Too cool, part 34: A chronicle in amber

I had initially said that I wasn’t timely on this, and that was even a few days earlier when I’d started to type this up, but then I realized how much I was falling for the same trap that has made “news’ the pathetic state of affairs that it is now. Scientific findings of this nature don’t have this bare moment of interest, like a celebrity doing something stupid, but a lasting impact and a value of fascination that persists. So regardless of how many times it’s been covered by other outlets, if you haven’t looked at the details of this yet, you really should.

Lida Xing, a paleontologist from China, had been poking around a market in Myanmar looking at fossils for sale when he spotted a remarkable specimen: a portion of a dinosaur’s tail preserved in amber. Now, this alone is a so-far unique find, since what we’ve been working on for the past century have been fossilized bones and the occasional body imprint in mud, so a portion of intact body – skin, muscles, and so on – is pretty sweet in itself. But this one has feathers, and in remarkable detail too. Why Evolution is True has more information, and I encourage everyone to go check it out, because I’m not going to even come close to doing it justice.

The most frequent comment on the entire subject is how the images of ancient sauropods have changed in the personal timeline of just about everyone. While the idea that birds evolved from some branch of dinosaurs has been around for a long time, it was a casual theory for many decades, having little impact on the popular perception of dinosaurs as just variations of reptiles. The idea started slowly gaining ground and attention as more fossil finds began to fill in the missing factors that distinguished the similarities between the two. In the past thirty years or so, the evidence started to become overwhelming, both in the form of matching skeletal structures like the clavicle/furcula, and in the finding of primitive feathers, mostly in mud impressions. Just a few years ago, I remember reading about one study that had started piecing together the colors of these feathers by noting that black feathers in modern birds contained a significant amount of a certain element; by testing the sedimentary deposits in the mud casts of some of the feather impression fossils, the same elements were found, but only in certain locations within the cast. The feathers, as they decayed, would give up their mineral composition to the surrounding mud, and so finding the same elements among only certain feather impressions gave evidence that at least some of the feathers were black. While far from proven, it remains a remarkable application of forensics, and provides the first inkling of what colors at least some dinosaurs might have been – before that, we only had guesses based on existing reptiles and the supposition of what would have worked best in certain circumstances. Was camouflage an issue? What about sexual selection, like how current birds select mates based on how brilliant their plumage is?

The newest find in amber not only shows excellent resolution of the feathers, including the fine structure of the barbs and vanes, but also a hint of colors. Quite notably, it provides enough detail to start filling in the finer points of how feathers themselves evolved, especially since, at present, it appears as if they have only evolved once in history (as opposed to flight, which has evolved four separate and distinct times, for birds, insects, bats, and pterosaurs, which were separate from the theropods believed to be the ancestors of modern birds.) But feathers have to have a specific interlocking structure to be useful aerodynamically, to form a smooth and manipulable surface, and such flight feathers are distinctly different from even the down feathers of modern avians. Within each feather are ‘arms’ (barbs) that radiate from the central ‘quill’ (shaft,) and then smaller ‘barbules’ that branch from those – these are the ‘teeth of the zipper’ that allows the flight feather to become one smooth structure rather than simply fuzzy like a down feather, and they must alternate position on either side of the barb in order to interlock. One of the questions about the evolution of feathers is if either the alternation, or the ability to interlock, developed first, and this specimen lends weight to the latter; this was definitely a flightless species with feathers that could not form a smooth surface, yet the structures that could hold the barbs together can be made out in examination, without the necessary alternation that would permit a contiguous surface. In other words, the teeth of the zipper are there, but not yet placed where they could fit together.

photomicrographs of amber-preserved coelurosaur tail and feathers

© 2016 Elsevier Ltd.

As may be seen when inset G is closely examined, some of the barbs are alternating, and some are not, giving an indication that these are in transition. It’s not just flight that requires this structure, but also rainproofing and the ability to retain body heat. ‘Fuzzy’ feathers can still serve this purpose to some extent, which is likely why feathers even began to develop (and why so many species have fur,) but it becomes much more efficient when a more cohesive structure evolves. And flight itself really needs developed feathers – air cannot bleed through or be induced towards turbulence by more than a few percent of the flow before efficiency drops too low to be considered “flight” (as opposed to “greater hang time” when jumping to escape predators or capture prey.)

Whooping crane Grus americana portrait through fenceJust stopping to consider the various aspects of it leads to countless further questions, and Wikipedia’s page on the origins of avian flight provides a lot of the speculative detail (and while we’re here, the origin of birds page is cool too.) But here’s one aspect that I’ll provide, first introduced to me by my raptor rehabilitation training many years back. Most modern birds have scaly legs, which is not unlike the scaly skin of many modern reptiles, and curiously, the scales tend to follow the same distribution pattern of feathers in nearby areas while not found beneath feathered areas at all (nor on the bare face or head areas of species like vultures and cranes.) They are largely the same types of cells, and closely related to the cells that produce fur in mammals despite the wide gulf in evolutionary development between the classes; there is a simple genetic mutation that can produce feathered legs in birds. In other words, the DNA of the cells contains the ability to produce either, with indications that only a small number of proteins regulates how they become either simple scales or elaborate feathers.

It’s definitely worth checking out the original paper, and especially seeking out the full-resolution version of the image that shows the electron microimages, solely for the detail obtained from this amber specimen. And if you’re curious about feathers and skin, this page briefly explains a lot of details about the functions and properties of both.

Comments are closed.