Large plant-eating dinosaurs are typically thought of as strict vegetarians, but an analysis of fossilised dino poo from the Cretaceous Period suggests some of these creatures also feasted on crustaceans. The study, published this week in Scientific Reports, suggests these occasional lapses in vegetarianism might have had something to do with dinosaur reproduction.
Digging for fossilised dino poo: CU Boulder associate professor Karen Chin at work. (Image: University of Colorado)
"From what we know about dinosaurs, this was a totally unexpected behaviour," said study author Karen Chin, curator of palaeontology at CU Boulder's Museum of Natural History, in a statement. "It was such a surprising discovery we wondered what the motivation could have been."
The evidence comes via 75-million-year old poo fossils known as coprolites. Ten different samples of dino poo were discovered across three stratigraphic layers at Grand Staircase-Escalante National Monument in southern Utah, by a team from the Denver Museum of Nature & Science.
Dark crustacean shell fragment embedded in coprolite, or fossilised dinosaur poo. (Image: University of Colorado)
The coprolites found in Utah greatly resembled others examined by Chin a few years ago in Montana, both in terms of size and the presence of fossilised fragments of rotting wood. The Montana samples were produced by duck-billed dinos known as hadrosaurs — common herbivores of the Cretaceous that grew up to 9.14m long and weighed up to 3t. These creatures featured specialised teeth and jaws for munching on fibre-rich plants.
But as the new analysis suggests, these beasts also ate some seafood. Close examination of the Utah coprolites revealed thick bits and pieces of fossilised shell, in addition to the rotten wood. Chin and her colleagues sliced the coprolite samples into thin sections, and then analysed the slices with an electron microscope to determine chemical composition, which included plenty of calcium. Chin theorises that the crustaceans provided the hadrosaurs with valuable protein and calcium.
The researchers don't know the exact species of crustaceans consumed by the dinos, but modern examples include lobsters, crab, shrimp, and crayfish. Analysis of the crustacean remnants in the dino poo suggest they were about two inches in length, maybe larger. Present day Utah was situated near a sea during the Cretaceous, which explains the source of the crustaceans.
Hadrosaur skeleton. (Image: Daryl Mitchell/Flickr)
"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 says the seafood-eating habit may have been a seasonal thing, a behaviour tied to breeding and egg-laying. She points to modern birds, which are known to eat more protein and calcium during the breeding season to support successful reproduction. And as we all know, birds are descended from dinosaurs.
It's a neat finding, one that reminds us that strict herbivore/carnivore categorisations don't always hold up on closer inspection. When it comes to survival, an animal's gotta do what an animal's gotta do.