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.
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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