Big bird tracks in NZ river among oldest moa material in existence

About 3.57 million years ago, a stately moa picked its way across the probably slippery, silty-clay surface of an ancient river plain in New Zealand, leaving a trackway of 7 steps.

The extinct bird’s footprints – measuring roughly a ruler’s length (260-294mm) and width (272-300mm), sinking 46mm deep in the clay – are the first ever discovered on the South Island, and the second oldest fossil record of moa in the country.

At the same place, and about the same time; another, larger bird left a single print nearby, measuring 448mm wide and 385mm long.

According to the team of New Zealand and Australian researchers publishing in the Journal of the Royal Society of New Zealand, these ancient footprints (known as trace fossils) were probably buried quickly under glacial windblown dust, at a site now known as the Kyeburn River in Central Otago.

In March 2019, the footprints were spotted – in fine-grained, compacted clay at the bottom of a swimming hole approximately 1.3m deep – by farmhand Michael Johnston when he was taking his dogs for a swim.

Kane Fleury, natural history curator (and first author of the paper) at Otago Museum was the first to respond to Johnston’s call. 

Fleury tells Cosmos he soon visited the site to verify the find and has led the project ever since, organising everything from resource consents and permissions to excavations and ongoing research.

He says the place where Johnston found the moa tracks “was very accessible […] in the first upstream meander from the major bridge of the state highway”.

“The river is amazingly clear, and you could just see them from the side of the river bank, which was pretty impressive,” Fleury recalls.

He suspects the tracks were probably exposed by a huge flood a few months prior. “The river normally runs pretty low most of the time,” he says. “But when it rains, there’s a horrendous amount of water that can come down that river.”

In discussion with local iwi, the decision was made to excavate the footprints, particularly given one had already been heavily damaged by the river, Fleury says. 

That wasn’t a simple task, he says. It required the entire river to be diverted around the site, with a model of the site captured using photogrammetry before the tracks could be excavated.

Further analysis involved a technique called cosmogenic nuclide dating, comparisons with moa foot skeletons from museums, and the use of algorithms and models to estimate measurements of the birds’ weight, hip height and speed of movement.

The paper concludes the 7-print trackway was likely left by a moa from the Emeidae family, probably the genus Pachyornis – by a bird weighing an estimated 84.6 kilograms travelling at a steady pace of 2.6km/h in roughly metre-length strides. The same genus includes species such as the heavy-footed moa and crested moa.

The second bird, which left the much larger footprint, was likely from the family Dinornithidae and the genus Dinornis, a relation to the South Island Giant Moa, weighing an estimated 158kg. “That’s a big, big bird,” Fleury says.

He says, the fossil find “provides the first evidence that moa, especially the Dinornis species, had attained that ginormous size by that point in time”.

Cosmogenic nuclide dating is a technique which measures the decay of isotopes that have formed in quartz as a result of radiation from the sky. “If that rock has been buried, you can look at the decay of those isotopes through time to give you a predicted burial date of when that rock was last exposed to the surface,” explains Fleury. 

Fleury says the dating technique was “incredibly important” in assigning about a 3.57 million year old average minimum average age, most likely from the Late Pliocene. He says, it’s a period “which is a bit of a dark hole in New Zealand’s evolutionary record” from which not much is recorded or known about. 

“It makes them some of the oldest moa material in existence,” he says. Only some small fragments of bones and an egg from the St Bathans material are older, at about 12-15 million years old.

There were 9 species of moa (Dinornithiformes) known to occur in New Zealand, ranging in size from that of a large turkey to a 3-metre tall giant moa. The birds were an important natural food source for Māori until their extinction around 600 years ago.

The measurements of the fossil site were compared to foot skeletons from moa known to live on the South Island: little bush moa (Anomalopteryx didiformis), stout legged moa (Euryapteryx curtus gravis), eastern moa (Emeus crassus), heavy-footed moa (Pachyornis elephantopus) and South Island Giant Moa (Dinornis robustus). As well as upland moa (Megalapteryx didinis).

While the moa skeletons held in museums are nowhere near as old as the Kyeburn River fossils, they can provide a useful reference point for the moa genera and family, Fleury says.

He says it’s important to recognise the role of two local iwi groups in the research: Kāti Huirapa Rūnaka ki Puketeraki and Rūnaka o Ōtākou. 

And also sadly, the recent death of one of the paper’s co-authors, Emeritus Professor Ewan Fordyce from the University of Otago. “This was one of his last large excavations that he worked on,” Fleury says.

The trace fossils from the excavated trackway are now held in the Tūhura Otago Museum in Dunedin.