Is it ok for vegetarians to eat fish? Turns out this dietary dilemma may date back further than we thought – to around 75 million years ago, in fact. Paleontologists studying the droppings of what was thought to be a strictly herbivorous dinosaur have found evidence that they enjoyed the odd cheat day, something they believe may have provided a nutritional boost to help reproductive activities.

Fossilized feces from herbivores are harder to come by than those from carnivores, which are better preserved thanks to minerals in the bones of consumed animals. But Karen Chin, a paleontologist at the University of Colorado Boulder, has had some recent luck in this area.

In 2007 she found some ancient herbivore droppings at the Two Rock Medicine rock formation in Montana that contained fossilized chunks of rotting wood. She has also found similar samples since in the Grand Staircase-Escalante National Monument in southern Utah. Chin believes they came from dinosaurs, most likely the large and common hadrosaur, which were chowing down on rotting logs in hope of finding some dietary supplements around 75 million years ago.

The hadrosaurs were large plant-eating dinosaurs that lived on land and spent most of their time close to bodies of water. Known as "duckbilled dinosaurs" for their wide and flat snouts, the hadrosaurs were such proficient plant-eaters that they were the world's dominant herbivores for millions of years leading up to their extinction at the end of the Cretaceous period, around 66 million years ago.

"You can't imagine a 20-foot (6-m) hadrosaur going after a butterfly," Chin says. "They would go for some place that had a predictable, concentrated source of food – some place like rotting logs."

Insects, small organisms and other sources of protein inside the logs were thought to be the dinosaur's motivation, but examination of the latest feces samples has also revealed thick pieces of crustacean shell. And not just a one-off either, but in at least 10 different samples spread out over a distance of 13 mi (21 km).

"If we found one coprolite with a crustacean fossil in it, that would be a really interesting scientific discovery," Chin said. "But it wouldn't necessarily indicate a recurring feeding behavior. We now have multiple coprolites with crustacean fossils, showing that at least some types of herbivorous dinosaurs occasionally engaged in this unanticipated feeding strategy."

The bits of shell indicate the crustaceans were at least two inches (5 cm) in length, making them between 20 to 60 percent of the width of a typical hadrosaur beak. The researchers say this suggests that they weren't swallowed by accident, and may have been eaten intentionally as part of a seasonal dietary shift, much like some birds today seek out more protein and calcium as they are looking to reproduce.

"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."

The team has published its research in the journal Scientific Reports.

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