Snake in a lake

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Palaeontologists in this country face a problem. Despite an extensive marine fossil record that includes some of the world’s best examples of marine verte­brates, such as whales and penguins, the record of terrestrial vertebrates is pitiful. We have dinosaur fossils from about 70 million years ago (Ma)—not long after New Zealand split from Gondwana—but over the huge gulf of time between then and 100,000 years ago, there are few fossils of land animals. We have an abundance of fossils from the last few tens of thousands of years back to 100,000 years ago, but they are all species that either exist today or became extinct following human arrival.

As a consequence, very little is known about what animals might have lived here over the past 80 million years, and hypotheses about the origins of our existing terrestrial vertebrate fauna are difficult to test.

Recent discoveries in Central Otago, however, may alter the palaeontologi­cal balance sheet.

For some time, it has been known that a large freshwater lake, known as Lake Manuherikia, existed in this area, dating from the lower to mid-Miocene, some 20–15 Ma. The characteris­tic white sediments that betray the lake’s existence are perhaps most famously exposed in the gold fields about St Bathans, but they can also be seen about 50 km away at Bannockburn, near Cromwell—so it must have been a large lake. Abundant macro-and microfossil remains of plants provide a detailed picture of the environment at the time: the climate was some 7–80 C warmer than at present, and plants we take for Australian natives—eucalypts and casuarinas—along with palms, lived around the lake, along with many other scrub and rainforest species. Modern taxa such as fuchsia, supplejack, flax, kamahi, red and silver beeches were also present in the region.

The remains of animals living in the lake’s vicinity were first discovered by Barry Douglas and Jon Lindquist in 1978. Further material was collected by Ewan Fordyce and others in the early 1980s, including, among the many fish bones, about a dozen bones of at least two kinds of waterfowl. Views about what taxa were in New Zealand’s palaeofauna received a jolt in 1997, when Ralph Molnar and Mike Pole reported a crocodilian bone that Pole had found in these same lake sediments in 1989. Where there was one crocodilian bone there were bound to be more, so the stage was set for a re-examination of the sediments. In December 2001, Jim McNamara (South Australian Museum), Craig Jones (Institute of Geological and Nuclear Research), Alan Tennyson (Museum of New Zealand/Te Papa Tongarewa) and Trevor Worthy (Palaeofaunal Surveys), visited the area and began a specific search for fossil vertebrates. Their method was to locate a layer in which bones had accumulated, remove the sediment and wash it through screens to concen­trate the fossils. Big bones were immediately obvious, but most of the “good stuff” was only revealed back in the lab, where many hours were spent picking tiny bone fragments from the silt.

The researchers found fossil vertebrates other than fish in three distinct sites and in different strata at two of the sites. The remains were surprisingly abundant. Apart from thousands of fish bones, at least a couple of hundred potentially identifiable non-fish bones were recovered.

Among these—and of considerable interest—were the remains of several reptiles. The crocodilian gained a few teeth and skin osteoderms (scutes) to add to the single cranial element previously found.

Even so, we still lack something diagnostic by which to relate the animal to other crocodilians in the south-west Pacific, including those now known from New Caledonia, Vanuatu and Fiji, in addition to those from Australia.

A tiny bone fragment bearing three acrodont teeth (teeth which are part of the jaw, rather than insertions or additions) suggests tuatara ancestors were present. If confirmed as sphenodontid, this discovery would be the first fossil record of the group between the end of the Cretaceous, 65 Ma, and the Recent to be found anywhere in the world.

But the real surprises were two fragments bearing snake teeth. Snakes, of course, are not part of New Zealand’s terrestrial fauna, and have no previous fossil history here. But these tiny remnants clearly indicate that snakes lived with crocodilians and ancient tuatara in the Miocene. Also discovered were fragments from two kinds of bat, which preliminary work suggests were related to our mystacinids, or short-tailed bats.

Along with these various reptile and bat remains were relatively abundant bones of at least a dozen types of bird: five types of waterfowl, a couple of waders, four passerines, or songbirds, a crake-sized rail and a parrot. Bird eggshell was common, and some of it may even belong to moa ancestors. None of these taxa are the same as any that are living today.

So, New Zealand finally has a Cenozoic record in the form of a diverse fossil fauna that reveals what terrestrial vertebrates lived in New Zealand for at least one period prior to the Recent.

The study of this fauna has just begun, and it is to be expected that many more surprises are yet in store. But the question about how much of the present fauna was derived from Gondwana at the time New Zealand split away 82 Ma, and has been resident on this land ever since, is beginning to get tangible answers from the rocks.

Similarly, suggestions that various components of our fauna are the result of recent colonisation by over-water dispersal may face a challenge from the fossils. The “Oligocene bottleneck” hypothesis argues that the extensive submergence of New Zealand during the Oligocene (30–24 Ma), when land area was reduced to about 18 per cent or less of what we have at present, filtered out much of the diversity of life New Zealand took with it from Gondwana. But the animals revealed in Central Otago’s fossil lake already challenge this idea.

Snakes and crocodilians clearly survived here after the great inundation, but now they are gone. The questions of when and why they disappeared will occupy palaeontologists for a while yet, but it seems likely that the evolution of some of New Zealand’s unique endemics may yet get a fossil history.

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