S is for Skin

Dinosaurs had a lot of skin in the game, even though it’s rather hard to find it 200 million years later. They did have a lot of skin in general. That process of fossilization I described back in letter “F” replaces bone with minerals, though. Skin has to be preserved in a completely different way. It must be mummified, i.e., geologically captured in a unique set of circumstances that don’t allow it be replaced with anything. Skin is tricky to discover.

But there are a few examples, and they can tell us plenty about what the reptile rulers were like.

The preserved Borealopelta, photo on Reddit.


It’s difficult for dinosaur skin to be found intact, but miners in Canada managed to find an exquisitely preserved–it can hardly be called anything else–nearly full skin, complete with horn edges and the face of a type of ankylosaur. This was a nodosaur that they called Borealopelta, meaning “northern shield,” in reference to its discovery origin and covering.

The circumstances of preservation were unusual. Our Borealopelta fell into water when it died, then flipped upside-down. That wasn’t particularly odd as corpses do float after death, typically filling with gases. Perhaps because its thick top layer was so much heavier than the dead, soft underside, the dead animal turned belly-side up, then sank into the mud. It was held fast and did not decay, the sands inside the water acting in this case as a mummifying agent rather than as a conduit to replace living tissue with rock.

When the Canadian paleontologist pulled it out they saw a beautiful set of scales, clearly defining the horns that had long been known as stubborn resistance to predators. Yet they also found something else in the skin. There were remnants of skin-marking melanosomes, the organic things that create coloration and pigmentation.

From Current Biology analysis by Brown et al.

The scientists described how they determined that this animal had “countershaded camouflage.” In other words, in addition to the thick, thick skin and spikes and being low to the ground… Borealopelta also blended into the foliage. Just a bush, folks, just a bush. This one died of something, but its preserved skin suggested that it probably hid well from the big heads and teeth.

Dinosaur skin had tremendous variation.

What’s Skin For?

Skin does act as camouflage. But skin has a lot of other purposes, which is why the skin samples that researchers have found come in so many different patterns, shapes, and sizes. Skin has both unique texture and elasticity–some of it might be smoother and stretchier, and others, like our nodosaur friend’s, bumpy, or pitted. Reptiles were known for scales–thin, flat, sometimes overlapping or patterned structures.

Thicker skin was protective. Something thinner might even help thermoregulate, that is, stay cool in heat and provide a little warmth when it’s cooler. Some skin was more waterproof. A lot of skin might have been sensitive to the touch, allowing some animals to work their way through darker environments.

But the melanin receptors in skin shows that it could have taken on different colors. Some of that would be to hide. But some also might have been to show off.

I was writing, a few posts ago, about how they know dinosaurs had color vision. This came by comparing their skeletal anatomy with modern animal groups and drawing conclusions. The dinosaurs who preceded paraves, the line that led to birds, were thought to see in color because birds see in color. Birds see in color, in part, because they themselves have colors. Boy birds need to find girlfriends; they show off their plumage. They need the feathers to be colored.

Pigmentation in Plumage

The dots start to connect a little tighter now. The first time bone-diggers found the famous Archaeopteryx, the dinosaur fossil with feathers, they recognized the connection to birds. More significantly, as they have found more fossils, they have found tons of examples with feathers or protofeathers.

This Jurassic dinosaur had skin with features that held feathers. It also had the skin cell remnants that suggested colors, and specific colors. The reconstruction of its plumage was based on differentiated melanosomes under the skin. In other words, this isn’t a fanciful, CGI, Internet-artist guess about coloration. This is very likely how that coloration pattern originally existed on Anchiornis huxleyi. (Thomas Henry Huxley was a Darwin contemporary who studied, debated, and furthered biological ideas of development, including variations of evolution.)

Scales to Feathers

One last point about skin and skin evolution. Bumpy, horny skin and scales is one type of reptilian skin, the original type for most of the ancestral lines of our dino buddies. One line of these creatures developed into birds. Down at the end of the tree, or top of the tree if you wish to think about it that way, came the birds. Birds had feathers; the immediate ancestors of birds had feathers, too.

So where did the feathers “come” from?

Feathers have their own structure of anatomy, which would take a whole ‘nother post to go into, so I will refrain. But what they were curious about was how the feathers got there. Feathers are kinda sophisticated, and fish didn’t have them. So they had to get there somehow.

Yet another batch of scientists researched “integumentary” structures–another three-dollar word–which just means covering. Now, I gotta be honest. I met my match with this particular study, as it had almost too many big words for me to understand. Is there a Google Translate for scientific speak?

But I can read a chart. Here’s the question they asked. Filaments in the skin are a beginning structure for feathers. Instead of skin being flat, it starts to develop spots that develop a place for fibers to grow. In humans, that turns into hair, in birds into feathers. That’s oversimplifying it, but you get the idea.

If feathers develop out of filaments, and filaments develop out of scales, then did the original ancestor of dinosaurs have the filaments or not? If they did, then maybe other lines and branches of the dinosaur tree might have had feathers, besides the birds. If the ancestors of the other flyers–the pterosaurs like Quetzacoatlus–also had feathers, maybe lots of dinosaurs really did have feathers. Maybe they all had feathers instead of scales to start with?

Evolution of dinosaur epidermal structures,” of course!

What this tree says, and you can follow the colors, is were there only filaments and feathers on the bird wing or where they everywhere? It’s comparing a possibility (a) with (b). Unfortunately, to our imaginations, (b) won. That is, you can only find the filaments and feathers distinctly way far down on the tree. You had to get to coelosaurs, the direct cousins and uncles of birds, in order to get feathers themselves. And you had to get way down the tree on the other flying reptiles for them to develop feathers.

No crocodiles with feathers. But don’t give up on the T. rex yet because Mr. T is way down the ancestral line.

Speaking of the letter “T”…

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