Feathered dinosaur tail in amber!

In a market in Myanmar, the Chinese scientist Xing Lida, shown in the picture below, found a piece of amber about the size of a dried apricot, and it had an inclusion. The seller, thinking the inclusion was a piece of plant, raised the price, for biological items in amber dramatically increase its value. Still, Xing bought the piece at a relatively low price, for the seller didn’t realize that the inclusion was not a plant, but part of a theropod dinosaur! And so it was: part of the theropod’s tail, which was sprinkled with feathers. The specimen turned out to be from the mid-Cretaceous, about 99 million years old. It’s a remarkable piece:

The specimen: a bit of theropod dinosaur tail with very clear feathers

Ryan McKellar and Xing Lida (discoverer of the specimen) with some amber from the site. Photo from CNN.

That specimen tells us something about the nature and evolution of dinosaur feathers, which evolved long before the feathers were used for flight in the birds that evolved from theropods. The function of these feather rudiments still isn’t known, but they were likely to be for thermoregulation and could also have served as ornamentation. (Sexual selection is probably ruled out since there doesn’t seem to have been sexual dimorphism in the feathers.)

The paper, by Lida Xing et al. (reference below, along with link that may or may not allow you to get the full pdf), is the first to describe not only feathers in amber, but also mummified skin and skeleton.  It apparently belonged to a non-avian coelurosaur, the group of feathered dinosaurs from which birds are descended (not all paleontologists and ornithologists agree about that scenario, though most do). Based on the tail, the animal was very small: a CNN report on the finding says the specimen could fit in the palm of your hand, and was about the size of a sparrow. Can you imagine a dinosaur that small?

The remarkably preserved feathers were examined with phase-contrast X-ray scanning (right below), which showed a paired series of feathers along the midline of the dorsal (top) part of the tail (the bottom is sparsely feathered). Some color can be discerned, suggesting the dinosaur was white and chestnut brown, also like a sparrow. In B, below, you can see some of the vertebrae; there are eight full ones and part of a ninth—a remarkably large section of tail, and showing that the bird was indeed small. (All photo captions are from the original paper.)

Photomicrographs and SR X-Ray μCT Reconstructions of DIP-V-15103 (A) Dorsolateral overview. (B) Ventrolateral overview with decay products (bubbles in foreground, staining to lower right). (C) Caudal exposure of tail showing darker dorsal plumage (top), milky amber, and exposed carbon film around vertebrae (center). (D–H) Reconstructions focusing on dorsolateral, detailed dorsal, ventrolateral, detailed ventral, and detailed lateral aspects of tail, respectively. Arrowheads in (A) and (D) mark rachis of feather featured in Figure 4A. Asterisks in (A) and (C) indicate carbonized film (soft tissue) exposure. Arrows in (B) and (E)–(G) indicate shared landmark, plus bubbles exaggerating rachis dimensions; brackets in (G) and (H) delineate two vertebrae with clear transverse expansion and curvature of tail at articulation. Abbreviations for feather rachises: d, dorsal; dl, dorsalmost lateral; vl, ventralmost lateral; v, ventral. Scale bars, 5 mm in (A), (B), (D), and (F) and 2 mm in (C), (E), (G), and (H).

For reference: here are the parts of a modern bird feather; the important parts are the rachis, or main shaft, the barbs, branches off the shaft, and barbules, the smaller branches off the barbs bearing hooks that hold the barbules together—like Velcro—into a single apparatus.

Parts of a feather: 1. Vane,  2. Rachis, 3. Barb, 4. Afterfeather, Hollow shaft, calamus

This picture shows a series of rachis-like structures that splay out from a single place, and each of those is covered with branches, which the authors interpret as barbules:

Photomicrographs of DIP-V-15103 Plumage (A) Pale ventral feather in transmitted light (arrow indicates rachis apex). (B) Dark-field image of (A), highlighting structure and visible color. (C) Dark dorsal feather in transmitted light, apex toward bottom of image. (D) Base of ventral feather (arrow) with weakly developed rachis. (E) Pigment distribution and microstructure of barbules in (C), with white lines pointing to pigmented regions of barbules. (F–H) Barbule structure variation and pigmentation, among barbs, and ‘rachis’ with rachidial barbules (near arrows); images from apical, mid-feather, and basal positions respectively. Scale bars, 1 mm in (A), 0.5 mm in (B)–(E), and 0.25 mm in (F)–(H). See also Figure S4.

Below is a close-up of the feather, which shows a “weakly-developed” rachis off of which ramify alternately-placed barbs, themselves bearing barbules.  According to the authors, this supports one of two alternative forms of feather development proposed by evolutionists, with both shown in the bottom part of the figure below. In one scenario (top), the barbs ramify from a developmental focus, then coming to branch directly opposite each other off a rachis, withe the barbules evolving later, becoming asymmetrical to form a flying surface.

The second scenario, which the authors say this specimen supports, is the development of barbules on the barbs before one of them (I think) evolves into a rachis with alternatingly-oriented barbs (that’s this specimen, circled in the figure as an intermediate). Then the barbs evolutionarily move to positions opposite each other on the rachis. Thus, this intermediate supports the bottom evolutionary scenario.

I have to admit that I’m not familiar with the controversy about feather development, and if there are facts to add here I’ll leave them to more knowledgable readers.

DIP-V-15103 Structural Overview and Feather Evolutionary-Developmental Model Fit (A and B) Overview of largest and most planar feather on tail (dorsal series, anterior end), with matching interpretive diagram of barbs and barbules. Barbules are omitted on upper side and on one barb section (near black arrow) to show rachidial barbules and structure; white arrow indicates follicle. (C) Evolutionary-developmental model and placement of new amber specimen. Brown denotes calamus, blue denotes barb ramus, red denotes barbule, and purple denotes rachis [as in 5, 12]. Scale bars, 1 mm in (A) and (B).

Finally,  you might say, “Well, this may not be the developmental pathway for modern bird feathers, but only for the lineage that contained this species.” But that’s unlikely since paleontologists and developmental biologists tell us that feathers evolved only once, so this specimen does have a bearing on feather evolution. (By the way, the supposedly unique evolution of human intelligence is often used by theologians to claim that that our intelligence, with the ability to apprehend the divine, must have itself been promoted by God. But feathers and elephant trunks are evolutionary one-offs, too! Could it be that God is a bird?)

Thoth, an Ibis God of ancient Egypt.

h/t: Nicole Reggia ♥

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Xing, L., R. C. McKellar, X. Xu, G. Li, M. Bai, W. S. I. V. Persons, T. Miyashita, M. J. Benton, J. Zhang, A. P. Wolfe, Q. Yi, K. Tseng, H. Ran, and P. J. Currie. A Feathered Dinosaur Tail with Primitive Plumage Trapped in Mid-Cretaceous Amber. Current Biology. 26, 1–9 December 19, 2016 http://dx.doi.org/10.1016/j.cub.2016.10.008