“We propose that New Zealand is likely to be where the earliest ancestor to all crested penguins lived, and where the ancestor of all penguins lived,” Thomas said.
Ancient penguins roamed the lost continent of Zealandia
E. atatu lived in New Zealand tens of millions of years after the rest of Zealandia sank. But the researchers think its ancestor evolved about 60 million years ago, suggesting that penguins likely once wandered the continent while the rest of its surface lay above sea level.
These ancient penguins may have been gigantic. In 2017, researchers found that prehistoric “mega-penguins” stood 5 feet 8 inches tall and weighed 220 pounds.
A lot there is open to question. Was it ever above water? Did it ever become submerged? How would that happen? Are there any terrestrial sediments there? And why is this continental crust so much thinner than other continental crust?
During glacial periods, more of Zealandia becomes a terrestrial rather than a marine environment. Zealandia was originally thought to have no native land mammal fauna, but the discovery in 2006 of a fossil mammal jaw from the Miocene in the Otago region shows otherwise.
It would therefore appear that it was glaciers melting at the end of the last Ice Age that likely sunk it.
Addendum: This would probably put it in roughly the same category as Doggerland in the North Sea, except I rather suspect that, at its greatest extent, it was likely still partially (mostly?) submerged.
No, you have misunderstood. The claim is that the whole thing, specifically including New Zealand, was underwater 23 million years ago, not that prior to that the whole thing was above water. Further, that statement is supported by a newspaper article and a popularization, and there is considerable doubt about whether all of New Zealand was ever submerged.
That’s the contention that goes along with “23 million years ago”, however. Nobody is claiming that Zealandia was not mostly submerged before 23 million years ago. I haven’t so far been able to find any clear statements of when if ever it was mostly above water.