Some large plant-easting dinosaurs also snacked on crustaceans

"From what we know about dinosaurs, this was a totally unexpected behavior," Karen Chin, curator of paleontology at the University of Colorado's Museum of Natural History, said in a news release. "It was such a surprising discovery we wondered what the motivation could have been."

Researchers found decaying, coniferous wood inside the fossilized dino dung. And a closer look revealed fragments of fossilized shells. Paleontologists found crustacean remains in 10 coprolite samples spread across 13 miles -- proof, they say, that the dinosaurs were targeting crustaceans on purpose.

The fragments were also large enough to suggest the shells were at least two inches long, some more than half as long as the beak of a common hadrosaur -- suggesting the dinosaurs didn't accidentally swallow a shell.

Hadrosuars' beak-like bills and thousands of grinding teeth evolved to clip and break down sinuous plant fibers, but they could have easily doubled as shell crushers.

"While it is difficult to prove intent regarding feeding strategies, I suspect these dinosaurs targeted rotting wood because it was a great source of protein in the form of insects, crustaceans and other invertebrates," said Chin. "If we take into account the size of the crustaceans and that they were probably wriggling when they were scooped up, the dinosaurs would have likely been aware of them and made a choice to ingest them."

Chin and her colleagues detailed their logic in a new paper, published this week in the journal Scientific Reports.

Chin suggests the feeding behavior may have be seasonal, or even tied to breeding or egg-laying. Contemporary bird species, distant relatives of dinosaurs, often up their protein and calcium consumption during breeding season. Additional analysis of the coprolites revealed significant amounts of the calcium.

Most hadrosaurs grew to roughly 30 feet in length and were marked by crests on the top of their heads. Paleontologists believe the bipedal dinosaurs traveled in herds of a few dozen, and nurtured their young in a communal environment.