Landis and his colleagues looked at the evidence for dry land during the Oligocene period in order to better estimate the location and proportion available for the flora and fauna to have survived on. One major piece of evidence that they used was the Waipounamu erosion surface. These surfaces are flat areas in the landscape that represent wave cut platforms created as Zealandia was slowly inundated by sea. They also used sedimentary deposits, terrestrial fossil sequences and geomorphological evidence to reach a conclusion that there was no hard evidence for ANY land during the late Oligocene. The authors argue that the New Zealand region may have completely submerged or, at most, persisted in tiny remnants of land during the peak inundation. This has obvious implications for New Zealand’s biology as it suggests that all, or at least the vast majority, of species have colonised New Zealand over the last 22 million years across an ocean gap rather than simply persisting here for 80 million years in splendid isolation.
So much for the geology, what does the biology tell us? We know that the New Zealand biota can change very quickly. New Zealand’s forest composition has changed remarkably over the last tens of thousands of years. Fossils from a lake bed at St Bathans from 16 million years ago show evidence of a more tropical fauna complete with crocodiles, parrot and bat species not found in New Zealand today and even a mammal bone! So 22 million years is long enough for all sorts of things to happen. Molecular data, which can estimate when common ancestors lived, also overwhelmingly shows that closest relatives to taxa living in New Zealand (generally found in Australia) diverged from them well within the last 20 million years, implying that they arrived and colonised here. The only biological evidence that challenges the idea of submergence is that of ancient taxa, like tuatara, leiopelmatid frogs and the recently extinct moa. Their presence in New Zealand and nowhere else suggests that they inhabited Gondwanaland and were isolated here for 80 million years. Tuatara and the frogs are both groups that can persist on small islands and may represent true ‘ghosts of Gondwana’ that made it through the submergence. Moa are species of ratites and we know that another ratite group, the kiwi, did successfully colonise New Zealand. Perhaps moa did the same.