Everyone loves a good dinosaur story and they don’t come much better than the dramatic dinosaur stampede found in Queensland’s outback. But did a stampede really happen?
In the late 1970s at Lark Quarry, about 110km southwest of Winton, central-west Queensland, half a hillside was removed to reveal a surface of rock pitted with thousands of three-toed dinosaur tracks.
There was a series of large tracks heading off to the southwest and thousands of smaller ones, most of which are directed to the northwest.
The original interpretation that was published in 1984 proposed that the set of large tracks were made a large-bodied carnivorous theropod dinosaur, perhaps something akin to the mighty Tyrannosaurus. The smaller tracks were thought to belong to small-bodied coelurosaurian theropods and herbivorous ornithopods.
It was estimated that there were more than 150 of the smaller dinosaurs. Given that nearly all their tracks are aligned in the same direction, and that some of them were imprinted into the larger tracks, it was proposed there must have been a stampede, most likely triggered by the approach of the large meat-eating theropod.
Stampede story a popular attraction
Tourists love it as well, with many going out of their way to visit Lark Quarry and have the story told to them by professional guides.
The state and federal governments liked it too. Approximately $A3.2 million was put into the construction of purpose-built building to protect the site, and in 2004 it was given National Heritage Listing and renamed the Dinosaur Stampede National Monument.
But did a dinosaur stampede really happen at Lark Quarry?
The greatest story ever told … was just a story
Around 2010, we began to look more closely the large Lark Quarry dinosaur tracks. When various proportions of the outlines that had been published in 1984 were compared alongside other dinosaur tracks, they appeared to belong to a plant-eating ornithopod rather than a theropod.
Could it be that the large Lark Quarry trackmaker wasn’t a theropod?
If it was a large-bodied ornithopod, maybe something similar to Muttaburrasaurus, which we know inhabited this part of Queensland around the time the Lark Quarry tracks were made, would its approach have caused the smaller dinosaurs to stampede?
When we published our results in 2011, many of our colleagues thought it was great; they’d also had doubts about the identity of the large track-maker.
A stampede of dispute
But others were aghast. Chief among these was Dr Tony Thulborn, formerly at the University of Queensland and lead author on the original 1984 publication in which the tracks had been described, and the stampede scenario first proposed.
In a response that came out in 2013, he labelled our claims as “iconoclastic”. He considered the methods we had used to analyse the track outlines to be flawed, and that in obtaining our results we had in some way “fabricated” our data.
His main gripe seemed to be that we had altered the outlines of some of the tracks to suit the analytical method we chose to employ.
The analytical method we used required the use of complete track outlines. The outlines that had been published in 1984 had dotted portions to them where a definite outline was presumably hard to make out.
There were different versions of some outlines, and the dotted parts weren’t always in the same place. In subsequent publications by Thulborn and others, some of the track outlines had been filled with colour, while others had the dotted parts joined. After convincing ourselves that the 1984 outlines were good approximations of the tracks, we decided to join the dots.
What the ensuing controversy highlighted was just how problematic the construction of track outlines can be. The majority of dinosaur tracks are 3D structures, and any representation of them in 2D means that information relating to depth is lost or compressed.
Creating an outline therefore depends on how the investigator interprets the track, making them highly subjective. This was clearly the case with the large Lark Quarry tracks. Where some saw dots or clear, unbroken lines, others saw red.
Using digital photogrammetry, we were able to approach the problem in a far more objective way.
This technique involves taking a series of photographs and then stitching them together to create a 3D model. The results can easily be reproduced, and none of the 3D information relating to track depth is lost. If you consider an outline, it’s simply a matter of selecting a contour line around or within part of the track.
The 3D photogrammetric analysis of the large tracks that we published this year in Cretaceous Research shows them to be very different to how they were first portrayed in 1984. The dinosaur that made these tracks had broad feet with flat, rounded toes.
There is no evidence of separate pads beneath the toes or the heel, and there is no evidence of sharp claw impressions; characteristics that would normally be expected had they been made by a theropod.
Overall, they are a good match for tracks from Europe and Asia that are thought to have been made by large-bodied iguanodontian ornithopods, animals similar to Muttaburrasaurus.
Something else revealed in the images
The 3D images also showed something else. Most of the large tracks have raised margins around them. These “displacement rims” typically occur when the ground in which a track is made is saturated, but not covered in water.
This suggests that the large Lark Quarry track-maker walked across the site when the surface was wet, but probably not underwater.
The displacement rims around some of the large tracks are disrupted by drag marks, probably caused by partially floating vegetation. These could only have been made when water covered the site some time after the large dinosaur traversed it.
Since both the large footprints and drag marks have small dinosaur tracks overprinting them, the dinosaurs that made the smaller tracks must have crossed the area much later. From research that we published in 2013 we also know that some of these smaller dinosaurs were running, some were wading and some were even swimming.
No evidence of a stampede
The timing between the formation of the larger tracks, the vegetation drag marks and the smaller tracks could be anywhere from hours to days.
The most likely scenario that could account for the series of events that produced Lark Quarry is probably a gentle rise and fall in the water level of a river that flowed through the area approximately 93 million years ago.
During this time various dinosaurs and a few floating bits of vegetation crossed the site. Some point after all the tracks were formed the entire surface was probably exposed and allowed to harden, with another rise in water eventually burying it in layer of sand before any features in it weathered away.
We may never really know exactly what happened at Lark Quarry all those millions of years ago. But we can present interpretations of what we think happened based on the evidence at hand.
In the case of Lark Quarry, the evidence is written in stone. As scientists, we just need to learn how to read it, and the way we read the evidence can change as science progresses. If new techniques allow us to refine the way we see things, then our interpretations will likely change. That’s science.
What the scientific community and the public at large make of our reading of the evidence is something we can’t always control. But at some point, the dust, or in this case, mud, will eventually settle.