Not Fleeing, Swimming

Lark Quarry wouldn’t be a fun place to work at the moment, what with the temperatures at nearby locations over 46°C earlier this month and stunning deluges not that far away it must feel rather like being back in the era when the dinosaurs that made it famous were laying down their tracks. However, some interesting research came out this month about the reason it’s a focal point for palaeontologists.

Located in central western Queensland where directions are based on a town more than 100km away with fewer than 1000 people the quarry hosts an astonishing concentration of 3300 dinosaur footprints (possibly with many more still to be excavated). Most of the prints are from small creatures, estimated to range from the size of chickens to that of emus, but there are a few very large prints, once thought to belong to a relative of the Tyrannosaurus family. This led to the conclusion the arrival of the huge predator had provoked a stampede amongst terrified smaller creatures.

Doubt was cast on this idea a year ago when University of Queensland scientists produced evidence the large prints were more likely to come from a Muttaburrasaurus, or related herbivore, than a terrifying killing machine. You wouldn’t want to get underfoot of such a beast, but chasing was unlikely, and therefore little chance of a stampede.

Now PhD student Anthony Romilio has shifted our view of the tracks even further by suggesting the dinosaurs were not running but swimming. Many of the tracks are scored lines in the mud rather than proper footprints, and others look like the maker was walking on tiptoes. Given it is unlikely that the dinosaurs were trying to sneak past a snoozing giant in the style familiar from comics, swimming makes more sense, with feet occasionally catching the bottom

Previously it was thought that all the tracks were made pretty much at once, during a brief period when the surface was suitable for taking prints. However, after estimating the size of the animals that made each print from their foot size, Romilio and his supervisor reached the conclusion that the water depth must have varied during the time the prints were made. At some times the water would need to have been just 14cm deep in order for the smallest dinosaurs’ feet to touch the bottom. At other times, larger creatures appeared to only just be leaving scorelines, suggesting the river depth must have been around 40cm, while the smaller creatures entirely water-borne. This also leads to the conclusion that the trackmakers were even more numerous than previously thought – as often the smaller animals would have left no trace.

Romilio’s theory is that the area was part of a wide river and the dinosaurs were either crossing, or using the current for transport. Evidence for the latter comes from the fact that some of the marks are far enough apart as to suggest the dinosaurs were being swept substantial distance between each step. Unfortunately, all we have is the small piece of the puzzle represented by the area where the soil was suitable for preserving tracks so we have no idea how large the river was or any idea of its wider shape.

The UQ team seem a little defensive, apparently fearing that rebutting the idea of a dinosaur stampede will downgrade Lark Quarry’s significance in the eyes of the public. I however, am even more taken by the idea of hundreds of chicken-sized dinosaurs crossing an obstacle a hundred million years before anyone would question their motives.

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About Stephen Luntz

I am a science journalist, specialising in Australian and New Zealand research across all fields of science. My book, Forensics, Fossils and Fruitbats: A Field Guide to Australian Scientists is out now through CSIRO Publishing. I am also a professional returning officer for non-government organisations. I'm very politically active, but generally try to restrict this blog to scientific matters.
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