Between 25 and 21 million years ago, New Zealand was largely submerged beneath the Pacific. However, a study of the relationships between some tiny birds has demonstrated that the immersion was not complete. At least one small island survived the deluge, and on it these birds thrived.
Acanthisittid wrens get their name from a resemblance to wrens in other parts of the world. They are, however, a different family from the “true wrens” or Troglodytidae. Five of the seven acanthisittid species that existed when humans arrived in New Zealand have become extinct, contributing to considerable ignorance about the birds. A study of their genetics aiming to address this has revealed something important about the history of the islands on which they live.
Dr Kieren Mitchell of Adelaide University compared the genomes of the surviving rifleman and south island wren with preserved DNA from three extinct species. The comparisons included Lyall’s wren, which was famously largely extinguished by the efforts of the lighthouse keeper’s cat that discovered it. “We found that some of the wren species were only distantly related to each other, potentially sharing a common ancestor over 25 million years ago,” Mitchell said in a statement.
Lyall's wren: An extinct acanthisittid wren, was discovered when a lighthouse keeper’s cat brought some home. Sadly, the cat drove the birds most of the way to extinction. Public Domain, John Gerrard Keulemans
If New Zealand was entirely submerged in the intervening time, these birds could certainly not have survived there. Consequently, the ancestral wren must either have reached New Zealand from somewhere else after the islands resurfaced, or a part of one of the islands served as a natural ark for the birds through the deluge.
“As the wrens were largely very poor fliers, or even flightless, some land must have remained throughout that period,” Mitchell concluded, publishing his findings in Molecular Phylogenetics and Evolution.
Although the possibility that the wrens did in fact reach New Zealand later can't be ruled out, perhaps having been better fliers at the time, Mitchell told IFLScience this is unlikely. “Fossils from 16-19 million years ago look very similar to the rifleman, limiting their ability to fly long distances,” he said.
The evidence of widespread immersion can be seen in sedimentary records. The fact that many New Zealand species, such as the now-extinct Moa, appeared to evolve from species that arrived shortly after the land re-emerged led to a popular theory that complete submersion wiped clean the slate of New Zealand’s flora and fauna.
“A debate has been going on for about a decade, with bits and pieces of evidence each way,” Mitchell said. “We think we have put a nail in the coffin of the complete submersion theory.” Instead, the wrens’ ancestors probably reached New Zealand before it broke free from the supercontinent Gondwana and have been evolving ever since.
At this stage, there is no way of knowing what part of the islands remained above the waves, let alone how large the refuge might have been. Meanwhile, the south island wren (Xenicus gilviventris) is listed as vulnerable, with limited alpine habitat and decreasing population. The rifleman (Acanthisitta chloris), on the other hand, is considered to be safe for the moment. Despite falling numbers, it should survive – potentially as the last living reminder of the rare survivors of New Zealand's great flood.