Large Cover-up

A remarkably well-preserved Borealopelta markmitchelli fossil shows the dinosaur’s armored body in great detail.

Royal Tyrrell Museum of Palaeontology, Drumheller, Canada

Borealopelta markmitchelli was a heavily armored, spiny, plant-eating ankylosaur that stretched 5.5 meters long, weighed over 1,300 kilograms, and lived in the early Cretaceous Period, about 110 million years ago. Its fossil was found in 2011 at an oil-sands mine in northeastern Alberta, Canada. The species was named for the technician who spent over five years cleaning and preparing it for study. The conditions under which B. markmitchelli fossilized were incredibly rare, resulting in remarkable three-dimensional preservation of soft tissues, including the skin. A study of the skin has led to an unexpected discovery about the dinosaur’s coloration.

An international team of scientists, including paleontologist Caleb Brown of the Royal Tyrell Museum of Paleontology in Drumheller, Alberta, used mass spectroscopy to look for chemical signatures of pigment in the organic films that covered the dinosaur’s scales and other parts of its skin. Chemicals indicating a reddish-brown pigment were prominent on the back of the dinosaur but disappeared towards its sides and belly. This pigmentation pattern is consistent with a type of protective coloration called countershading, designed to break up the outline of an animal’s body and make it more difficult for predators to detect.

“It’s the most common form of camouflage we see in animals today,” says Brown. Such camouflage, however, is not seen in modern large herbivores, such as elephants and rhinos, because they are big enough to no longer require protective camouflage. B. markmitchelli’s strong countershading, despite its larger size and thorny armor, suggests that it experienced enough predation to warrant keeping its camouflage. “That just tells you how nasty some of these predators were back in the Cretaceous,” says Brown.

One such predator may have been Acrocanthosaurus, a five-ton theropod that measured up to 11.5 meters long and resembled T. rex. A smaller, possibly pack-hunting raptor, Deinonychus, may also have fed on B. markmitchelli. More evidence is needed to confirm which predators contributed to the evolutionary pressure for such a large and heavily-armored dinosaur to also need visual camouflage. Still, the fact that any predators chose to regularly feed on B. markmitchelli surprised the team. Says Brown, “It would not have been an appetizing meal.”  (Current Biology)