More than 100 million years ago, on a muddy stretch of land that is now Australia, nearly two-dozen species of dinosaur once roamed.

There were duck-billed ornithopods, which left long, three-toed tracks in their wake. Heavy armoured dinosaurs pressed large, tulip-shaped prints into the soil. Predators scratched the ground with their talons. And the feet of gigantic, long-necked sauropods created bathtub-sized depressions in the dirt.

Asteroids struck, continents moved, sea levels rose and fell. What was once a damp, forested environment surrounded by shallow seas became the hot, rugged coastline of northwestern Australia.

Richard Hunter lies next to a 1.7 metre dinosaur footprint belonging to a sauropod. The scale Richard is holding is 40cm long. Photo: Steve Salisbury

But the dinosaurs' tracks remained. The footprint assemblage, which contains evidence of 21 species, is the most diverse in the world, researchers reported Friday in the Journal of Vertebrate Paleontology.

One of those tracks is the largest dinosaur print ever recorded: a 5-foot-9-inch (1.75-metre) print from a sauropod, or long-necked dinosaur. The tracks also provide the first evidence that spiky-tailed stegosaurs lived in the land down under.

The Yanijarri-Lurujarri section of the Dampier Peninsula, Western Australia. Dinosaur tracksites are scattered all along this stretch of coast, intermittently exposed at low tide on shore platforms and reefs of the Lower Cretaceous (Valanginian-Barremian) Broome Sandstone. Photo: Supplied

"The tracks provide a snapshot, a census if you will, of an extremely diverse dinosaur fauna," lead author Steve Salisbury, a palaeontologist at the University of Queensland, told Gizmodo. "Twenty-one different types of dinosaurs all living together at the same time in the same area. We have never seen this level of diversity before, anywhere in the world. It's the Cretaceous equivalent of the Serengeti. And it's written in stone."

There are thousands of marks along the 24-kilometre stretch of coastline, called Walmadany by the indigenous Goolarabooloo people and labelled James Price Point on most maps. Salisbury likened the region to "Australia's own 'Jurassic Park.' "

Exposures of LFA-2 preserving evidence of heavy 'dinoturbation'. Left, showing a likely sauropod thoroughfare; right, showing partly eroded tracks of large ornithopods and sauropods. Photo: Supplied

The Goolarabooloo have known about the fossil trackways for millennia. The massive markings, which are visible only at low tide, are featured in Goolarabooloo oral histories, or "song cycles," Salisbury told the BBC.

"They relate to a creation mythology, and specifically the tracks show the journey of a creation being called Marala — the emu man. Wherever he went he left behind three-toed tracks that now we recognise as the tracks of meat-eating dinosaurs," he said.

It's the Cretaceous equivalent of the Serengeti. And it's written in stone

Steve Salisbury

Walmadanyichnus hunteri, ichnogen. et ichnosp. nov., from the Yanijarri-Lurujarri section of the Dampier Peninsula, Western Australia. Left pedal impression, topotype UQL-DP11-5, preserved in situ as A, photograph; B, ambient occlusion image; and C, schematic interpretation. D, silhouettes of hypothetical Walmadanyichnus trackmakers based on UQL-DP11-5 and UQL-DP11-8, compared with a human silhouette. Photo: Steve Salisbury

In 2008, Walmadany was selected as the preferred site for a natural gas plant. Worried that the sacred and scientifically significant site would be lost, the Goolarabooloo reached out to palaeontologists and asked them to look into the tracks.

"We needed the world to see what was at stake," Goolarabooloo leader Phillip Roe said in a statement.

The area was listed as a natural heritage site in 2011, and plans for the natural gas plant fell apart two years later.

Examples of tracksites preserving evidence of dinosaurs that have had to negotiate the gently undulating topography of LFA-1 and LFA-2. A, Dr. Steve Salisbury with his right foot on one of several horizontally emplaced sauropod tracks (UQL-DP9-10) that traverse a gently sloping surface. Photo: Damian Kelly

Working alongside the Goolarabooloo, who are considered the region's "traditional custodians," Salisbury and his colleagues spent 400 hours investigating the markings. Each one was measured with three-dimensional photogrammetry, a technique used to build a 3-D reconstruction of an object by taking photographs from a variety of angles. For some tracks, the scientists also made casts out of flexible silicon, which can later be used to produce museum replicas of the prints.

According to Salisbury, most other Australian dinosaur fossils come from the continent's eastern side and date back to the mid-Cretaceous, about 90 to 115 million years ago. These tracks, which are between 127 and 144 million years old, represent the only fossil evidence from the early Cretaceous and are some of the oldest dinosaur remains in Australia, he said.

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