Fossilised shell parts in an ancient stool sample Karen Chin/CU Boulder

Plant-eating dinosaurs may not have been the strict vegetarians we thought. Some of these species seem to have occasionally dined on shellfish, as revealed by traces of seafood meals found in their fossilised faeces.

In recent years, many coprolites – fossilised excrement – have been found in Utah. The ancient faeces were deposited between 76 and 74.1 million years ago, during the late Cretaceous Period about 10 million years before the extinction of the dinosaurs.

Karen Chin at the University of Colorado Boulder examined 15 coprolites recovered from sites as much as 20 kilometres apart and from different points in history.


The dinosaurs had mostly eaten rotten wood from conifer trees – but in 10 of the samples, Chin also found shell fragments.

Chew on a crab

“At first, we thought we were looking at fossil eggshell inside the coprolites,” says Chin. “I pursued that line for quite some time because they really, really look like fossil eggshell.”

But some of the pieces had oddly irregular curves and shapes. So Chin consulted crustacean expert Rodney Feldmann at Kent State University in Ohio, who helped her realise what she had found: the remains of ancient crustaceans.

It is clear that the shells had been eaten, says Chin, because they had been thoroughly disintegrated and were distributed through multiple coprolites from different locations.

We don’t know exactly what crustacean was eaten, or by what plant-eater. However, Chin says the most likely scenario is that a crab or crab-like creature was eaten by a hadrosaur, or “duck-billed dinosaur”. These flat-snouted dinosaurs were common in the area at the time.

Hungry duckbills

Nobody knows why hadrosaurs might have embraced a flexible diet. Perhaps egg-laying females feasted on crustaceans as an additional source of protein and calcium to make their eggs. Alternatively, shellfish-eating may have been a seasonal habit, perhaps when other food was short.

“They have good evidence of the dinosaurs eating this stuff, which is a very unusual and slightly unexpected finding,” says Paul Barrett at the Natural History Museum in London.

A site in Utah where ancient dinosaur faeces containing shells were found Karen Chin/CU Boulder

Barrett and Chin both say that the dinosaurs probably didn’t munch on crustaceans regularly, but that seemingly plant-eating dinosaurs may have been a bit more flexible than we thought. Modern birds are descended from dinosaurs, and Chin points out that many plant-eating birds today are known to be occasional omnivores.

However, dinosaurs may not have been as flexible as modern birds, which have beaks that are well-adapted to an eclectic diet. For instance, urban pigeons will peck and eat practically anything. In contrast, hadrosaurs and other plant-eating dinosaurs often had teeth that were only suited to grinding plants. A hadrosaur’s mouth was “a much more specialist piece of cutlery”, says Barrett.

Journal reference: Scientific Reports, DOI: 10.1038/s41598-017-11538-w