The poop on what some dinosaurs ate

Study: Some plant eaters dined on shellfish

Giant herbivore dinosaurs, despite a reputation as strict plant-eaters, did not always stick to their greens. Some ate crustaceans, according to research published Thursday in the journal Scientific Reports. Paleontologists discovered bits of animal shell embedded in fossilized dung.

“We aren’t claiming that these dinosaurs were not herbivorous,” said Karen Chin, study author and paleontology curator at the University of Colorado Museum of Natural History in Boulder. “The point is their diets were more complex than we can usually learn from the fossil record.”

To help fill these gaps, scientists compared ancient giants with today’s plant-eating behemoths, like elephants and hippopotamuses. Elephants, the largest land animals, eat fruits and grasses. Hippos are considered herbivores, but a 2015 study found they eat a surprising amount of flesh. “Modern life isn’t as black and white as we would like,” Chin said.

Nor was life in the past.

Chin studies coprolites, a term of art for fossilized poop. Though Chin can’t say for certain what species of dinosaur produced the coprolites she studies, she said the animals were big.

Ten years ago, she discovered fossilized wood in coprolites from a Montana geologic formation. This was odd. Wood isn’t very nutritious, as most vertebrate animals can’t digest the tough binding molecules that pack its cellulose together. Chin at first hypothesized that dinosaurs were accidentally biting trees while munching on short foliage like juniper or cedar needles.

That idea didn’t pan out. There were too many wood fragments and a strange absence of chewed-up twigs. She realized the dinosaurs were likely eating rotted wood, predigested by fungi. There’s a modern example for that, too: Ranchers in Chile, Chin said, break open logs struck by white rot fungus — the soft wood is a favorite snack for their cows.

Chin kept searching for wood in coprolite deposits. In Utah, in the Kaiparowits Formation, she found wood and something stranger along with it. The dung contained what looked almost like eggshells.

Examining the fossils with Kent State University paleontologists Rodney Feldmann and Jessica Tashman, Chin determined these shells came from crustaceans. The scientists do not know what animals these were — perhaps something like crayfish, perhaps marine crabs that had scuttled inland. But their exoskeletons were at least two inches long, the scientists estimate.

Chin offered three possibilities for why herbivore dung contained crustacean exoskeletons. At one end of possibility, “they were really dumb dinosaurs and ate anything,” she said. At the other extreme, these were selective hunters. In the middle, and what she considers mostly likely, is that the dinosaurs grazed where crustaceans gathered, in rotted wood, as a seasonal meal.

Chin hypothesized that the dinosaurs ate crustaceans during breeding season. Herbivorous birds change their diets before they lay eggs, ingesting more protein and calcium.