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Tuatara Ancestor Adds to "Sunken New Zealand" Debate

Dave Hansford in Wellington, New Zealand
for National Geographic News
January 30, 2009
 
Fossil remains of an 18-million-year-old reptile found in New Zealand have rekindled debate about whether the island nation was underwater for millions of years.

Three jawbone fragments belonging to an ancestor of the present-day tuatara were recently unearthed at a site near Saint Bathans, Otago, on South Island (New Zealand map).

The tuatara is indigenous to New Zealand and is the only living member of the Sphenodontia family, which has existed for at least 200 million years.

The new find bridges a 70-million-year gap in the Sphenodontia fossil record, said the international team of paleontologists who described the work in last week's issue of the Proceedings of the Royal Society B.

It also casts doubt on a theory that the ancient landmass that includes modern New Zealand completely submerged between 22 million and 25 million years ago, drowning all of its inhabitants.

"Drowned" Continent

New Zealand is only a small part of Zealandia, a sunken continent larger than India.

Geologists think Zealandia drifted across the Tasman Sea after breaking away from the southern supercontinent Gondwana about 80 million years ago (explore a plate tectonics map).

But debate has raged for years over whether New Zealand also went underwater during part of this migration.

Submergence advocates think all of New Zealand's ancient plants and animals were wiped out by the inundation, and that the ancestors of today's wildlife must have later flown, drifted, or been blown to the islands.

Critics of this idea maintain that much of the country's current inhabitants, including the ancient sphenodontid line, were already aboard Zealandia when it broke away.

The study authors—based at University College London, the University of Adelaide, and the Museum of New Zealand Te Papa Tongarewa—say the new fossils confirm the presence of sphenodontids back to the early Miocene epoch some 19 to 16 million years ago.

This suggests that enough of Zealandia—around 20 percent of today's visible land area—remained above the waves to provide a home to "some of New Zealand's iconic biota," said Te Papa fossil curator Alan Tennyson.

The discovery doesn't totally quash the theory that New Zealand was once fully underwater, he said, but "this evidence makes it much less likely."

That's because the find drastically narrows the window when wind- or current-assisted colonizers could have arrived following the islands' submergence.

Tuatara Rafting?

Still, critics of the submergence theory maintain that it also creates the question of how sphenodontids such as the tuatara would otherwise have gotten to New Zealand after land resurfaced.

"If the continent of Zealandia was completely submerged, the sphenodon would have had to recolonize it by ocean rafting" on deadwood or other drifting debris, said lead study author Marc Jones, a postdoctoral fellow at University College London.

"If we look at the transoceanic capabilities of the modern [tuatara], it can swim but only short distances. It is able to survive without food for several months, but dehydration would be a serious problem for a long journey."

Paleontologist Ewan Fordyce, a professor at the University of Otago who was not involved in the study, said another problem with the theory is an apparent lack of a mainland tuatara population that could have recolonized the islands.

"If tuatara had actually migrated here after [Zealandia went underwater], we would expect to find a fossil record in nearby land areas like Australia," he said, "and they're just not there."
 

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