When New Zealand was colder, and central Otago was more like the Russian steppes than it is today, a moa tramped through soft mud. The bird was as heavy as a tall man, and its feet sank deep into the silt, so it took each step slowly, carefully.
None of us would have recognised the landscape it walked through. The Southern Alps were taller, thickened by glaciers. The wind was full of the dust of rocks ground down by ice. The moa would have looked quite funny to us: a long neck protruding from a thick, drum-like body, set atop spindly legs.
The consistency of the mud that day was perfect, like the clay used to record a baby’s handprint, and it captured all the details of the moa’s feet, down to the pressure of its metatarsal pads—its knuckles—and its claws.
Eventually, the moa stepped back onto solid ground and out of history. Its footprints dried in the sun, but were soon covered up, either by water or by dust. They lay, locked in the earth, for more than three million years—until the beginning of 2019, when a flood uncovered them.
It’s an ordinary river, the Kyeburn, and it was an ordinary autumn day, sweltering with heat. Farmhand Michael Johnston thought he might as well take his boss’s dogs for a swim. On a bend in the river, just off State Highway 85, he saw footprints leading down into the water. They were bigger than his hands.
Johnston snapped some phone pictures and sent a Facebook message to Otago Museum. Kane Fleury, who is now the museum’s curator of natural science, knew exactly what he was looking at, and how fragile it was. He organised for the footprints to be cut out of the riverbed and protected.
At first, it looked like the track of a lone moa, but it turned out there had been two birds, not one.
Analysis of the footprints published in November reveals that the moa that made them probably weighed around 85 kilograms and was strolling at the extremely leisurely pace of 2.6 kilometres per hour. One ghostly footprint, also recorded by the silt, belonged to another bird, thought to weigh 158 kilograms.
Estimating these sizes involved plugging measurements into existing algorithms that predict bipedal animals’ speed.
“You can estimate, based on the size of the steps and the size of the feet, how big the hip would have been, and how high it would have sat,” says Fleury. “And then when things walk, there’s an aspect of gravity influencing the footfall. The speed is pretty well constrained within that. And so you can plug the distances between the steps into the equations and work out how fast they travel.”
Determining the weight of the moa involved a different equation tailored to bird proportions, which involved measuring the length of the moas’ toes. “They’re probably not the most accurate,” says Fleury, “but I think they’re really important for giving people a picture of how big an animal is.”
Dating of the riverbank mud suggested that the birds walked through the silt at least 3.6 million years ago. No other moa remains from this period have been found.
“It’s a window into, I suppose, what you call the Dark Age of moa evolution, where we don’t have fossils, and our understanding of moa evolution is all based on DNA,” says Nic Rawlence, who directs the palaeogenetics lab at the University of Otago. “You’ve got heaps of stuff up to 60,000 years old, and then it becomes really patchy.”
Fleury and Rawlence didn’t realise this at the time, but Johnston’s discovery marked the onset of dramatic change for fossil finds.
Since 2019, extreme weather events and increased rainfall mean fossils have been popping out along rivers and coastlines at a rapid rate—and they’re now appearing so quickly that curators and scientists are struggling to get to them in time.
Recently, Rawlence was alerted through Oamaru Museum by a member of the public to a moa skeleton that had appeared in a coastal bank, as though emerging from the mud. “We had some of the vertebra, part of the pelvis, the legs, and we had like a femur sitting right on top of a tibia on top of fibula,” he says. “This was like, wind howling, beginning to rain, tide coming in, and I’m sitting there, trying to get this thing out.”
Next to the moa bones, from a different time period, were the remains of an umu, or oven, in cross-section.
“That entire coastline, from Oamaru down to, like, Karitāne, there’s just these moa bones eroding all out of there,” says Rawlence. “You can get skeletons eroding out of there that will be observable on one low tide, and the next low tide, they’re either washed out to sea or covered in tonnes of sand.”
Earlier this year, a cyclone uncovered large marine reptile fossils in a North Island riverbed—the same area frequented by fossil hunter Joan Wiffen, who discovered the first dinosaur bones in New Zealand. Last year, fossils hit the news when a collector on the West Coast chainsawed a whale fossil out of a river bank, sparking outrage and police action.
Rawlence hopes that interested members of the public will help out by providing a “fossil forecast” of newly uncovered remains. Recently, he made an appearance on a Christchurch fossil hunter’s YouTube channel to talk about ethical collecting.
“Private collectors are not all bad,” says Rawlence.
“Joan Wiffen was a private collector. And we need private collectors and members of the public to tell us what’s eroding out—otherwise the moa footprints wouldn’t have been found.”
Fleury says it was a series of lucky coincidences that allowed the moa footprints to be preserved. “It was just an absolute fluke. They’re incredibly fragile, and the chances of them actually persisting any longer in that river would have been extremely low.”
Fossils are safest when they’re buried; when they’re exposed to sunlight and air “they’re in the absolute most vulnerable state”, says Fleury. Still, he encourages people to get in touch with people like himself or Rawlence before moving any items out of harm’s way. “That scientific context of where the fossil is found is super critical to its story. A rock being found a metre here or a metre there can change its age by multiple millions of years.”
What’s going to be uncovered by the next flood? What’s just beneath the surface, waiting for one good rainshower to crack the silt lid of millennia and fill in more details of this country’s weird past?
“We rely on the public to find these things,” says Fleury, “and they play an incredible role in the documenting of New Zealand’s natural history.”