It’s amazing how much you can learn about an animal from the tracks it leaves behind. In the case of dinosaurs, tracks that have lasted for millions of years tell us how fast they moved or whether they travelled in groups. Now, a unique set of tracks discovered in Spain tell us that at least some types of dinosaur could swim*.
The track in question is preserved in the sandstone of the Cameros Basin, one of the richest known sources of dinosaur tracks from the Cretaceous period. It stretches across 15 metres but consists of just six pairs of footprints; their maker was clearly a large animal.
The ‘footprints’ are few in number, but their size and shape speak volumes. Each is actually a series of two or three long, slender scratch marks. That rules out a walking animal or a tip-toeing crocodile, both of which would have produced a broader, flatter print.
Ruben Ezquerra from the Fundación Patrimonio Paleontológico de La Rioja, who discovered the tracks, thinks that they are clear signs of a paddling carnivorous dinosaur.
During the late Cretaceous, these sandstone flats would have been submerged under metres of water. As the predator swam through the lake, its torso would have floated near the surface while its legs propelled it along. As it swam, the tips of its toes lightly scratched at the sediment, creating the tracks that exist today.
Each of its paddling strides spanned about 2.5 metres; this was a large animal. Even so, its tracks suggest that it swam with exaggerated walking motions, in the same way that modern (and less fearsome) water-birds do.
The tracks even tell Ezquerra that the predator was swimming against the current. They are asymmetric with the right prints angled forty-five degrees to the left. These were caused by the animal pushing harder with its right foot, while its body was slightly angled against upriver.
In a way, we shouldn’t be surprised. The dinosaurs filled ecological vacancies that modern mammals now inhabit, and many large mammals from bears to (surprisingly) elephants prove to be surprisingly capable swimmers.
Some dinosaur species were even thought to be specialised fishermen and one of these, Baryonyx (above), lived in Spain during the early Cretaceous. Could it have made the tracks that Ezquerra found?
Reference: Ezquerra, Doublet, Costeur, Galton, Perez-Lorente. 2007. Were non-avian theropod dinosaurs able to swim? Supportive evidence from an Early Cretaceous trackway, Cameros Basin (La Rioja, Spain). Geology 35: 507-510.
Drawing: by Guillaume Suan, University Lyon.