Borealopelta mitchelli found its way back into the sunlight in 2017, millions of years after it had died. This armored dinosaur is so magnificently preserved that we can see what it looked like in life. Almost the entire animal—the skin, the armor that coats its skin, the spikes along its side, most of its body and feet, even its face—survived fossilization. It is, according to Dr. Donald Henderson, curator of dinosaurs at the Royal Tyrrell Museum, a one-in-a-billion find.
Beyond its remarkable preservation, this dinosaur is an important key to understanding aspects of Early Cretaceous ecology, and it shows how this species may have lived within its environment. Since its remains were discovered, scientists have studied its anatomy, its armor, and even what it ate in its last days, uncovering new and unexpected insight into an animal that went extinct approximately 100 million years ago.
Down by the sea
Borealopelta is a nodosaur, a type of four-legged ankylosaur with a straight tail rather than a tail club. Its finding in 2011 in an ancient marine environment was a surprise, as the animal was terrestrial.
A land-based megaherbivore preserved in an ancient seabed is not as uncommon as one might think. A number of other ankylosaurs have been preserved in this manner, albeit not as well as Borealopelta. Scientists suspect its carcass may have been carried from a river to the sea in a flooding event; it may have bobbed at the surface upside-down for a few days before sinking into the ocean depths.
It would have been kept at the surface by what’s referred to as “bloat-and-float,” as the buildup of postmortem gasses would keep it buoyant. Modeling done by Henderson indicates its heavy armor would have rolled it onto its back, a position he suspects may have prevented ocean predators from scavenging its carcass.
Once the gasses that kept it floating were expelled, Borealopelta sank to the ocean floor, landing on its back.
“We can see it went in water deeper than 50 meters because it was preserved with a particular mineral called glauconite, which is a green phosphate mineral. And it only forms in cooler temperatures in water deeper than 50 meters,” explained Dr. Henderson.
He also told Ars that this environment probably also discouraged scavenging, saying, “It was probably a region where [long-necked] plesiosaurs and big fish didn’t like to go. It was too cold and too dark, and [there was] nothing to eat. And there were very few trace fossils in the sediments around it. So there wasn’t much in the way of worms and crustaceans and bivalves and things in there to further digest it. It was just a nice set of conditions in the seabed that had very low biological activity that led to that preservation.”
But none of this was known when the animal was discovered. Although it’s not entirely unusual to find dinosaur remains in marine environments, it’s also not very common. Henderson and Darren Tanke, also from the Royal Tyrrell Museum, walked onto the site fully anticipating that they would excavate an ancient marine reptile.
The two had consulted on fossil discoveries at other open-pit mines within the province. However, this was their first visit to Suncor, a mine in the northeast of Alberta, Canada. Everything about this mine is enormous. Massive machinery is constantly in motion, scooping out rock, sand, and gravel from surrounding cliffs, while other equipment clears it away, all with the goal of uncovering the deeper oil sands for fuel.
“It’s just unbelievable, the scale of the place,” Dr. Henderson said. “And it goes 24 hours a day, 365 days a year.”
Despite the pace of operations, one particular shovel operator, Shawn Funk, happened to notice something after taking a big chunk out of the cliff. It was thanks to him and several people within Suncor that operations stopped in that area and the Royal Tyrrell was notified.
It may have been their first visit to the Suncor mine, but the rock formation worked by the mine was well-known to Henderson and Tanke. This ancient marine environment had produced fossils of plesiosaurs and dolphin-like ichthyosaurs in other locations, so the researchers expected that any new find was going to be something similar.
“Our thinking was biased,” Henderson explained. “Everybody who looked at it here thought, ‘This is a plesiosaur flipper!’ Because what else could it be? When you looked at the bones, it wasn’t making sense as a plesiosaur. It is amazing how your thought process wants you to see something. Eventually you have to admit: It’s not there.”
A bit of luck
Henderson described the “series of coincidences” he and his colleagues recognized as being critical to this fossil surviving.
Suncor Energy and others who have mine claims, he said, won’t dig right up to the edge of the mine, as this can impact the landscape next to it. Rather, they leave approximately 10 meters or so as a buffer zone. Borealopelta was discovered not too far from this buffer zone.
“If it had drifted another 10 meters in the past,” Henderson remarked, “it would never have been exposed because it would be sitting in their buffer zone. And no one would have ever seen it!”
Shawn Funk had visited the Royal Tyrrell Museum only a few weeks before. “So he had some sort of vague search image in the back of his head about what to watch for,” Henderson noted.
“That place runs 24 hours a day. What if it was the night shift and it was like minus 20 or colder, as usual, and the shovel operator said, ‘Ah, it’s just some wood’ and kept digging?”
Out of the rock
One of the reasons this fossil was so well-preserved is because it was covered in a very thick, very hard concretion—a solid mass that sometimes forms around fossils. The concretion maintained the fossil in 3D, unlike the typically 2D-flattened fossils that occur after millions of years of pressure from overlying rock. Henderson said the concretion helped preserve the skin, preventing even bacteria from breaking it down.
Perhaps most amazing of all, the position in which the animal lay was preserved within the mine itself. When Funk sliced through part of the fossil, it caused the loss of its tail—an appendage Henderson dismissed with understated humor as “some sort of bland thing that tapers down to a point in some armor.” Arguably the most significant part of this fossil is its preserved face, which was positioned safely into the cliff.
It took the researchers 14 days to excavate the find and bring it back in separate enormous blocks to the museum. There, senior preparation technician Mark Mitchell was tasked with separating the fossil from the stone. This was no small endeavor, taking Mitchell seven hours per day over five and a half years. That task, he wrote in an email, took him a staggering 7,000 hours. The length of time it took and the quality of his work are why this dinosaur was named after him (he’s the “Mitchell” in the Borealopelta mitchelli).
“During preparation,” Mitchell explained, “I would piece together the blocks like a puzzle, and the animal started to really take shape. It got me excited to start on the next block to see the animal come together. Right before Christmas one year, I had pieced together both sides of the neck and the head, and you could really appreciate the impressiveness of the specimen and that this was a living creature with astounding preservation.”
Few people can claim to be the first to see the actual face of an extinct animal with no modern analogs. Mitchell described that experience as “absolutely amazing. This was the first dinosaur I’ve worked on with skin actually covering the skull, so being able to see what this animal looked like when it was alive was really cool.”
But he was also “amazed at the skin impressions on the bottom (pad) of the foot. These matched the patterns seen in footprints left behind by other ankylosaurs preserved in Alberta [and British Columbia].”
A complete map
Freeing the fossil from its stony tomb not only revealed one of the rarest dinosaur specimens in the world for everyone to behold; it also opened it up for scientific research. The researchers began with the outside of the dinosaur first.
Dr. Caleb Brown, the curator of dinosaur systematics and evolution at the Royal Tyrrell Museum, is the lead author of three subsequent papers published about this remarkable fossil.
“Generally, when an important new specimen is being researched,” he wrote in an email, “some of the first research that is done is the anatomical description and taxonomy. This is because we need to know what the animal is before much of the other research can be done.”
Armored dinosaurs like nodosaurs and ankylosaurs are covered in bony structures over their skin called osteoderms. Skin rarely survives fossilization, which means these osteoderms are usually found scattered around ankylosaur bones. And while there are a handful of ankylosaur specimens that preserve osteoderms in-situ, Borealopelta offered the rare opportunity to see exactly where every osteoderm and thick spike (parascapular spine) is located on most of its body. Brown took an incredible 605 measurements of 172 different osteoderms on this fossil.
For some, that might have seemed daunting, but Brown says he enjoyed it.
“Data collection and making figures are two of my favorite parts of my job,” he said. “I love when large sample sizes exist for dinosaur fossils because we can collect a large amount of data and test hypotheses. This usually means many specimens of a certain type of animal but in this case was one specimen preserving many different features.”
“I should also point out that the work measuring and drawing the specimen pales in comparison to the amount of work that went in to preparing it,” Brown added in a nod to fossil preparator Mark Mitchell.
Understanding where these osteoderms and spines are located on the animal can help paleontologists reconstruct the tissues on ankylosaur fossils that aren’t found with these bony structures intact. Like an enormous prehistoric puzzle, knowing where each piece fits offers insights into the structure of the animal and the function of those structures. Or, as Henderson explained, “For the previous 100–120 years, we’ve known about [ankylosaurs and nodosaurs]. The armor has always been sort of our best guess. And here we had it all in place.”
“Many armored dinosaur skeletons are preserved disarticulated, meaning their bones are all jumbled up,” Brown told Ars. “So while much of the osteoderms are preserved, we do not necessarily know where each of those osteoderms would be placed in life. Having the osteoderms preserved in life position in this specimen, and other specimens, can give us clues as to how to reconstruct those specimens where the position of the armor is less clear.”
More research needs to be done, but at the moment, the authors suggest that the spines along Borealopelta’s side may not have been defensive; instead, they may have been for display—a way of attracting a mate.
The intense study of this fossil’s anatomy, however, didn’t elucidate its sex.
“We know very little about how to tell the difference between male and female dinosaurs,” Brown admitted. “In almost all cases, we have no idea which specimens are male and which are female. There are rare exceptions to this. One is when we find preserved eggs still inside a dinosaur. Another is when we find medullary bone (associated with egg laying) preserved inside the bones of a dinosaur.”
Without either of these, whether Borealopelta is female or male, therefore, remains a mystery.
A last supper
But Brown and his colleagues discovered something completely unexpected: Borealopelta’s coloring may have helped keep it camouflaged in its environment. The team found evidence that the animal used countershading, meaning that the coloration of its body was darker on the top and tapering into a lighter underbelly. It’s a strategy used by a number of species today to help hide them from predators. Among extant terrestrial herbivores, countershading is seen in smaller prey animals, but it’s not seen in animals similar in size to ankylosaurs, like elephants and hippos.
Because this nodosaur was not found on land where it lived, the authors looked to rock formations of a similar age to tell them about the terrestrial animals that existed at the same time as Borealopelta. Numerous theropod fossil footprints and body fossils, such as those of Allosaurs and Carcharodontosaurs, suggest that Borealopelta co-existed with giant apex predators. Weighing approximately 1,300 kg (almost 3,000 pounds), with a length of about 5.5 meters (about 18 feet) and covered in armor, Borealopelta was a large herbivore. And yet it seems this animal also needed camouflage, offering chilling insight into survival in the Cretaceous.
And this species was definitely a herbivore. The team uncovered Borealopelta’s stomach contents, a discovery almost as rare as Borealopelta itself. Paleontologists often turn to things like dinosaur teeth, the type of fossil plants found in association with fossil bones, or to fossilized feces (coprolites) to understand dinosaur diets. This team, however, was able to uncover several fascinating details: What this animal ate shortly before it died, what that vegetation says about its surroundings, and even the season in which this animal died.
Most of what Borealopelta ate in its last moments were fern leaves, but it also nibbled on woody stems and ingested burnt plant fragments. The cross-section of one particular stem had growth rings indicating late spring to mid-summer, thereby setting the time in which Borealopelta met its end. But the burnt plant fragments are especially telling in that they point to a potential recent wildfire, one in which ferns may have been bringing life back into a ruined landscape. The authors note that research suggests Early Cretaceous forests were subject to frequent wildfires, potentially every 20 to 40 years. Ferns, they added, may have flourished after these events, providing potential feasts for ankylosaurs and other herbivores.
Do the stomach contents reflect the food available to this nodosaur at the time? Another team, including Brown, studied the type of plants that grew close to where Borealopelta was found and compared them to what was found in its stomach. Their results indicate that despite a variety of flora available, this animal may have specifically preferred ferns. The vegetation ingested before it died may have been regrowth from a wildfire, but fossil plants suggest this area offered other nutritious options.
Still, it’s important to note that these stomach contents are from one animal, and the theories may not necessarily hold true for other nodosaurs (let alone the usual diet of Borealopelta, as it only records the last several hours prior to death).
Further research on the stomach contents and the skin is expected in the future, according to Brown. For his part, Henderson would like to be able to study the parts that are normally all we get of dinosaurs—the skeleton. “This isn’t your average dinosaur,” he said. “The skeleton is completely covered up by skin and armor!”
“We took the skull to a place in Vancouver where they did industrial-grade CT scanning,” he added. “They regularly CT scan blocks of iron to look for air bubbles and cracks. This rock is so dense that their high-powered X-rays could not penetrate the skull.” He hopes CT scanning technology will improve in the near future.
“The specimen is impressive in its own right, even without any of the research,” Brown wrote. “The combination of preserved soft tissues and retained 3D shape results in the animal looking much like it did back in the Cretaceous… I think ongoing and future research, specifically looking at features such as the preserved skin and stomach contents will continue to add to our understanding of this animal.”
Henderson compares this discovery to lightning striking just once. “They’ve been digging for 11 years [at the Suncor mine], everybody knows what to look for”—and yet nothing. Moreover, he said, “you could roughly squeeze that whole Borealopelta body in just over one and a half cubic meters. If you figure they’ve dug one and a half billion cubic meters, that fossil truly is one in a billion.”