Diplodocus may have been one cool dinosaur—thanks to its skin

Finding any fossil skin is extraordinary; finding dinosaur skin is that much more rare. So when Tess Gallagher and her mom excavated patches of skin from one of the largest dinosaurs to exist, there was reason for jubilation.

More than a year later, that glee disintegrated—right along with the skin they excavated. But what could have been the end of a sad story was merely the beginning of another exciting chapter, one that could potentially broaden our understanding of how these enormous creatures cooled themselves.

Found and lost

Gallagher, now a paleontologist and paleobiology graduate student at the University of Bristol, and her mother, Lisa Marshall, were part of a team excavating a site called the Mother’s Day Quarry in Montana. The site has produced, among other things, 15 individual Diplodocus juveniles from about 145 million years ago.

Prior to their discovery, the only examples of sauropod skin came from embryos and juveniles at other sites, along with some foot pad impressions in trackways. After the skin was discovered, Gallagher applied glue to the skin surface to help preserve it. This is a normal process in excavation, as initial contact with the air can make fossils fragile. She and her colleagues wrote a paper later that year on their discovery, describing six different types of sauropod scales they observed on the skin. Four of them were new to science.

But then COVID hit. For several logistical reasons, no one could go back to excavate what had been discovered that season.

When the scientists returned more than a year later, they found the skin cracked and disintegrated—a result of the glue’s prolonged exposure to the elements. Gallagher, describing that moment, didn’t mince words. “I’ve never felt more defeated in my life,” she admitted.

She took a handful of sauropod scales found among the fragments with her back to Union College as “mementos.” It was only when she showed them to her then-advisor, Dr. Anouk Verheyden, that the story took another turn—one that eventually led to an entirely new discovery and one Gallagher discussed at this year’s annual meeting of the Society of Vertebrate Paleontology (SVP) in Toronto. The meeting is attended by more than 1,000 people from over 40 countries.

A series of tubes

It began when Verheyden encouraged her to examine the approximately 3-millimeter scales under a microscope. Gallagher’s first observations left her perplexed. Sauropod scales are often polygonal and contain protuberances referred to as "papillae." But the view under the microscope revealed a series of black dots unlike anything she’d ever encountered.

“I was so confused with these structures I was seeing,” she explained, “and I really wanted to figure out what was going on here.”

At first, she thought the dots were papillae. “But that didn’t really make sense,” she said. Upon further study, she noticed “this ring connecting to one of these black dots, which means they are not papillae. So what are they?” After looking at the other specimens, she saw a pattern: “The black dots are actually connecting to each other,” she said. The black dots and connections were, she realized, the sediment filling in holes within the skin; the skin was porous.

Or, she added, for clarification, “if you had a very open pore foam and filled it in with sediment, you wouldn't see the pores. Instead, you would only see the very tips of the strands that connect and make the pores poking out of the sand, so from the top, you would see a bunch of dots. This is how I visualized the black dots in the sediment within the skin.”

Which led to an obvious question: If the skin was porous, what would that mean for behemoths that could reach lengths of over 24 meters (80 feet)?

She looked to today’s wildlife for possible answers, and that exploration eventually led her to a paper discussing how enormous animals—extinct and extant—keep cool. Specifically, it compared the extant Asian elephant to the similarly sized but extinct Edmontosaurus, a type of duck-billed dinosaur. The evidence suggests that, like today’s elephants, Edmontosaurus would need help staying cool under extreme heat, perhaps wading in local water sources or staying in shaded areas.

“But this,” Gallagher noted in her SVP presentation, “begs the question: If a 3-ton Edmontosaurus is having trouble exchanging heat out of its body, then what in the world is a 13- to 20-ton Diplodocus supposed to do?”