Giant herbivore dinosaurs, despite a reputation as strict plant-eaters, did not always stick to their greens. Some of them ate crustaceans, according to new 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.”
Most dinosaur diets are a puzzle. Researchers often resort to skeletal clues: Grinding teeth or broad jaws suggest that the species is specialized for dining on foliage. Bite marks on bones of a prey species can be matched to a predator’s fangs. A dinosaur’s stance hints at whether it grazed up high, giraffe-like, or rooted in the underbrush. Some remains show a dinosaur’s last meal, though fossilized stomach contents are exceptionally rare.
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. (You won’t find the remnants of a crab snack in elephant dung.) 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. There’s a lot of data in waste, some of it clearer than others. Though Chin can’t say for certain what species of dinosaur produced the coprolites she studies, she said the animals were big.
In Utah, she found tiny fragments of feces as well as what she called “latrine deposits” — too much for a single animal to produce in one sitting. In Montana, by contrast, the typical dung pat was a more reasonable seven liters. “Seven liters is about comparable to what an elephant deposits in a single sitting,” she said.
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, who come running to eat it.
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 crustaceans are fragmented as though eaten, and too widely dispersed to have crawled in the dung and then been stepped on and shattered. The scientists do not know what animals these were — perhaps something like crayfish, perhaps marine crabs that had scuttled inland. But their exoskeletons were sizable, at least two inches long in one dimension, 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.
The crustaceans were too big to be ingested by accident, she said. Even largest dinosaurs in the area, hadrosaurs and ceratopsians, would have felt a two-inch crustacean wriggling in their beaks; the shell was somewhere between 2o to 60 percent as wide as these dinosaurs’ mouths. “They would have had the ability to spit out or avoid those crustaceans,” Chin said.
Chin hypothesized that the dinosaurs ate crustaceans during breeding season. Herbivorous birds change their diets before they lay eggs, ingesting more protein and calcium.
But mega-herbivores alive today don’t feed on rotting wood to get at invertebrates. Chin said she wondered why that ability was lost.
Or might it be, she said, that we don’t know living herbivores as well as we think we do? “We are living in a snapshot of life through time. Understanding more about the past — and that includes dinosaurs and other kinds of prehistoric animals — helps us evaluate our place in the ecosystem,” Chin said. “There’s still a lot we don’t know about modern life.”