The fossil of Borealopelta markmitchelli; (top) an artist’s impression of the species and possible predators. © Royal Tyrrell Museum of Palaentology, Canada
Exquisitely preserved with its skin and armour intact, it’s a one-in-a-billion dinosaur find, in the words of scientists who have studied the fossil. And the 5.5-metre giant, now on display at the Royal Tyrrell Museum of Palaeontology in Canada, has revealed a secret that was unexpected of an armoured dinosaur this size: it was possibly under stress from predators, and used camouflage to evade them.
Declared a member of a new species, Borealopelta markmitchelli, the dinosaur was reddish-brown on top and a lighter shade on the underside, researchers say. This could have been to conceal itself from predators, they suggest in a description published last week in Current Biology. When light falls on an animal from above, having a dark back and a lighter belly helps conceal its three-dimensional shadow, explained Caleb Brown, a palaeontologist at the Royal Tyrrell Museum and lead author of the study.
“It is the most common form of camouflage seen today,” Brown told The Indian Express. “You see it on animals from deer and rabbits to penguins and sharks.” Such a detailed study of pigmentation was made possible by the state of exceptional preservation in which the dinosaur was found. In March 2011, workers of a Canadian petroleum company, Suncor, found the remains while excavating at the Millennium Mine in Alberta. “Our workers… noticed a large lump of dirt with an odd texture and diamond patterns, quite unlike the usual oil sands clay,” Suncor said in a statement that April.
Word was sent to Royal Tyrrell Museum, and curator of dinosaurs Donald Henderson arrived on the spot with a technician. Henderson, who is one of the co-authors of the study, was struck by what he saw in 2011 — “a perfectly three-dimensionally preserved, uncrushed, armoured dinosaur complete with all the armour in place, original scales perfectly aligned with the armour, all the fingers and toes (very rare), and probable stomach contents”, he wrote in The Guardian in 2013.
The dinosaur belonged to the Early Cretaceous period, between 146 million and 100 million years ago. The area where it was found was then part of a sea. The 1,300 kg carcass arrived at the seabed on its back, the impact deforming the sediment and resulting in its burial. “When found, the fossil was completely encased in a very dense and strong but brittle siderite concretion,” the study authors write.
Over the six years subsequent to the discovery, museum preparator Mark Mitchell spent 7,000 hours of work removing the hard rock encasing the fossil. The species name “markmitchelli” honours his effort. The genus name, “Borealopelta”, is derived from “borealis” (Latin, “northern”) and “pelta” (Greek, “shield”).
In May this year, the museum put the dinosaur on public display. “The animal looks almost the same today as it did back in the Early Cretaceous,” a PTI report quoted Brown. “You do not need to use much imagination to reconstruct it; if you just squint your eyes a bit, you could almost believe it was sleeping. It will go down in science history as one of the most beautiful and best preserved dinosaur specimens — the Mona Lisa of dinosaurs.”
Because of the condition of the specimen, the researchers could use chemical analysis to infer the dinosaur’s pigmentation pattern. Analysing organic compounds that had been preserved in a coating of film surrounding the exterior, the researchers attributed the reddish-brown colour to a form of melanin called pheomelanin, and found that the concentration of the pigment decreased from the back down the sides.
Such a condition with darker backs and lighter bellies is called countershading. Today, most herbivores living on land have countershading; the exception is animals that are too large to worry about predators, those are one solid colour. “Think elephants, rhinos, hippos, etc,” Brown said. The presence of countershading in a large, heavily armoured herbivorous dinosaur, therefore, provides a unique insight into the predator-prey dynamic of the Cretaceous Period, the study notes.
“Previously some had suggested that because of their size and huge armour, armoured dinosaurs were attack-proof and did not need to worry about predation,” Brown told The Indian Express. “We are suggesting that this may not have been the case, and they may have been preyed upon by large carnivorous theropod dinosaurs.”
Not all other scientists are convinced, however. Alison Moyer of Drexel University, a researcher who has studied fossilised tissues, told National Geographic that the study doesn’t fully address whether the film covering the exterior is really fossilised skin or the remains of a bacterial film that grew over the decaying dinosaur. And since the preserved skin doesn’t extend to the underbelly, she’s not convinced that this region lacked the pigmentation, the magazine said.
