For the biggest animals, bulk is a great defense. Other creatures need tricks, like the snowy coat of an arctic hare or a hagfish's choking slime clouds. But elephants and rhinoceroses get by with tough hides and sheer size. They have little to fear from predators and little need for camouflage — hence their dull gray skin.
A new fossil analysis reveals that things were different in the Cretaceous period, 110 million years ago. Even massive dinosaurs with thick skin and long spikes needed to avoid hungry eyes. Under pressure from ferocious predators, these herbivores evolved camouflage, according to a study published Thursday in the journal Current Biology. For the first time, paleontologists detected color in the fossil of a giant armored dinosaur called a nodosaur.
The nodosaur was the sort of creature that wouldn't bother with a disguise if it lived today. It had horns and scale plates for defense. And it was huge — 2,900 pounds or more when full-grown, bigger than a black rhino. But size and armor were not enough.
The authors discovered chemical traces of pheomelanin, the same pigment that gives redheads their hair color, within the dinosaur's fossilized hide. The nodosaur was darker red and brown on top than on the bottom. This pattern, one seen today with deer and antelope, obscures a creature's silhouette.
“It gives you a sense of how nasty the theropod predators would have been back in the Cretaceous,” said Caleb M. Brown, a paleontologist at the Royal Tyrrell Museum of Paleontology in Canada and an author of the new report.
That the scientists could find color at all, let alone a pattern, surprised other experts.
“I never seriously thought that color preservation on this scale would have been possible,” said Thomas R. Holtz, a vertebrate paleontologist at the University of Maryland who was not involved with the study. “This skeleton is truly spectacular in terms of the quality of completeness.”
The dinosaur died in 3-D, still covered with fossilized skin and with the remnants of its last meal in its stomach. “There's no other dinosaur specimen like it,” Brown said. And, rather than the severe contortion of many dinosaur skeletons, this nodosaur looks almost peaceful — as though its last act was a nap on Medusa's porch.
A man digging in Canadian oil sands in 2011 struck the fossil with his backhoe, as The Washington Post reported in May. A sea once covered that region in northeastern Alberta. People had discovered bones there before, mostly aquatic reptiles like plesiosaurs and ichthyosaurs, but the nodosaur was the first dinosaur discovered in the Alberta mine. Researchers hypothesize that the animal tumbled into a river and died. Its body floated out to the sea, where it sank. There, layers of sand kept scavengers at bay as the carcass settled into rock.
“It is an unusual depositional environment for a dinosaur,” Holtz said. But it was a lucky one for paleontologists. Scientists have found color traces in fossils before, but most of those were small animals dug up from ancient lakes.
After the miner found the nodosaur, a fossil preparer named Mark Mitchell worked for more than five years to expose the creature within the rock. The animal's scientific name, Borealopelta markmitchelli, honors his 7,000 hours of labor.
When Brown and his colleagues examined the fossilized skin, they found molecular signatures left by pheomelanin pigment. What's more, these analyses showed the reddish-brown was more pronounced on top than on the bottom, a pattern known as countershading.
“The further you went toward the belly, the less and less of this stuff there was,” Brown said. “It was darker pigmented on top and lighter on the bottom.” The pattern obscures an animal's outline, brightening bellies in shadow and darkening where light falls from above. It's a common camouflage, found in chipmunks, gazelle and giraffes — but never before in a land animal the size of a rhino.
Even though this was the first time this pattern had been found in a large dinosaur, the discovery was “incredibly reasonable” in Holtz's view.
“The largest living land predators — big bears and tigers — are dwarfed by the giant dinosaur predators,” he said. (The latter really were nasty: The 20-foot-tall Tyrannosaurus rex, although it lived a bit later than this nodosaur, could pulverize bones with a powerful chomp.) “We have plenty of other evidence of big predatory dinosaurs having attacked living plant-eaters,” with those other prey fossils showing partially healed bite marks.
This won't be the last time we hear about this nodosaur. Thanks to the detail of the fossil, “you can test some ideas and aspects of their biology that weren't possible before,” Brown said. The paleontologists plan to examine the preserved stomach contents to see what it ate before it died. “There’s going to be a lot more work on this particular animal.”
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