significant, implying that archaic mammals that appeared at the time of the dinosaurs have been living as 'survivors' on New Zealand for probably a hundred million years. It holds enormous implications for future field work.
The find is also a strike against another theory that has been used to explain the lack of New Zealand mammals, Archer says. Geologists have argued that New Zealand was submerged beneath the sea from 25 to 30 million years ago, and re-colonised by plant and animal species from nearby landmasses like Australia once it re-emerged.
But the new fossils suggest that the ancient mouse-sized creature would have been incapable of crossing the sea to colonise a newly emerged New Zealand and must have lived there all along, says Archer. The top of the thigh bone tells us a great deal about how the animal moved. It did not swim. It did not fly. It had to waddle, and it wouldnt have been capable of rafting there.
Biogeographer Jonathan Waters of the University of Otago in Dunedin, New Zealand, is not convinced, however. There are enough cases where small vertebrates have managed to colonise oceanic islands by rafting, he says.
Waters points to the case of the Chatham Islands, 600 kilometres to the east of New Zealand, which emerged only a few million years ago and are home to lizards that could only have got there by rafting.
Journal Reference: Proceedings of the National Academy of Sciences (vol 103, p 19419)