The latest issue of Science contains a paper by McKellar et al, showing a number of examples of dinosaur and early bird feathers preserved in late Cretaceous amber (about 70 million years old) from Alberta, Canada (h/t to artist Kalliope Monoyios at Symbiartic for calling this work to my attention). The feathers show almost all the stages in the evolution of those structures—beginning with simple filaments, presumably for thermoregulation, to coiled filaments used by diving birds like grebes to modify their buoyancy by absorbing water, to more complicated feathers similar to those in modern birds.
The hypothesized stages in the evolution of modern feathers are shown below (figure 1 from the paper); all of these stages are seen in the amber specimens. The authors describe the stages:
The currently accepted (11, 12) evolutionary developmental model for feathers (Fig. 1A) consists of a stage I morphology characterized by a single filament: This unfurls into a tuft of filaments (barbs) in stage II. In stage III, either some tufted barbs coalesce to form a rachis (central shaft) (IIIa), or barbules (segmented secondary branches) stem from the barbs (IIIb); then, these features combine to produce tertiary branching (IIIa+b). Barbules later differentiate along the length of each barb, producing distal barbules with hooklets at each node to interlock adjacent barbs and forma closed pennaceous (vaned) feather (stage IV). Stage V encompasses a wide range of additional vane and subcomponent specializations. Most modern birds possess stage IV or V feathers or secondary reductions from these stages (11, 16).
And more complicated feathers:
How do we know these feathers are from theropod dinosaurs (the ones that evolved feathers and whose descendants became modern birds) rather than more modern birds living at the time. The authors give several reasons, including the fact that some of the feather types are found only in fossilized dinosaurs like Sinornithosaurus millenii. But I’m not convinced that at least some of the more complex feathers didn’t derive from what we’d consider “birds”, especially since bird fossils do occur in the area.
In his a Perspective piece accompanying this paper, Mark Norrell is more judicious, concluding that “complex ‘modern’ feather adaptations had already appeared before the extinction of the nonavian dinosaurs.” Norrell also sees the most surprising finding as that of feathers with coiled “barbules” (side structures in stage 3b above) suggesting that some of the early feathered dinos were diving animals.
If you want just a summary of the pictures and a brief description, check out Kapi’s post at Symbiartic (she was the illustrator of WEIT). Here’s one of the photo she presents, showing the feather in situ in an amber chunk, with the caption she gives:
Within this amber piece, six feather fragments partially overlap each other. The beaded appearance of the barbules (finest structures) in this image is a result of pigments concentrated within just a portion of each of the segments that make up the barbules. These barbule segments or “internodes” are connected in a fashion similar to the segments in a bamboo shoot. Photo: Ryan McKellar
McKellar, R., B. D. E. Chatterton, A. P. Wolfe and P. J. Currie. 2011. A diverse assemblage of Cretaceous dinosaur and bird fossils from Canadian amber. Science 333:1619-1622.