Dinosaurs are always going to be more popular than rays. There’s just no contest. I’m not saying this to be cruel to the chondrichthyans. I quite like the squishy, bat-winged fish. Nevertheless, it seems that the public would much rather imagine dinosaurs wading through the Cretaceous shallows than the voracious grubbing of hungry rays. I’m not going on an abstract tear here. I know that rays can’t beat dinosaurs because of an unpopular interpretation of a fossil tourist attraction in Spain.
Around 66 million years ago, the area around Isona, Spain was on a Cretaceous coastline. And, at one famous locality, it would appear that dinosaurs had a dance party. The site’s wide swath of sandstone is pocked by scores of rounded divots – potholes apparently created by dinosaurs as they trod along the shore. But, try as you might, you can’t see any claw marks, toe impressions, or any other details of dinosaur feet in the fossils. These tell-tale features were not recorded in the fossil record. The only clues that dinosaurs had made the impressions were bones, eggs, and other fossils found at other fossil sites nearby.
As paleontologist Jordi Martinell and colleagues found out when they scrutinized the site, though, the dinosaurs were not in the details. The site was not a heavily-trod dinosaur sidewalk. Hungry rays had created the thousands of divots in the Cretaceous sand.
Ichnology – the study of tracks and other trace fossils – can be a tricky science. A single animal can make multiple kinds of marks on a surface, all depending on the anatomy of that organism, the behavior of the creature, and the characteristics of the substrate. Think of the neat footprints you leave behind on wet sand versus the ugly, messy holes you create while walking through ankle-deep mud. Your anatomy is the same in each situation, but your behavior and the substrate are different, creating different tracks. Therefore ichnologists often look to the traces left by modern animals to interpret what prehistoric creatures were doing, and it’s fortuitous when modern animals behave much in the same way their prehistoric relatives did. Just such a correspondence is what revised our understanding of what happened at Isona.
The association of large depressions at Isona with other trace fossils provided Martinell and collaborators with an important clue. The larger pits were surrounded by thousands upon thousands of tiny burrows. These are some of the most common of all trace fossils, and they were made by small crustaceans. You can see similar tunnels along tidal flats and other nearshore environments today, and, in some places, rays swim in during high tide to flush the arthropods from their homes. During high tides, when the flats are covered by shallow water, the rays glide in and flap their wings to scatter the sediment over the crustacean burrows. Once they have their prey, they move off, leaving behind a pothole.
When some of Martinell’s co-authors saw the holes modern rays left behind on the shores of Puerto Peñasco, Mexico, they were reminded of the Isona mystery traces. And the modern traces compare very closely with the fossil versions. Cretaceous rays, not dinosaurs, made the depressions.
The ray interpretation also explained two other peculiarities about the site. If dinosaurs made the impressions, paleontologists would expect to see defined trackways of animals moving in a particular direction, as well as overlapping tracks where one animal stepped on the trail of another. Yet the holes don’t show any trackway pattern, and there is almost no overlap among the divots. There were multiple problems with the dinosaur hypothesis, and the ray reconstruction was more consistent with the environment and details of the trace fossils. And even though no ray body fossils were found at the site, fossil teeth from three ray species were found in adjacent deposits. All of them have modern relatives which flap their wings to flush out burrowing crustaceans, and so it seems that at some of these Cretaceous rays were doing the same thing.
If you visit the spectacular trace fossil site at Isona today, however, you will still see signs about dinosaurs. Some of the local people were not exactly thrilled that their dinosaurs had been replaced by bottom-feeding rays, and so the signs have remained the same. And as much as I adore dinosaurs, their intense cultural pull can be more than a little frustrating in situations like this. Visions of dinosaurs plodding through the Cretaceous surf have squashed a much more accurate reconstruction of a ray feeding bonanza.
Martinell, J., Gibert, J., Domenech, R., Ekdale, A., Steen, P. 2001. Cretaceous Ray Traces?: An Alternative Interpretation for the Alleged Dinosaur Tracks of La Posa, Isona, NE Spain. PALAIOS, 16 (4) DOI: 10.2307/3515580
[This post was originally published at National Geographic.]