Lucky find offers our first look at a dinosaur tail, complete with feathers

Geoscientist Lida Xing was shopping at an amber market in Myitkyina, Myanmar in 2015 when he saw an unusual piece of amber. Trapped inside was a small object that the amber merchants thought was a sprig of leaves. But Xing thought something much more interesting was going on, so he decided to take a closer look. What he found could change our understanding of how feathers evolved.

Xing had discovered eight fully preserved vertebrae from a young, non-avian dinosaur called a coelurosaur. As an adult it would have been about the size of an ostrich, but this juvenile was still tiny enough to get trapped in tree sap and never escape. Feathers covered its tail, but at the tip they fluffed out in a pattern that suggested this animal may have had a fan-shaped tail. After Xing convinced the Dexu Institute of Paleontology to buy the amber, he and an international group of colleagues in China, England, and Canada examined it closely, using a number of imaging techniques that allowed them to generate 3-D reconstructions of the tail structure.

Xing worked on imaging the tail with Ryan McKellar of the Royal Saskatchewan Museum in Canada. In a statement, McKellar confirmed it is unmistakably a dinosaur: "We can be sure of the source because the vertebrae are not fused into a rod or pygostyle as in modern birds and their closest relatives. Instead, the tail is long and flexible, with keels of feathers running down each side." Chemical analysis of the remains also showed traces of iron, meaning that it probably contained hemoglobin when the dinosaur was alive. Previous studies have shown that other dinosaurs, including the infamous T. rex, had hemoglobin as well.

In a paper for Current Biology, Xing, McKellar, and colleagues describe "the spatial arrangement of follicles and feathers on the body, and micrometer-scale features of the plumage," which led them to a new insight about how feathers likely evolved. Modern bird feathers have a thick central quill called a rachis, and from that branch barbs covered in the soft barbules provide the feather with color and a structure that enables flight. This young coelurosaur's tail has barbs and barbules only, though one central barb is in the same position that a rachis would be on a modern bird.

What this means is that feathers appear to have started out as what some paleontologists call "dinofuzz," a soft, downy covering for warmth.

For decoration, not flight

The tail was also colorful, likely a rich, chestnut brown on top and a creamy white underneath. The researchers explain that the closest modern analogue to these feathers would be ornamental, decorative ones: "The open, flexible structure of these feathers is more analogous to modern ornamental feathers than to flight feathers... If the entire tail bore plumage similar to that trapped in DIP-V-15103, the feather bearer would likely have been incapable of flight."

Bristol University paleobiologist Jacob Vinther told NPR's Rae Ellen Bichell that this structure also meant that colorful, iridescent feathers may have evolved before ones capable of flight. "I think the fact that the finest branches, which could have harbored this bright iridescence, got established before we got very robust feathers—that could potentially lean toward this idea that feathers were mainly used to show off before they got used to fly with," Vinther said. "Perhaps a greater number of dinosaurs, and more primitive dinosaurs, could have been iridescent."

So at least in feathered dinosaurs, beauty came before flight on evolution's road.

Current Biology, 2016. DOI: 10.1016/j.cub.2016.10.008

Listing image by Royal Saskatchewan Museum (RSM/ R.C. McKellar)