Dawn bathes the floodplain in amber light and pink-hued clouds decorate the distant horizon. The air is sticky with humidity. Buzzing insects have broken the silence of the night and something enormous is on the move; the vibrations are felt first, the massive footfalls of creatures up to 70 tonnes in weight and 30 metres in length.
Across this river delta about 10 km from the coast on the supercontinent of Gondwana a herd of long-necked sauropods is pacing. They cross sandbars between braided river channels. Their long tails flick back and forth, leaving deep impressions in the soft mud. Travelling among them are other herbivores – stegosaurs, armoured ankylosaurs and small, beaked ornithopods, moving swiftly on their hind legs.
The delta itself holds little attraction for these dinosaurs but acts as a great thoroughfare between fern and cycad forests on either side. Today the mud is just firm enough to record the paths these dinosaurs take. Before rain can rinse those tracks away, a layer of sand will wash over them, preserving the impressions for millennia until Australia’s first human inhabitants notice shapes marked in sandstone and weave them into their Dreamtime narratives.
The scene is fanciful but something like this must have happened about 130 million years ago to leave traces of 21 kinds of dinosaur in the rock around Walmadany, about 50 km north of Broome in northern Western Australia. The rock is part of a formation geologists call the Broome Sandstone, with outcrops along more than 300 km of the Kimberley coast, straddling the town of Broome.
The menagerie identified around Walmadany includes gargantuan long-necked herbivorous sauropods (think Brachiosaurus); other herbivores ranging from the size of kangaroos to bigger than elephants; the first-ever evidence for stegosaurs in Australia; and five kinds of carnivorous theropods (think Velociraptor). At 70 sites along a 100 km stretch of the coast, tens of thousands of footprints offer a snapshot of a time when Australia was joined to South America, Antarctica and the other southern land masses that then made up Gondwana.
“It would have been like looking out over the Serengeti,” says Steve Salisbury, a vertebrate palaeontologist at the University of Queensland who has led the documentation of at least 70 sites within this fossil trove. “It’s a complete dinosaur ecosystem preserved in these rocks.”
The extent of the footprints was revealed in papers published early in 2017 in the Memoir of the Society of Vertebrate Palaeontology and PeerJ. They made headlines worldwide, because some of the sauropod prints – big enough to fit a person – were the largest dinosaur prints ever discovered. These footprints are also significant because they offer a window into Australia in the Early Cretaceous period – a period for which we have almost no fossil records because there are very few rock outcrops of this age in Australia.
To date, almost all knowledge of Australian dinosaurs comes from the east of the continent – and mainly from a time slice covering only 90 to 115 million years ago. “Outside of that bracket we know very little,” Salisbury says.
Phil Bell, a palaeontologist at the University of New England in Armidale, NSW, describes the diversity of species recorded in the Kimberley sandstone as “simply astounding”, adding that it is satisfying to finally see the scientific work done and the results published. “We’ve known about these tracks for decades,” he says, “but they were never studied in detail and their worth never fully appreciated.”
Palaeontologists may have been in the dark but the tracks were no secret to the local Goolarabooloo people. “To the Goolarabooloo the dinosaur tracks are part of their culture,” Salisbury says. “They feel a strong link to these tracks and have a very deep knowledge of where they are and what’s happened to them over hundreds of generations.”
In 2011, the Goolarabooloo approached Salisbury to document them. They recognised the theropod tracks to be bird-like (birds are, in fact, living theropod dinosaurs). Both the tracks and the impressions of cycad-like plants are woven into Goolarabooloo creation mythology, forming part of a songline about the journey of a being called Marala, the emu man. “Marala was the lawgiver,” explains Goolarabooloo elder Phillip Roe. “He gave country the rules we need to follow. How to behave, to keep things in balance.”
The plant fossils are seen as impressions of Marala’s tail feathers where he sat to rest. The Goolarabooloo believe Marala passed into the sky and settled into the Milky Way – in keeping with how many Aboriginal cultures see the dark space between the stars in the Milky Way as an emu.
While their traditional view of the footprints is quite different to the scientific conclusions, this hasn’t been a problem for Goolarabooloo, such as Roe, who have a close working relationship with Salisbury and his colleagues. Says team member Anthony Romilio: “They see ours as a different interpretation, but it’s not an unwelcome one. Goolarabooloo are of the attitude of inclusion – of sharing knowledge and beliefs.”
In 2011 the Goolarabooloo were in crisis. James Price Point, the European name for the Walmadany headland where many footprints are found, had been selected three years before by the West Australian government as the site for an onshore liquid natural gas (LNG) processing facility. The plan had burgeoned into a $45 billion project, including a harbour, led by Woodside Petroleum and backed by the state government. Much of the coastline was to be developed and the Goolarabooloo had little say in it.
Government-backed scientists had surveyed for footprints in 2009-10 but reported there was little of scientific interest, much to the surprise of the traditional custodians. “We needed the world to see what was at stake,” Roe says. They called Salisbury and asked him to come and look at the tracks.
Salisbury jumped at the chance. He arrived in 2011 at the height of the campaign to save the region. “These people were being told they had no right to speak for this land that they’d looked after for thousands of years,” he recalls. “We knew the best thing we could offer was the science.” To this end, he spent much time with Roe and fellow indigenous lawman Richard Hunter, learning how to recognise the difficult-to-spot impressions in the rocks: “I discovered there was a lot there, much more than had been let on.”
Then the federal environment minister, Tony Burke, sent a delegation to assess the area. Salisbury’s team convinced the delegates of its tremendous scientific and cultural significance. A month later Burke announced the greater West Kimberley, including the dinosaur tracks, would be National Heritage listed. The LNG project proposal finally collapsed a few years later, in 2013, when Woodside pulled out.
With the protection of National Heritage listing, Salisbury and his team got to work. For the past five years the scientists have returned to the site about three times a year, timing visits to coincide with the confluence of the lowest tides occurring during dawn or dusk, when long shadows help the eye pick out the elusive footprints in the red-brown Broome sandstone.
It is a challenging environment. The Kimberley coast boasts daily tides of up to 10 metres and the tracks are found up to half a kilometre out from the shore. “They are only exposed for a short period when the tide retreats, so you can’t go out there, lay your tape measures and take your time recording them,” Salisbury says. “You have to be able to do it fast.”
“The tide turns really, really quickly,” Romilio affirms. “We often need to wade through belly-deep water. On one occasion, after working on a particular reef, we came back the next day to have one of the campers tell us they’d seen a three-metre croc swimming where we’d been just a day earlier.”
On another occasion, a bushfire encircled the researchers’ beachside camp. “There were flames surrounding us on three sides,” Roy recalls. “I even found a scorch mark on my tent the next morning.”
To record such tracks researchers have historically made drawings and created casts. There is little time to do that with shifting tides and circling crocodiles. Luckily technology has come to the rescue. Using drones equipped with 3-D laser scanners and high-resolution photography, Salisbury says researchers now have a detailed digital record of the entire coastline. They have also used rapid-setting silicone to take impressions of prints in order to create lightweight plastic replicas back in the lab.
What they are learning is fascinating. Of 21 identified trackmakers (the animals that left the prints), six are herbivorous sauropods, five are carnivorous theropods, four are bipedal herbivorous ornithopods (some similar to Muttaburrasaurus), and six are armoured ‘thyreophoran’ dinosaurs – either stegosaurs or ankylosaurs.
“In South America, the US and China there are sites with a large number of different track types, but the Broome Sandstone surpasses these,” Romilio says. The diversity of armoured dinosaur tracks is especially surprising given that hitherto only the fossils of two such species had been found in Australia – the ankylosaurs Minmi and Kunbarrasaurus, both located in Queensland.
The carnivore tracks indicate animals ranging from the size of emus up to species six to seven metres in length. Some of them might have been ‘megaraptorid’ theropods, similar to Australovenator from Winton in Queensland; or they might have been ‘abelisaurs’ – more primitive carnivores with tiny arms most prevalent in South America, Africa and Madagascar.
Another observation is that the stegosaur prints appear to show animals walking on their hind legs for some of the time – definitely not the way Stegosaurus appears in illustrations and toys. “The hand prints all of a sudden disappear, and then it’s just these big, fat, three-toed tracks,” Salisbury says. “All we can work out is that the stegosaur that made these tracks was somehow capable of getting up onto its hind legs, a bit like a pangolin, because there’s no other way it could have left those tracks. The rocks don’t lie.” Stegosaur tracks in the US and Spain point to similar behaviour.
Also of surprise is the scale of the 1.7-metre footprints, which could only have been made by the very largest of dinosaurs – perhaps something like the huge titanosaur discovered in Patagonia several years ago. The particular sauropod that left this huge footprint, estimated to have been 30 metres in length and 70 tonnes in weight, is yet to be officially named and described as a species.
Often footprints indicate what the animals were doing; in other cases it can be confusing: “It’s like if you went down to the dam on a farm and the cattle and kangaroos have all made a mess in the mud,” Salisbury says. “The sauropods have trampled each other’s tracks and walked in wiggly lines, making it really hard for us to make sense of them.”
In one case, though, Romilio says “we knew exactly where it was going and what it was doing.” The animal’s tracks told scientists it was slipping and struggling to get a grip on the mud as it moved down a slope: “Its footprints are dragging here and there. It was sliding down, slipping and sometimes making double steps.”
Because trace fossils – such as footprints, burrows and nests – are the products of behaviour, they offer special insights into animal lives millions of years ago, says Anthony Martin, an expert on footprints and other trace fossils at Emory University in Atlanta, Georgia. “Footprints, for example, can inform us of whether an animal was walking, running, stopping, changing directions, swimming or reacting to other animals; and they are normally in the same place where dinosaurs made them, like a lakeshore or a streambank. In contrast, bones can only tell us so much about their behaviours, as they were often moved far away from their original environments.”
While the 21 trackmakers identified so far have been determined from a 25 km stretch of coast around Walmadany, the scientists have been studying footprints along a 100 km stretch. There are almost certainly going to be new animals announced among the trackmakers from the wider area.
So far the research has involved documenting the prints and identifying the kind and number of animals that created them, but much work lies ahead. “We would like to understand the behaviour of these trackmakers,” Romilio says. “Did they move in herds? Were they solitary? Why were some types of dinosaur, such as the carnivores, more prevalent in some areas than others?”
To answer such questions, Salisbury and his team must continue to return to the Kimberley to collect more data. “It’s an environment that’s changing all the time. Some areas are getting buried by sand and others are exposed as the sand moves,” Salisbury says. “I don’t think we’ve covered all of it, not by a long shot.”
One question yet to be definitively answered is why there are no fossil bones. The scientists suspect the environment was too acidic to preserve bone.
As far as they can determine, the footprints were left in a very narrow time window – perhaps as little as a few days or weeks. They might also very easily have been obliterated forever with a few days’ work from bulldozers. “Thankfully we can talk about this area having the world’s most diverse dinosaur track fauna and not an LNG precinct,” Salisbury says.