(CNN) A dinosaur with impressive armored plates across its back became mummified around 110 million years ago after enjoying one last meal before dying.
And now we know what it ate for its last meal.
Dinosaur stomachs and evidence of their diets are rarely preserved. Occasionally, seeds and twigs have been found in the guts of dinosaur remains, but never conclusive evidence about the actual plants.
In this case, a muddy tomb encased and preserved the dinosaur so well that even its stomach contents remain to tell us that it was a picky eater.
The details of this dinosaur's plant-based diet were published Tuesday in the journal Royal Society Open Science.
"The leaf fragments and other plant fossils were preserved down to the cells," said David Greenwood, study coauthor, Brandon University biologist and University of Saskatchewan adjunct professor, in an email.
The nodosaur, known as Borealopelta markmitchelli, was found in 2011 during mining operations north of Fort McMurray in Alberta, Canada.
After its death, the dinosaur's remains ended up in what was an ancient sea, landing on its back in the muddy sea floor and remaining undisturbed until nine years ago.
It's been on display at the Royal Tyrrell Museum of Palaeontology in Alberta since 2017. The fossil was unveiled after museum technician Mark Mitchell dedicated six years to painstakingly revealing the dinosaur's preserved skin and bones from the marine rock in which it was encased.
In life, the dinosaur — a type of ankylosaur — weighed more than a ton. But it lived off of plants and favored ferns, based on the contents of its stomach. The chunk resembling its stomach is about the size of a soccer ball.
"The finding of the actual preserved stomach contents from a dinosaur is extraordinarily rare, and this stomach recovered from the mummified nodosaur by the museum team is by far the best-preserved dinosaur stomach ever found to date," said Jim Basinger, study coauthor and University of Saskatchewan geologist, in a statement.
"When people see this stunning fossil and are told that we know what its last meal was because its stomach was so well preserved inside the skeleton, it will almost bring the beast back to life for them, providing a glimpse of how the animal actually carried out its daily activities, where it lived, and what its preferred food was."
This discovery sheds light on definitive evidence of what a large herbivorous dinosaur ate — in this case, a lot of chewed-up fern leaves, some stems and twigs. The details of the plants were so well preserved in the stomach that they could be compared to samples taken from modern plants today.
"We could see the different layers of cells in a leaf fragment including the epidermis with the pores, called stomata, through which plants take in carbon dioxide," Greenwood said. "We could also see the surface patterning of the epidermis cells, which was like a jigsaw pattern that we see on many living ferns."
This discovery changed what the researchers know about the diet of such large herbivores, and the plant material revealed more about the dinosaur's interactions with its environment.
This nodosaur was picky. The researchers compared the contents of its stomach with fossil leaf studies from the same time period and region. The nodosaur specifically ate the soft leaves of certain ferns and largely neglected common cycad and conifer leaves.
Overall, they found 48 microfossils of pollen and spores including moss and liverwort, 26 club mosses and ferns, two flowering plants and 13 conifers.
"The lack of horsetails, and rarity of cycads and conifers is surprising, given that these are very common in the surrounding flora," said Caleb Marshall Brown, study author and curator of dinosaur systematics and evolution at the Royal Tyrrell Museum of Palaeontology, in an email. "Even within ferns, it looks like Borealopelta may have had a preference for certain types of ferns, while ignoring others."
The preservation of the plant material in its stomach suggested that the dinosaur died and was buried soon after eating. Based on the growth rings and maturity of some of the plant material, the researchers were able to determine that the dinosaur's death likely occurred between late spring to mid-summer, Brown said.
Then there's the matter of charcoal, which was also found in its stomach. That likely meant that it was grazing in an area hit by wildfires. The researchers know that forest fires were common in forests during the early Cretaceous period, 100 million to 145 million years ago. And after forest fires, ferns flourish low to the ground.
"When you think about it, this may actually make a lot of sense," Brown said. "If you are a nodosaur, you can only feed close to the ground. This new growth will also be more palatable and has a higher nutrient content than established growth [like conifers]. As a result, many large mammal herbivores we are familiar with today will seek out recently burned areas in both grasslands and forests, as they provide unique feeding opportunities."
Wildfires likely occurred in the area where the nodosaur was grazing in the previous six to 18 months, Greenwood said. That's enough time for lush ferns to pop up.
"The discovery of charcoal together with a fern-filled stomach opens a window into the biology of this large herbivorous armoured dinosaur as it suggested Borealopelta was likely a keystone herbivore that shaped the landscape by its grazing, and that it also grazed on the ferns growing in open areas created by wildfires," Greenwood said. "That is so cool."
Paleobotany, which is the study of plant fossils, provides insight that dinosaur skeletons simply can't, Greenwood said.
Gizzard stones, much like those swallowed by birds to help with digestion, were also found in the dinosaur's stomach.
The researchers will continue to study the dinosaur to see what other secrets they may unveil, like how this nodosaur was able to thrive and achieve such a large size with relatively poor quality food, Brown said. Greenwood's graduate student, Jessica Kalyniuk, is studying fossil plants from the Gates Formation in Alberta's Rocky Mountain Foothills to learn more about the forests where the nodosaur lived.