Swim Tracks Undermine Dinosaur Stampede

In 2004, the Australian government established Dinosaur Stampede National Monument in central Queensland. At this one place, preserved in the 95 million year old stone of the Lark Quarry, over 3,000 small dinosaur tracks are scattered across the rock surface. The tracks have traditionally been thought to be the only existing evidence of a dinosaur stampede. Australia’s ABC Television reconstructed the event like this:

But, paleontologists have found, there was no mixed herd, no marauding predator, no lake, and, in fact, no stampede. This exceptionally-rich, scientifically-important site has been drastically misinterpreted. If University of Queensland paleoichnologist Anthony Romilio and coauthors are correct, the park should be renamed something along the lines of “Swimming Dinosaur National Monument.”

Romilio and his adviser Steven Salisbury started systematically challenging the interpretation of Lark Quarry a few years ago. In popular descriptions of what happened at the tracksite, a huge theropod was often implicated as the inspiration for the stampede. Large, three-toed tracks – called Tyrannosauropus by students of trace fossils – put a formidable carnivore at the scene. As Romilio and Salisbury found, however, these tracks could very well have been made by a large herbivore – a dinosaur akin to the Australian iguanodont Muttaburrasaurusa dinosaur akin to the Australian iguanodont Muttaburrasaurus.

Richard Thulborn strenuously objected to this new interpretation. Along with Mary Wade, Thulborn helped pioneer the study of Lark Quarry during the 1970s and 80s, and laid the scientific groundwork that eventually allowed the site to be preserved as a national monument. In a Cretaceous Research preprint, Thulborn not only questioned the scientific conclusions of Romilio and Salisbury’s study, but charged that their “iconoclastic” hypothesis was based on “fabricated data” and the desire to have something original to say to justify Romilio’s PhD project. In the course of his unprofessional reaction, Thulborn also pointed out that the large, tridactyl prints were left at a different time than the stampede. It was only in popular accounts that a vicious carnivore was envisioned as charging a gaggle of small dinosaurs.

Thulborn’s paper was withdrawn before publication. Nonetheless, Romilio and coauthors nod to the controversy in a new Journal of Vertebrate Paleontology study that further undermines the quarry’s cherished story. “[W]e are aware of the fact that we may be considered iconoclasts by some members of the paleoichnological community and public alike, many of whom have become enamored with the idea that the site preserves a dinosaurian stampede,” Romilio and colleagues note. Beloved hypotheses are often slain by new data, although the new interpretation of Lark Quarry still presents an unusual look at dinosaur behavior. In place of a dinosaur stampede, the researchers envision a fast-moving river that little herbivorous dinosaurs frequently had to ford.

When they looked at the geology of the quarry, Romilio and collaborators didn’t find evidence of a prehistoric lake. Instead, the sediments reflected an ancient channel where water flowed at different depths and speeds. Invertebrate burrows associated with fast-moving bodies of water, as well as drag marks created by plants on the Cretaceous riverbed, indicate that the site was once a channel that wound through a prehistoric floodplain.

Many little dinosaurs had to swim to cross the river. Rather than recording the frantic runs of scared dinosaurs, some long-stride trackways were made by dinosaurs that floated across the channel and pumped their legs to maintain their course. Other dinosaurs were able to walk and even run across, perhaps because of their larger size or lower water levels at the time they crossed.

Romilio and coauthors didn’t find evidence of a coordinated run in any direction. Rather, the scores of little dinosaurs crossed the river at different times and under varying conditions. Based on track size and presumed skeletal proportions, the researchers estimated that the dinosaurs stood between five inches and five feet at the hips. And contrary to previous interpretations, only one species of dinosaur was responsible for the abundance of tracks.

Earlier analyses by Thulborn and Wade identified two particular types of small dinosaur tracks at Lark Quarry. There was Wintonopus, left by a small herbivorous ornithopod similar to Dryosaurus, and a different kind of track called Skartopus that was thought to have been made by a bird-like theropod dinosaur. In their reanalysis, Romilio and colleagues couldn’t find any real differences between the two. All the small dinosaur tracks were made by the Wintonopus trackmaker. The tracks did vary from one to the next – depending on the nature of the substrate and the behavior of the animal leaving the tracks – but the basic anatomy of the dinosaurian feet was consistent across the traces.

Lark Quarry does not record a single, frenzied moment of prehistoric time. As Romilio and coworkers envision the scene, “Lark Quarry most likely represents an accumulation of tracks over a period of time (perhaps days), during which water level fluctuated, with the majority of the smaller animals swimming or wading, and the larger animals walking or wading, and many animals using the current to assist their movements.”

The new scenario may not be the sort of story that inspires bloodthirsty documentaries or even songs, but, according to the authors of the study, the site preserves “one of the highest concentrations of dinosaur swim traces in the world.” Even though dinosaurs were not the semi-aquatic, swamp-bound animals they used to be characterized as, such rare traces are starting to show paleontologists how a surprising array of dinosaurs actually swam. And, of course, there’s the question of why so many ornithopods crossed the Cretaceous river. Could Lark Quarry preserve part of a local dinosaur migration, or some other social phenomenon? Tracks are fossil behavior, after all, and Lark Quarry undoubtedly holds additional secrets about dinosaur lives.

Reference:

Romilio, A., Salisbury, S. 2010. A reassessment of large theropod dinosaur tracks from the mid-Cretaceous (late Albian–Cenomanian) Winton Formation of Lark Quarry, central-western Queensland, Australia: A case for mistaken identity Cretaceous Research. 32, 2: 135-142

Romilio, A., Tucker, R., Salisbury, S. 2013. Reevaluation of the Lake Quarry dinosaur tracksite (late Albian-Cenomanian Winton Formation, central-western Queensland, Australia): no longer a stampede? Journal of Vertebrate Paleontology. 33: 1, 102-120

Thulborn, R. 2011. Lark Quarry revisited: a critique of methods used to identify a large dinosaurian trackmaker in the Winton Formation (Albian-Cenomanian), western Queensland, Australia. Cretaceous Research. Withdrawn.