Armoured dinosaur's last meal preserved in stunning detail

Fossilised gut contents reveal details of the prehistoric creature's daily life—and even the season it died.

By Michael Greshko
Published 3 Jun 2020, 09:54 BST
Photograph by Illustration by Julius Csotonyi

On a summer day 110 million years ago, an armoured dinosaur likely ambled through the remains of a wildfire in what is now Alberta, Canada, gobbling up delicate green ferns peeking out from the ash. Somehow, shortly after, the dinosaur ended up dead in a river among the Cretaceous landscape and was swept out to sea. The ancient creature remained entombed in marine sediments until 2011, when an oil-sands miner stumbled across the remains: the best-preserved dinosaur of its kind ever discovered.

Already, the fossil has shed new light on how armoured dinosaurs’ hardened exteriors looked and functioned. Now, scientists studying the extraordinary fossil have made a new discovery: a ball of plant matter in the dinosaur’s gut that not only reveals the animal’s diet, but also chronicles the season the dinosaur died.

Hours after Borealopelta enjoyed its last meal, the dinosaur somehow got swept out to sea. The dinosaur’s undersea burial in what's now northern Alberta preserved its armor—and its stomach contents—in exquisite detail.

Photograph by Robert Clark, Nat Geo Image Collection

Borealopelta's gut contents included stones the dinosaur ingested to help break down its food.

Photograph by Royal Tyrrell Museum Of Palaeontology

By embedding some gut-content fragments in resin, scientists were able to slice them thin enough to study under a microscope, revealing thousands of tiny plant fossils.

Image by Royal Tyrrell Museum Of Palaeontology

“The preservation is just so great, we can actually say something about the stomach contents,” says lead study author Caleb Brown, the a dinosaur curator at Alberta’s Royal Tyrrell Museum of Palaeontology.

The research, announced today in Royal Society Open Science and partially funded by the National Geographic Society, provides an unprecedented look at the world this large plant-eating dinosaur inhabited—down to charcoal bits it swallowed.

“It paints a really evocative picture of this environment that this dinosaur would have been passing through,” says Victoria Arbour, the curator of dinosaurs at Canada’s Royal BC Museum. “You can envision the very specific event that happened in this dinosaur’s lifetime, and I thought that was really, really cool.”

A prehistoric meal

In general, finding fossilised gut contents is rare. Fossils that unequivocally preserve herbivores’ final meals are rarer still. The chemical conditions that preserve bone also tend to break down plant matter, and plant material can often get swept into a fossil animal’s body during burial, making it hard to judge what’s a meal and what’s infill. Only one other armoured dinosaur, the Australian Kunbarrasaurus, has been found with digested plant matter in its stomach. But the Albertan dinosaur, Borealopelta markmitchelli, was bigger, roughly 18 feet long and nearly 3,000 pounds in life, and its stomach contents were better preserved.

Borealopelta was a nodosaur, a type of armoured dinosaur that lacked the tail club of its better-known cousin Ankylosaurus. It lived about 110 million years ago in what is now northwestern North America. The dinosaur fossilised under remarkable circumstances: Somehow, the animal ended up dead in a river and was swept more than 100 miles into a seaway that once split North America in two, from the Gulf of Mexico to the Arctic Ocean.

The dinosaur’s improbable burial at sea in what is now northern Alberta preserved its body in impeccable detail. Not only does its bony armour remain intact, but many of the keratin sheaths that covered it also fossilised. These clues are helping scientists understand how the dinosaur’s plates appeared and functioned, as well as providing possible evidence of its skin color. (Take a virtual 3D tour of Borealopelta, including the fossil's stomach contents.)

Borealopelta was freed from its stony tomb in 2011, when an excavator at Suncor’s Millennium Mine, an oil sands operation, in northern Alberta uncovered the fossil as he was digging. A crew from Alberta’s Royal Tyrrell Museum of Palaeontology flew up to the mine to excavate it, and museum preparator Mark Mitchell spent the next six years painstakingly removing excess rock from the fossil with hand tools. The dinosaur’s species name is markmitchelli in his honor.

When Borealopelta was unveiled in 2017, scientists marvelled at the quality of its preservation. Brown and his colleague Don Henderson, a fellow curator of dinosaurs at the Royal Tyrrell Museum, harboured suspicions that the fossil also contained stomach contents. The left side of the dinosaur’s chest cavity featured a curious mass of multicoloured pebbles, right about where the stomach ought to have been. So Brown and Henderson took small pieces of the mass that had fallen off, embedded them in resin, and made paper-thin slides that they could examine under a microscope.

The two researchers quickly recognised bits of fossilised plant matter under magnification, including chunks of leaves preserved at the cellular level, down to the pores they used to take up CO2. But Brown and Henderson are experts on dinosaurs, not plants. So in 2017, they reached out to Jim Basinger and David Greenwood, two of western Canada’s most experienced experts on ancient plant life.

“What is the landscape? The plants give us that time machine,” says Greenwood, a palaeobotanist at Brandon University. “How wet was it? Was it seasonally dry? Did it freeze in winter? And of course, with this study, what was this guy eating?”

Sifting through the guts of a dinosaur

From mid-2017 to late 2018, the team pored over the slides, led by Greenwood’s wife Cathy, a technician who painstakingly catalogued the thousands of plant fossil fragments. To better understand the environment where Borealopelta lived, the team also reviewed the plant fossils from the Gates Formation, a series of coal beds that formed in western Alberta around the time that Borealopelta lived.

In the mid-Cretaceous, northern Canada’s climate was much wetter and warmer than it is now, and lush forests and glades covered the landscape with a foliage wildly different from the wheat fields and forests of modern Alberta. Flowering plants were only beginning to spread 110 million years ago and remained rare. Instead, forests were dominated by conifers and palm-like plants called cycadeoids, with ferns and horsetails filling out the underbrush.

By comparing the Gates Formation plant fossils with those in Borealopelta, the team concluded that the short-limbed animal grazed for low-growing plants. But to the researchers’ surprise, the majority of its diet seems to have been a particular type of fern that it apparently sought out, ignoring other available vegetation. In addition, about six percent of the gut contents were bits of charcoal, a possible sign that Borealopelta was grazing on regrowth in an area recently hit by wildfires.

Further clues found in the tree rings of woody twigs that Borealopelta ate suggest that the dinosaur ingested the plants about halfway through their growing season, which would have lasted from late spring to the height of summer. In addition, the ferns Borealopelta ate had mature sporangia, the spore-launching organs on the undersides of the leaves. Taken together, the data suggest that Borealopelta ate its final meal in early to mid-summer—and died mere hours afterward.

Previously, researchers surmised that armoured dinosaurs went after ferns and other low-growing plants, and that narrow-jawed types such as Borealopelta selectively browsed the foliage like modern deer. Now, this new fossil provides confirmation of these past ideas, as well as a blueprint for how to evaluate future gut-content fossils.

“This is one particular specimen, and it’s one time slice throughout its entire life, so we don’t know if it’s representative,” Brown says. “But theoretically, if we had more specimens like this, you might be able to tease out differences between a summer diet versus a winter diet.”

The Alberta Borealopelta fossil could still have secrets to reveal as well. The animal was swept out to sea, so researchers don’t yet know precisely where the dinosaur lived—but the team not only has plant matter, but also pebbles the dinosaur swallowed to help break up its food, called gastroliths, like the gizzard stones of modern birds. Scientists know dinosaurs like Borealopelta lived in the Gates Formation, thanks to fossil footprints, and chemical tests may yet trace the gut pebbles back to specific outcrops.

Even without those extra details, Borealopelta remains a striking window into one summer day more than a hundred million years ago. “We get used to seeing [dinosaurs] as dead things, not as living things,” says Basinger, a palaeobotanist at the University of Saskatchewan. “This is a really important way to remind people that we’re actually dealing with things that wandered around the landscape and ate stuff ... not just bones in a museum.”


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