Tuesday, October 11, 2016

Psittacosaurus and the Colorful Dinosaur Revolution

Within the last decade, paleontologists have discovered how to unlock the color of extinct organisms, and I think it is one of the most exciting and thought-provoking modern advancements in the field. I’ve written about it no less than four times on this blog (one, two, three, four!), and now I feel obligated to write about it again.

It has been about a month now since the unveiling of the “most accurate depiction of a dinosaur ever,” and the fanfare was suitably loud. Now, I’d like to take a look over what we know about this dinosaur, and where it fits into this grand paleontological revolution we're living through.

The dinosaur: Psittacosaurus

Psittacosaurus is a small herbivore from the Early Cretaceous, a cousin of the famous Triceratops, who lived 50 million years later. This little ceratopsian could grow up to two meters long, though the specimen we’re discussing here wasn’t nearly that big.

This particular Psittacosaurus was discovered in the famous Jehol Biota, a fossilized ecosystem which has produced many other wonderfully-preserved fossils preserving feathers and coloration. These fossils come from northern China, but this Psittacosaurus has spent more than a decade as a prized part of the exhibits in the Senckenberg Museum in Frankfurt, Germany.

This is the famous Psittacosaurus specimen, in all its phenomenally-preserved glory.
You can see the skin outline, scales on the arms, color patterning, and those tail bristles! 

Photo from Vinther et al. 2016 [CC4.0]
And it’s beautiful. Even without microscopes or chemical analysis, you can see the impressions of the skin and scales spread out around the bones; you can see what’s left of the soft bristles that protruded from its tail, now preserved as impressions in the sediment; and you can even see remnants of the dinosaur’s color pattern.

As is so often the case in paleontology, extraordinary information comes to us from extraordinary fossils.

How do you colorize a dinosaur?

The examination of this fossil by Jakob Vinther and colleagues is particularly exciting, since most of the previous work on dinosaur color has looked at pigmentation in feathers. This Psittacosaurus is entirely featherless (except for those tail bristles – more on that later), so instead researchers identified the color in its skin and scales.

There are a few ways to determine what sort of pigment is preserved in a fossil. One way that seems to be gaining more and more traction is through chemical analysis. Unfortunately, the varnish covering this specimen prevents that approach, so these researchers went with good old-fashioned microscopy, identifying the tiny structures we now recognize as melanosomes, the organelles that hold pigment. The shape of these organelles indicates that they are phaeomelanosomes, which provide brown color.

Imaging the specimen with cross-polarized light photography and laser-stimulated fluorescent imaging (paleontology really isn’t just digging in the dirt anymore!) allowed the researchers to identify the precise patterning of the melanosomes across the body.

A brief note on just how much we can rely on fossilized color: the fossil record loves holding onto secrets. We understand a good deal about pigmentation, but the paper points out that “it is debated … how melanosome shape correlates to melanin chemistry outside of mammals and birds.” On top of that, many modern-day reptiles use special pigmented cells (chromatophores) for coloration rather than colored scales, fur, or feathers, and this is – so far – undetectable in fossils. This leaves open the two possibilities that a) we might not know exactly what colors are produced in dinosaurs by certain melanosomes and b) the skin may have produced additional colors that we cannot detect.

On the other hand, most of the pigment patterning detected in this fossil is "locked" within the scales sitting over the skin, which the authors point out is “incompatible” with chromatophore-coloration. That is to say, if the scales are colored, any pigmented skin underneath wouldn’t affect the animal’s outside coloring. So we can be reasonably certain at least that we aren't missing much of the original coloration of this animal, which is pretty remarkable. 

Prehistoric patterning

This incredible specimen preserves its scale and color patterns in fantastic detail.

The body is mostly covered in scales, superficially not unlike many lizards today. It had tinier scales across its fingers and toes, and blockier scales on its belly and underneath its tail. On top of its tail are long bristles, and parts of its legs and face are covered in scale-less, naked skin. (This is something people don’t often consider, that dinosaurs often had multiple types of integument – think of an ostrich, part-feathered, part-scaly [the legs], and partly ‘naked.’) It also has a flap of skin stretching from behind its hind legs (a patagium) which was quite an unexpected feature.

The color of the dinosaur varies, with some body parts darker than others. Along the torso and tail, the dorsal (back) surface is consistently darker than the underside, though the chest is more pigmented than the belly, and parts of its back and tail have faint stripe and spot patterning. On its arms there are clusters of dark scales, and its back legs have very fine striping.

Other parts of the body have particularly high pigmentation as well, including its ankles, shoulder scales, cloaca (that is, its dino-naughty bits) and the scales at the tip of the pubic bone. But some of these highly-melanized areas may have had purposes other than coloration, which brings me to…

The purpose of all that pigment.

Animals today use color for all sorts of reasons, from drawing attention to themselves to blending in. The main focus of Vinther et al's paper was that this Psittacosaurus exhibits counter-shading, being dark on the top and light on the bottom, a very common form of camouflage in modern critters. It is actually the first ever documented case of counter-shading in a terrestrial fossil animal (the condition has been identified in marine fossils, though).

Counter-shading is an extremely effective form of camouflage, and is found all
over the natural world, from goldfish to gazelle. Images from Pixabay [CC0]
How does counter-shading camouflage an animal, you ask? Well, when sunlight hits an animal from above, it illuminates their back and casts their belly in shadow, creating a light-dark contrast that identifies it as a three-dimensional object. When an animal’s natural coloration is the opposite, it lessens that contrast and makes an animal appear “flatter” and thus harder for predators’ eyes to spot.

Vinther and colleagues were even able to link Psittacosaurus’ particular brand of counter-shading to its habitat. With the help of paleoartist Bob Nicholls (he created that awesome artwork of the dinosaurs being struck by lightning in my last post), they created a life-size model of the dinosaur in gray-scale, photographed it in different lighting, and compared the shadows cast upon it to the coloration on the fully-colored model. The colors of the dinosaur matched the shadows cast by “closed canopy” forest-like environments (they actually photographed the model in the University of Bristol Botanical Gardens' “Cretaceous plants” room – how awesome is that?), indicating that this dinosaur had evolved its particular coloration to blend in with the mottled lighting of a forest, fitting with what’s known of the vegetation of the Jehol Biota. Intriguingly, knowing that this dinosaur camouflaged itself the same way modern-day animals do also tells us that predators of the time hunted using similar visual cues to modern-day hunters, which is not all that surprising but good to know.

This Psittacosaurus was using pigmentation for more than just blending in with the shadows. The authors of the paper point out that the dark coloration on its face is similar to the dark “masks” of many mammals, though the purpose of those masks is debated – they may be for display, thermoregulation, deflecting glare, or a number of other uses. It should also be pointed out that much of the face pigmentation is not locked into scales, leaving open the possibility that there were additional colors produced by those specialized chromatophores I mentioned earlier. That, for now at least, we cannot know.

It turns out melanin – the pigment granting all of this color – also has protective uses. It strengthens tissue, making it more resistant to abrasion and wear (this is part of the reason high-melanin fossils often preserve so well), which may explain the high density of melanin on the ankles and pubic edge, which the dinosaur probably rested upon while sitting. Melanin also protects tissues against microbial infection, which is why genital and anal skin are often heavily pigmented (Vinther informs a surprised Dave Marshall of this fact in this video), and it may be why Psittacosaurus had such a dark-pigmented cloaca, though I also like to imagine it showing off its colorful dino-booty during mating season.

What about those tail-bristles?

The tail bristles of Psittacosaurus have sparked much discussion. They’re kind of feather-y, but not really, though another recent study, examining this same specimen and conducted by many of the same researchers, considered it likely that the bristles are related to bird feathers, and noted that they are strikingly similar to the not-quite-feathers of peacock crowns and turkey beards.

Given that the bristles are apparently soft, and only cover the tail, their most likely use was for display, much like the similar structures in the birds I just mentioned. Although in his interview for Palaeocast, Vinther makes the intriguing suggestion that perhaps the bristles were used to make noise somehow. That’s a neat, if so far unsupported idea!

The big picture: What does it all mean for paleontology?

There are many reasons why this is a very exciting time for the field of paleontology, but among those reasons, my personal list is capped by the fact that we are able to reconstruct the appearance of prehistoric creatures – and make all the inferences that go along with that – better than ever before. Just as Sinosauropteryx and Anchiornis were milestones in the field for being the first dinosaurs to have their colors reconstructed from direct evidence, I believe our friend Psittacosaurus will go down in the prehistory hall of fame as the first dinosaur whose appearance was able to be reconstructed with such a staggeringly impressive level of detail.

Photographs of Nicholl's exquisite life-size model, the "most accurate dinosaur reconstruction ever!"
With extraordinary detail in color patterning and scalation.
Image from Vinther et al. 2016 [CC4.0]
This dinosaur is also a marvelous representative of the state of paleontological art. Bob Nicholls’ life-sized fiberglass construct was not only an integral part of the study, but it may very well be, as the headlines have claimed, the most accurate dinosaur sculpture to date, with everything from the stripes on its legs to the bumpy scales on its arms to that weird flap of skin beneath its tail drawn directly from the evidence in the fossil.

For those interested, Nicholls outlines his creative process in this video. It’s really cool to see the step-by-step creation of the model!

I think this dinosaur marks an awesome time for both the science and the art of dinosaur paleontology, with paleo-artists engaging in non-stop discussions over how best to portray accurate dinosaurs, interactive projects like Saurian utterly devoted to scientific rigor, and models like Psittacosaurus drawing from never-before-seen levels of detail in the fossil evidence.

In Elsa Panciroli’s article on this dinosaur, she quotes Nicholls in saying, “The days when a paleoartist could simply fabricate any colour pattern they wanted for a dinosaur are gone.” This seems to be becoming true for more and more aspects of dinosaur biology and appearance, and that’s a truly great thing.

Go look at the Guardian article by Elsa Panciroli, with lots of great artistic images. 

Check out Vinther, Nicholls, and co-author Innes Cuthill discussing the work in this video.

Watch Palaeocast’s Dave Marshall interview Vinther (and giggle about colorful anuses) in this video.

The scientific paper itself:
Vinther et al. 2016. 3D Camouflage in an Ornithischian Dinosaur. [PDF]
*Be sure to look through the Supplementary Material for more really awesome images of the fossil.

The Bristles paper:
Mayr et al. 2016. Structure and homology of Psittacosaurus tail bristles [Abstract]

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