A Cretaceous crime scene
Roughly 120 million years ago, during the Early Cretaceous, this region of Spain was a vast, swampy delta plain bordering the ancient Tethys Sea. Dinosaurs thrived on these marshy flats, and the soft mud recorded their daily lives with exceptional clarity. Over time, the footprints they left behind—known as ichnites—were baked by the sun, buried by sediment, and fossilized into stone. Today, La Rioja contains one of the largest concentrations of these trace fossils in the world, with more than 11,000 individual prints identified across over 140 sites.
The sites, particularly around the town of Enciso, preserve a remarkable prehistoric census. By counting the different types of tracks, paleontologists can reconstruct the ecosystem's structure. The data reveals a community dominated by large herbivores. For every footprint left by a carnivorous theropod, there are ten left by plant-eating sauropods and ornithopods. This 1:10 predator-to-prey ratio mirrors the ecological dynamics seen in modern large mammal communities, such as those on the African savanna. The footprints show the social structures and populations of the Early Cretaceous.
Reading the stone tape
Each trackway tells a story. At the Valdecevillo site, prints reveal a family of Iguanodons—two adults and a juvenile—walking together. Nearby, the tracks of a large carnivorous theropod, between 7 and 10 meters long, are clearly visible. The Virgen del Campo site, with over 500 individual footprints, shows even more complex behaviors. Preserved tracks suggest a confrontation between a meat-eater and a herbivore. Other marks at the same site include skin impressions, tail-drag marks, and scratches left by swimming dinosaurs clawing at the sediment for propulsion.
Researchers have even identified a new type of ichnite here, named Theroplantigrada encisensis, from a dinosaur that walked on the soles of its feet, unlike most that walked on their toes. The presence of a possible membrane between the toes suggests an adaptation for walking in wet, muddy environments. By measuring the stride length and pace of different trackways, scientists have calculated the running speeds of some dinosaurs. One theropod at a site in Igea was clocked at an impressive 45 kilometers per hour (28 mph), among the highest speeds recorded for this type of dinosaur.