

Wednesday, March 24, 2010

"What about bacteria?" (Dinosaur Color Part II)

Now, let's get back into the topic of dinosaur color. Maybe you're on the fence about the "Dakota" story I wrote about in a previous post, and not entirely convinced that pigment is something that can fossilize at all. Luckily for you, a paper was recently published in Nature showing that the color of an organism can in fact be preserved in the rock record. Even better, actually: it can be preserved in feathers.

Sinosauropteryx and other feathered dinosaurs

It's actually kind of crazy when you think about it. A few years back some researchers discovered melanosomes - cells in living tissue that contain melanin, a light absorbing pigment - in a 40 million year old feather under a scanning electron microscope. They determined that, like a modern European starling, this feather belonged to a bird that had dark, iridescent, plumage covering its body. This was great news for people interested in this type of thing, because it meant other researchers could use this technique to look at more fossilized feathers from both avian and non-avian dinosaurs and potentially determine their colors. And that's exactly what Zhang et al. did (2010).

For years now, beautifully preserved fossil specimens of avian and non-avian dinosaurs (as well as many other animals and plants) have been coming from the rocks of the Early Cretaceous Jehol Group of northeastern China (photograph of Sinosauropteryx fossil from National Geographic, courtesy of the Institute of Fossil Paleontology and Paleoanthropology, Beijing). Many of the dinosaurs there appear to have been covered in a fine, filamentous, integumentary structures commonly referred to as "proto-feathers" or "dino-fuzz". These structures were predicted by paleontologists long before they were discovered, partly because of the many other anatomical features that extinct species of dinosaur share with living birds, and the phylogenetic relationship that they share. There are however, many people out there that, despite the overwhelming evidence in proof of the hypothesis, believe that modern birds are not the living descendants of maniraptoran dinosaurs. And they like to argue that dino-fuzz is not the precusor to "true" feathers, or an analogous structure. Instead, they say that the structures seen in the fossils are collagen fiber bundles that have deteriorated after decomposition, and only resemble a filamentous, downy feather. Others have suggested that they are simply the outlines of colonies of bacteria that formed as the body of the animals decayed. (For those paying attention, this is the part of the post that makes the title relevant, and rather clever.)

Luckily for proponents of the scientific method everywhere, Zhang (et al. 2010) and colleagues were able to prove once and for all, and repeat through testable hypothesis, that these structures are in fact feathers and feather-like filaments. And that was in part due to the presence of melanosomes in the structures. The melanosomes appear inside the structures (not outside like a bacterial film would), they are arranged in nearly identical patterns as melanosomes found in modern bird feathers (which would be quite a coincidence for bacteria), and they are found in the same parts of the fossils as you would expect to find them in modern birds (again, not an impossible coincidence, but the evidence seems a little stacked in favor of the not-bacteria hypothesis). So there you have it. Definitely feathers. Definitely in (non-avian) dinosaurs. But that's not all!

Because Zhang and his team are too legit to quit, they went one step further. They figured since they had melanosomes in fossil feathers, and they knew what melanosomes looked like in modern feathers, they could compare the two and determine (by shape, size, and other features) what colors were present in different melanosome varieties. For example, rod-shaped eumelanosomes and spherical phaeomelanosomes could be observed in the basil pygostylian bird Confuciusornis. And it can be reasonable inferred that because these types or melanosomes are packed with the dark colored pigment melanin, that Confuciusornis probably had some dark feathers on it's body. They did a similar analysis of Sinornithosaurus, a non-avian dinosaur, and determined it probably had feathers of varied colors across the surface of it's body.

Still not enough for you? Well good. Because there is still more, and this last example is the kicker. To finish up, they took a look at one more Cretaceous critter: Sinosauropteryx. This theropod dinosaur is the first to have a portion of its body coloration scientifically established. Zang et al. didn't just hint that it may be a dark or light animal, but went as far as to conclude that, using a scanning electron microscope to examine the melanosomes in the proto-feathers of a new specimen, Sinosauropteryx "exhibited chestnut to reddish-brown tones" along it's body and bands of light and "dark colored stripes on the tail" of the animal (2010). Incredible! For the first time, scientists can answer a question many have long sought out the answer to: what color were the dinosaurs (illustration of accurately colored Sinosauopteryx courtesy of James Robins, from National Geographic) . This may only be one species in a vast assortment of extinct animals, but one is better than none, and this pioneering research will surely be followed by more. In fact it already has, but I'll save that for "Part III".

So there you have it. One paper that changes the way we look at dinosaurs. Obviously the color thing is a huge deal, but more importantly, the research has shown that, beyond a reasonable doubt, dinosaurs did in fact have feathers and feather-like structures. This should put the debate to rest, once and for all. At least until more repeatable observations can be made that prove otherwise. Because that's how science works. You don't just make a claim and back it with a bit of patchy evidence. Your observations have to be subjected to cross-examination, and experiments have to be repeatable. That is what makes science great. You can't just say whatever you want and get away with it. The peer review process will get you in the end (sorry "Ida").

As always, thanks for reading. I know that for many, this story is old news. And to be perfectly honest, there is so much going on in the world of paleontology right now, it's been hard for me to keep up! But I'll try to keep the posts coming as regularly as I can for the dedicated few out there that want to learn more about science and life on Earth.


Zhang, F., Kearns, S. L., Orr, P. J., Benton, M. J., Zhou, Z., Johnson, D., Xu, X., Wang, X. 2010. Fossilized melanosomes and the colour of Cretaceous dinosaurs and birds.
Nature 463: 1075-1078.

1 comment: