No one knows exactly how dinosaurian dominance came to an end. Paleontologists and geologists have identified the triggers for the end-Cretaceous mass extinction that devastated life on Earth 66 million years ago, yet, even with that knowledge, we still lack the resolution to comprehend how asteroid impact, volcanic activity, and climate change conspired to snuff out so many species while sparing others. Truly understanding a mass extinction goes deeper than pinning down ultimate causes of catastrophe. In fact, paleontologists still have a great deal of work to do in the effort to catalog who was around when the terminal-Mesozoic calamity struck, and the latest clue from the roll of victims is a succession of non-avian dinosaur tracks found among the southern Pyrenees mountains of Spain.
Spread across thirty different sites, the footprints described by paleontologist Bernat Vila and colleagues in PLoS One span millions of years of Cretaceous time leading up to about 300,000 years before the mass extinction. That sounds like quite a long time on a human timescale – our species has only existed for about 200,000 years – but through the lens of Deep Time, that brings us incredibly close to some of the last dinosaurs to have ever walked the Earth.
The tracks described in the new study are found within southwestern Europe’s Tremp Formation. They record the movements of dinosaurs that once trod along the margins of meandering and braided streams that flowed over 66 million years ago. Footprints from both long-necked sauropod dinosaurs and shovel-beaked hadrosaurs pock the sites, although hadrosaur tracks outnumber sauropod impressions by far.
Similar to tracks found elsewhere and designated Hadrosauropodus in ichnological terminology, the three-toed, blunt impressions found in Spain were left behind by hadrosaurs about half the size of the 40-foot-long Edmontosaurus from similarly-aged rocks in North America. Rather than representing juvenile animals, Vila and coauthors note, the comparatively small size of the footprints in the Tremp Formation matches a skeletal record of hadrosaurs that evolved comparatively tiny stature on the isolated islands of Cretaceous Europe.
The multiple tracksites do more than simply add to the number of dinosaur trace fossils known from the end of the Mesozoic. Footprints and other traces can act as signs of who was stomping around an environment at a particular time and, therefore, let paleontologists decipher the ebb and flow of dinosaur evolution. During the latest Cretaceous of Europe, for example, armored dinosaurs called nodosarids and an ornithopod called Rhabdodon are eventually replaced by the quite-common hadrosaurs. Trackways such as those in the Tremp Formation help establish the timing of this turnover. And, simply be virtue of their tracks being there, we can be sure sauropods and hadrosaurs were still walking across prehistoric Europe right up until the very end of the Cretaceous.
And in the global pattern of sudden extinction, the Tremp Formation tracks offer one more piece of evidence that will assist paleontologists in their efforts to understand the fate of the non-avian dinosaurs. Early in their paper, Vila and collaborators write “With the exception of data from the bolide impact zone and nearby areas (e.g. North America), little is known about how the last dinosaur faunas reached the boundary in most parts of the planet.” This is frustratingly true. We’re trying to understand a global catastrophe through the lens of a very small geographic area. But with finds such as the new tracksites in Spain and others around the world, researchers can begin to fill in the bigger picture of one of the most pivotal and disastrous events in the history of life on Earth.
Vila, B., Oms, O., Fondevilla, V., Gaete, R., Galobart, À., Riera, V., Canudo, J. 2013. The latest succession of dinosaur tracksites in Europe: Hadrosaur ichnology, track production and palaeoenvironments. PLoS ONE 8,9: e72579. doi:10.1371/journal.pone.0072579