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The myth of New Zealand's predator-free history

Scientists have found evidence of a bat three times the size of those around today. Illustration/Gavin Mouldey
Early New Zealand echoed to the sound of birds, secure from the furry predators of other continents, until the relatively recent arrival of predatory mammals over the past 800 years. Right? Wrong. Our mammal-free prehistory was blown out of the water in 2006, when researchers in Central Otago found the fossil of a small mouse-like creature that appeared to have pattered across our forest floor 16-19 million years ago. Different from placental or marsupial mammals, and completely unrelated to mice, it belonged to a primitive group living during the Mesozoic period before the Zealandia landmass split away from the Gondwana supercontinent.

Since then, the discovery of an unrelated molar suggests a second small land mammal was part of a diverse catalogue of early New Zealand wildlife that we now know included turtles, crocodiles, giant bats and flamingos.

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Over the past 17 years, an international team led by New Zealand palaeontologist Trevor Worthy, based at Flinders University in Adelaide, has been working on a 40m-thick sequence of sediment layers from the massive palaeolake, Manuherikia, that covered 5600sq km of Central Otago about 20 million years ago. Nine times the size of Lake Taupō, the lake bed has proved to be an extensive boneyard for our historic flora and fauna, overturning the notion that we were a land of birds, bats and insects. Now it appears our archetypal species – our tuatara, moa, kiwi, wrens and native frogs – evolved in a far more complex environment.

Zealandia’s birdlife was prolific. Analysis of several tonnes of sediment has revealed 40 kinds of birds, including ducks, geese, diving petrels (the oldest recorded globally), two types of herons, a flamingo, several waders (including representatives of groups now found only in Antarctica and arid Australia), an adzebill, a tiny wren, an eagle about the size of a small wedge-tailed eagle, another smaller hawk-sized bird, two species of rails, four species of parrot, an owlet-nightjar, a subtropical swiftlet and a number of as-yet-unidentified songbirds (expected to include the ancestors of the kōkako, piopio and huia). Alongside recent seabird and penguin fossils found in Canterbury and coastal Otago, including last year’s discovery of a towering 1.7m penguin, these findings show New Zealand played a major role in the history of bird evolution.

An owlet-nightjar. Illustration/Paul Martinson/Te Papa

“[New Zealand] was one of the hubs of bird evolution during the period immediately after the extinction of the dinosaurs,” says Paul Scofield, senior curator of natural history at Canterbury Museum. “We’re not saying New Zealand also had fossil penguins or a fossil seabird – these are the oldest penguins, the oldest tropical birds found anywhere. They are amongst the oldest fossil representatives of the animals we call birds.”

One of the most surprising finds in Manuherikia’s St Bathans fauna was the bone fragments of a small kiwi. About a third the size of the little-spotted kiwi, it could well represent an ancestral flying kiwi, backing theories that the kiwi ancestor was not a moa-sized bird dwarfed over time but that, like the moa, it flew to New Zealand and subsequently became flightless.

Among the thousands of bird bones and millions of fish bones, the team has also unearthed the fossils of a native frog, a giant land turtle, a close relative of the modern-day tuatara and two species of 3-4m crocodile, the first undisputed record of crocodiles in this country. Also emerging from the sediment are the bones of five kinds of bat, including, most recently, a new genus of burrowing or short-tailed bat three times the size of the average bat today.

This catalogue of wildlife lived in a climate close to that of a North Queensland rainforest. Average temperatures would have been in the 16-20°C range with high (1500-2500 mm) annual rainfall. Between 13 and 14 million years ago, temperatures dropped about 8°C. The Pleistocene ice ages added another 5°C fall, bringing about the loss of tropical plant species – the cycads, eucalypts, Casuarinaceae and palms – and an exodus of subtropical birds and animals. (These ice ages also marked the appearance of new species, including the ancestors of the Haast’s eagle, takahē and the large laughing owl.)

An artist’s impression of moa and kiwi in New Zealand’s prehistoric forest. Illustration/Vivian Ward

But their presence in the fossil record also challenges the theory that, 25-23 million years ago, the whole of the Zealandia landmass slid under the ocean, with subsequent plant and animal species recolonising the islands once the land resurfaced about three million years later. Much of New Zealand did go under water, but there is evidence that enough land remained for some species to survive, including our so-called ghost lineages – the tuatara, kākāpō and frogs – that occupied Zealandia before it separated from Gondwana.

“Certainly, land area was markedly reduced,” says Worthy, “so limiting the diversity of animals surviving in terrestrial environments, but there does seem to be growing evidence that several vertebrates passed through this evolutionary bottleneck.”

Joining this group are other, more recent, species that may have used the small peaks of a sinking Zealandia to walk, fly or float from Australia.

“When the zoologists saw tuataras and all these strange birds, some thought all these things could have been on Gondwana when it drifted away,” says Worthy. “Since then, it has become apparent that, for a heck of a lot of even New Zealand’s iconic species such as the laughing owl or piopio, the splits from Australia aren’t very old, maybe 20-30 million years old – not old enough to fit the period when New Zealand was rifting from Gondwana.”

There are still massive gaps in our terrestrial fossil record, thanks to 10 million years of geological upheaval, but each tonne of sediment, each painstaking hour washing, sorting and identifying, brings us closer to a new understanding of the diverse life forms that once lived on this continent.

This article was first published in the September 29, 2018 issue of the New Zealand Listener.