A moa walks across a vast flood plain, its feet sinking into the soft mud. Once buried by the next flood, the footprints left behind harden. Millions of years later, a farmhand taking the dogs for a swim on a hot day makes the discovery of a lifetime.
Seven footprints, around 25 cm in diameter, were recently discovered in the bed of the Kye Burn River, near Ranfurly, exposed after summer flooding. Such is the importance of this discovery, they represent the first moa trackway for the South Island and potentially the oldest in Aotearoa New Zealand. Thanks to the hard work of Otago Museum and scientists from the University of Otago’s Geology Department, this ara-moa has been saved from being erased from our biological heritage forever.
— Ian Griffin (@iangriffin) May 9, 2019
The lure of treasure
But what is it about trace fossils (or ichnofossils as they are scientifically known) that fascinates us so much? Trace fossils include trackways, feeding and resting traces, bite marks, gizzard stones, and even coprolites (desiccated poo), urine scour marks and dinosaur vomit.
Unlike the fossilised remains of prehistoric animals, trace fossils provide us with ethereal yet tantalising glimpses of something much more elusive: behaviour. More often than not, behaviour can only be inferred from bones. Yet it’s behaviour that brings us that much closer to reconstructing how prehistoric animals functioned in ancient ecosystems, as if looking along the fossilised moa trackway, just around the corner you will come face-to-face with its maker. Behaviour brings the fossil record to life, from ancient humans hunting a giant ground sloth, to a dinosaur stampede, and a man and his dog walking along Motutapu Island just after Rangitoto erupted, to intra-species competition (sorry wifey for the blatant plug of your paper).
Scientists can learn a lot from trackways including the speed and gait of an animal, body size and mass, and weight bearing adaptations. Some fossil trackways have become world famous like the dinosaur stampede in Winton, Queensland, and the dinosaur trackways of Western Australia’s Kimberly region. Others are more contentious, such as the supposed sauropod trackways from Whanganui Inlet in northwest Nelson.
Hide and seek
I’m at the University of Otago’s Geology Department with Marcus Richards, who was involved in the rescue excavation of the Kye Burn moa trackway. The prints were preserved in sediment the consistency of solidified wet toilet paper or old Christmas cake, and had already partially eroded since their fortuitous discovery only months before.
I’m staring up at a larger than life photograph spanning the entire wall Geology’s ornate wood panelled stairwell, when Marcus excitedly points to something. Jumping out at me, but imperceptible at the time of discovery as all attention was focused on the exquisitely preserved moa footprints, was a ‘ghost print’ in the same layer. This print was from a moa twice the size of the one that produced the trackway.
While the trackway was produced by a medium sized moa, similar in size to the mountain goat of the avian world, the Upland moa (Megalapteryx didinus), the identity of the feathered maker of this large ‘ghost print’ is the subject of collaborative research by Otago Museum and University of Otago scientists. The shear weight of this prehistoric giant would have compressed the younger layers of mud that covered the trackway, leaving the tell-tale impression of a footprint in the older layers that were exposed in the Kye Burn River. While the physical ‘ghost print’ was lost to the excavation process, it’s digital footprint has been preserved for future research.
The Dark Age of moa evolution
I ask Marcus how old he thinks the moa trackway is. The footprints are preserved in the Maniototo Conglomerate, which dates to around 1-11 million years ago, possibly long before the Kakanui Range that borders this area of Central Otago was even on the scene. In contrast, moa trackways from the North Island mostly date to the last 10,000 years.
So what were moa doing during this time and can we use the whakapapa of moa to inform the likely trackway culprit, and vice versa? The Kye Burn prints give us a unique glimpse into a Dark Age of moa evolution, from a time where only a few moa fossils have been found. Aotearoa’s fossil record of moa is only rich for the past 60,000 years – a blink of an eye in the geological history of New Zealand.
There are a handful of moa fossils older than this – the Martin Fauna (1 million years old) from the North Island, and some scrappy pieces of large moa bone and eggshell from the Miocene Wonderland of St Bathans (19-16 million years old) that represent the oldest confirmed presence of moa in New Zealand. For the majority of the 58 million years moa have been in New Zealand, based off genetic evidence, all that remains of this deep ancestry and whakapapa is the information hidden in ancient DNA, stretching back in time to where only ghosts remain.
The evolutionary tree genealogists have constructed suggests that the nine species moa that set eyes on humans around 750 years ago can trace their ancestry back to a moa living in the South Island around 4-8 million years ago. Hang on, I hear you say, didn’t you just mention moa have been here 58 million years, so why the discrepancy? It’s likely that moa underwent repeated evolutionary radiations and population bottlenecks during their long history in New Zealand. The moa we are all familiar with are just the latest models from the factory floor of evolution.
It also transpires that the oldest and most ancestral member of the recent moa posse is the South Island endemic Upland moa. In fact, moa from the South Island as a whole are more ancestral in the whakapapa to their North Island cousins. So why is this? The evolutionary radiation into the three main groups of moa (Megalapterygidae, Dinornithidae, and Emeidae) can be traced back to the uplift of the Southern Alps that created new habitats and job vacancies for moa to fill. It wasn’t until the North Island joined the South Island around 1.5 million years ago with the closure of the Manawatu Strait that moa moved into the North Island and diversified further. This was the first time since before the ‘Oligocene drowning’ around 23 million years ago, where 80-90% of New Zealand went under the waves, that the North and South Islands were joined. Any moa that may have survived the ‘drowning’ on the proto North Island didn’t contribute their genes to the moa we know and love today (if they had, the common ancestor of moa would have lived tens of millions of years ago). Further speciation and diversification occurred when Cook Strait formed around 500,000 years ago, and during repeated warm interglacial and cold glacial cycles of the Ice Age.
It makes perfect sense then that the oldest moa fossils and footprints in New Zealand would be found in the South Island, while the oldest moa fossils in the North Island (the Martin Fauna) is less than 1.5 million years old.
Pin the prints on the moa
At 1-11 million years old, the Kye Burn trackway could potentially pre-date the common ancestor – the moa’s mitochondrial Eve – providing a tangible link to a time beyond the reach of genetic genealogists. It’s equally likely they belong to one of the three main groups of moa, and if younger still, one of the six recognised species of moa from the South Island, though pinning the footprint on the moa is difficult due to geographical size variation and sexual dimorphism in these feathered giants.
For me, their identification to a species (or ichnospecies in the trace fossil world) is not important and that’s half the mystery. This ara-moa is a rare ethereal glimpse into a lost world that cannot be reconstructed from fossil bones. So now we wait, with bated breath, until the prints have been conserved by Otago Museum and scientists can tease out the secrets that they hold.
This blog has been updated to reflect ongoing research by Otago Museum and the University of Otago.