Some dinosaurs’ elaborate armour unquestionably helped them in a fight. But for at least one Cretaceous animal, a body covered in bony plates may have been vital for finding love, too.
Careful study of the armoured dinosaur Borealopelta markmitchelli confirms that spines along its neck and shoulder are highly exaggerated, perhaps to catch the eyes of mates and social rivals. The effect would’ve been heightened by soft-tissue sheaths covering the bony spikes, which we can see today thanks to the dinosaur’s eye-popping preservation.
About 110 million years ago, this plant-eating dinosaur died and wound up at the bottom of an ancient ocean. In 2011, miners in Canada accidentally dug up its remains, stumbling onto a 3-D fossil so well-preserved it looks as if the animal was instantly turned to stone. Since its unveiling in May, scientists have been poring over the spectacular remains to learn more about how it lived.
It isn’t entirely surprising that the protrusions on its armour would have been useful for both love and war. Elephants use their tusks as defensive weapons, but they also gauge other elephants’ tusks when they’re trying to find mates.
“For most of these elaborate structures in living animals—tail feathers in birds, colouration in lizards, horns in mammals—the driving [evolutionary] force is generally sexual selection,” says Caleb Brown, a Royal Tyrrell Museum researcher who is studying Borealopelta with support from the National Geographic Society. “That doesn’t preclude its function in defence or species recognition.”
Brown’s new look at Borealopelta, published on Wednesday in PeerJ, is one of the few studies of its kind on an armoured dinosaur—and the first to focus on soft tissues, since they fossilize only rarely.
“It’s hard to derive function from form, even in living animals,” says Victoria Arbour, an armoured-dinosaur palaeontologist at the Royal Ontario Museum who reviewed drafts of the study. “This is a cool starting point.”
When researchers study animals’ social and sex lives, they tend to look for structures that rapidly grow to extreme proportions once the animal hits sexual maturity, like horns in cows. But getting these growth rates in Borealopelta isn’t doable because that would require samples of the dinosaur’s bones. For now, the Canadian fossil is the only one known from this species, and much of its skeleton remains hidden by armour and skin.
Along Borealopelta's neck, the dinosaur's armour plates feature taller, longer spines than plates farther back. Note the banding of lighter, more flexible tissue between the tougher armour plates. - PHOTOGRAPH BY ROBERT CLARK, NATIONAL GEOGRAPHIC CREATIVE
Instead, Brown studied how the dinosaur’s armour plates change in appearance across its body. After carefully measuring each of the fossil’s 172 armour plates—about two-thirds of its estimated covering in life—he found that Borealopelta’s frontal spines were quantifiably taller and longer than the armour plates farther down its back.
In particular, its imposing shoulder spines resemble a bull’s horns—a far cry from the dinosaur’s flatter, tile-like back plates.
Brown also saw this exaggeration in the fossilized sheaths, once made of the protein keratin, that cover many of the armour plates. Some sheaths on the back plates added only an extra tenth of an inch in length. Some neck plates, however, had sheaths that added more than an inch.
Why was Borealopelta carrying so much bony bling near its head? Brown likens this elaborate armour to a billboard that may have helped it spot members of the same species, jockey for social standing, or find potential mates—the roles that tusks and horns play in living animals, beyond defence.
This interpretation gets a boost from Brown’s previous work, which hinted that Borealopelta’s shoulder spines were lighter in colour than the dinosaur’s red-brown skin, possibly making them stand out. (Borealopelta may also have used camouflage.)
Paleobiologist Jakob Vinther claims that the red-brown pigment pheomelanin covered much of Borealopelta except for its belly. If so, this two-tone colouration may have helped Borealopelta avoid predators. Its large shoulder spines also may have been lighter in colour. - MANUEL CANALES, NGM STAFF; PATRICIA HEALY. ART: DAVIDE BONADONNA. SOURCES: CALEB MARSHALL BROWN AND DONALD HENDERSON, ROYAL TYRELL MUSEUM OF PALAEONTOLOGY; JAKOB VINTHER; C. R. SCOTESE, PALEOMAP PROJECT
That said, researchers caution against reading too much into the dinosaur’s social life. For one, we don’t yet know how armour would have varied across the species, or whether Borealopelta males and females looked different from each other. (The fossil’s sex is unknown.)
“The study is very, very good and mostly convincing,” adds David Hone, a palaeontologist at Queen Mary, University of London who reviewed the study. “But at the same time, although this is the best you could possibly do, some more support form the fossils would help prove it.”
Future fossil finds could help clarify matters. Arbour is leading the study of Zuul, a different well-preserved armoured dinosaur that hasn’t yet been fully removed from the rock. She’s excited to compare her eventual examinations of Zuul’s armour to the results coming from Borealopelta.
“My hunch,” she says, “is that it’s a pattern that will hold up.”
Lead image: A right-side view of the dinosaur Borealopelta markmitchelli. Accidentally found by Albertan oil-sands miners in 2011, the fossil is the best-preserved nodosaurid ankylosaur ever found. - Photograph by Robert Clark, National Geographic Creative.