Midiaou Diallo
A single fossil toe is all we have of the ‘Mitchell ornithopod,’ the nickname of the first early Cretaceous dinosaur fossil found in Oregon in 2018. Ornithopods were enormous herbivores such as duck-billed dinosaurs and iguanodons, and Gregory J. Retallack, lead author of that discovery, wanted to find more of its skeleton. Three years later, he returned to the site, aided by over 80 volunteers who helped excavate in more detail.
No further ornithopod bones—indeed, no substantial dinosaur bones of any kind—were retrieved after two weeks of digging. It was, he said, “a failure” in that regard. What they found instead was a complete mess, a jumble of the remains of land-based and aquatic animals. And lots and lots of guano.
That came from extinct flying reptiles, known collectively as pterosaurs, and suggests these animals may have flocked together on the cliffs above the coast of Oregon.
A debris field
A recent paper published in the journal Lethaia describes what appears to have been a mass debris flow during the early Cretaceous, approximately 100-113 million years ago, carrying everything within its path into the sea. This debris flow explains the mix of terrestrial and marine life jumbled within the same stratigraphic level of sediment—none of which would normally be found together. And this is why a variety of plants and a 2.5 meter (about 8 foot) fossil log, for example, were preserved near molluscs, fish bones, and shark teeth.
More significantly, the sediment of this debris flow contained very high levels of phosphorus (levels not seen in the shale deposits above and below it). And phosphorus is a feature of the guano deposits left behind by birds, which may occupy some of the same ecological niches as pterosaurs.
Other remains at the site hint at what the pterosaurs were eating. Fractured shells from ammonites (ancient shelled cephalopods, similar to today’s Nautilus) were the most common remains present.
Fossil ammonites are often found intact and not crushed to small pieces, as they were at this site.
They are perhaps most recognized for the type with spiral shells, found throughout the world, including elsewhere in Oregon. In contrast to those finds, most of the remains in this deposit were the size of “corn flake breakfast cereal,” according to the authors.
Retallack, professor emeritus of the University of Oregon, said that these fragments were a surprise. “We have collected lots of ammonite localities and found nothing elsewhere like this,” he wrote in an email to Ars.
You look like what you eat
While we still have a lot yet to learn about pterosaurs, we can hypothesize about some of their diet based on their skulls. Dsungaripterus, for example, is a pterosaur with a beak-like rostrum that has shell-crushing teeth in the back of its mouth, exactly what you’d expect to be feeding on ammonites.
There is currently only one known pterosaur from Oregon, Bennettazhia oregonensis, but we don’t know what its skull and teeth looked like because all that we’ve found are a humerus and some vertebrae. So, while the team found some pterosaur teeth and a partial toothless (endentulous) mandible with grooves similar to those found in Dsungaripterus, we don’t know which species these belong to.
Although at least one dinosaur bone was found during the previous dig at this site, “no dinosaurs are known to be durophagous feeders,” Retallack explained, “and the one we found was evidently a herbivore.”
The high amount of phosphorus is another important clue, Retallack argued. “A deposit a meter thick would take a large population of seabirds for example. Huge colonies of penguins do not generate as much.” Coupled with the remnants of many pterosaur meals, this suggests a flock of pterosaurs may have gathered on the cliffs by the coast. It is that meal that rules out ancient birds as the guano producers; bird species of the time were not known to eat mollusks.