That’s no footprint, it’s got no toes

A favorite spot among paleontologists and paleotourists, the rock formations near Isona, Spain, are famous for fossils of large bones and eggs. However, the sign that tells tourists about dinosaur footprints in one area may need updating.

At the site in question, thousands of oval depressions currently identified as prints dot the sandstone landscape, says Jordi Martinell, a paleontologist at the University of Barcelona. Ripples in the rocks and fossil crustacean burrows suggest that the site was a lagoon more than 65 million years ago.

On average, the oval indentations are 43 centimeters long and 34 cm wide, and there’s about one of them every square meter over a broad area.

Scientists interpreted the pits as fossil footprints because of the area’s abundance of dinosaur remains.

Upon close inspection, none of the supposed tracks includes the hallmarks of a large animal’s footprint, Martinell notes. There are no traces of toes.

The pits have a U-shaped cross section, not the flat bottom expected from a big dinosaur foot. Also, sediments below the depression haven’t been deformed, which would be expected if a heavy dinosaur had stepped in soft mud. Furthermore, the pits don’t form clear trackways. Nor do many overlap–a situation unlikely if numerous dinosaurs had walked in the same area, says Martinell.

If dinosaurs didn’t make the oval depressions, then what did? In the August Palaios, he and his colleagues suggest that the fossils actually record the feeding behavior of stingrays.

Evidence for the new interpretation comes from living stingrays. In portions of the Gulf of California, for example, stingrays leave circular or oval pits 10 to 30 cm across in the mud as they forage in the burrow-riddled tidal flats.

Each square meter in these locations can hold up to three depressions, which closely resemble the indentations at Isona. Despite their abundance, the pits dug by living stingrays almost never overlap because the creatures usually don’t look for food where the mud is already disturbed.

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