Giant armored dinosaur may have cloaked itself in camouflage

Sometimes body armor just isn’t enough. A car-sized dinosaur covered in bony plates may have sported camo, too, researchers report online August 3 in Current Biology. That could mean the Cretaceous-period herbivore was a target for predators that relied on sight more than smell to find prey.

The dinosaur, dubbed Borealopelta markmitchelli, has already made headlines for being one of the best preserved armored dinosaurs ever unearthed. It was entombed on its back some 110 million years ago under layers of fine marine sediments that buried the animal very quickly — ideal preservation conditions, says study coauthor Caleb Brown, a paleontologist at the Royal Tyrrell Museum of Palaeontology in Drumheller, Canada. The fossil, found in Alberta in 2011, captured not only large amounts of skin and soft tissue but also the animal’s three-dimensional shape.
“Most of the other armored dinosaurs are described based on the skeleton. In this case, we can’t see the skeleton because all the skin is still there,” Brown says.

That skin contains clues to the dinosaur’s appearance, including its coloration. “We’re just beginning to realize how important color is, and we’re beginning to have the methods to detect color” in fossils, says Martin Sander, a paleontologist at Bonn University in Germany who wasn’t part of the study.

But despite ample tissue, the researchers didn’t find any melanosomes, cellular structures that often preserve evidence of pigment in fossilized remains. Instead, Brown and colleagues turned to less direct evidence: molecules that appear when pigments break down. The researchers found about a dozen types of those molecules, including substantial amounts of benzothiazole, a by-product of the reddish pigment pheomelanin. That might mean the dinosaur was reddish-brown.
The distribution of pigment by-products also gives clues about the dinosaur’s appearance. B. markmitchelli had a thin film of pigment-hinting organic molecules on its back, but that layer disappeared on the belly. That pattern is reminiscent of countershading, when an animal is darker on its back than its underside, Brown says. Countershading is a simple form of camouflage that helps animals blend in with the ground when seen from above or with the sky when seen from below.
This is not the first time countershading has been proposed for a dinosaur (SN: 11/26/16, p 24). But finding the camouflage on such a large herbivore is somewhat surprising, Brown says. Modern plant eaters that don similar camouflage tend to be smaller and at greater risk of becoming someone’s dinner. B. markmitchelli’s skin patterning suggests that at least some top Cretaceous predators might have relied more on eyesight than today’s top carnivores, which often favor smell when hunting, Brown says.

Some experts, however, want stronger evidence for the coloration claims. Molecules like benzothiazole can come from melanin, but they can also come from a number of other sources, such as oils, says Johan Lindgren, a paleontologist at Lund University in Sweden. “What this paper nicely highlights is how little we actually know about the preservation of soft tissues in animal remains. There’s definitely something there — the question is, what are those [molecules], and where do they come from?”

Sander does buy the evidence for the reddish tint, but it might not be the full story, he says. The dino could have displayed other colors that didn’t linger in the fossil record. But the countershading findings “point out the importance of vision” for dinosaurs, he says. Sharp-eyed predators might have made camouflage a perk for herbivores — even ones built like tanks.