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A small coelurosaur approaching a resin-coated branch on the forest floor.

A small coelurosaur, shown here in this artist’s representation, approaching a resin-coated branch.

Chung-tat Cheung

Dinosaur tail trapped in amber sheds light on evolution of feathers

Amber is often prized not just for its golden beauty, but also for the tiny creatures it contains, many of them millions of years old. Now, a chunk of this fossilized tree sap found at a market in Myanmar has turned out to contain a very rare treasure indeed: a slender piece of feathered tail that belonged to a small bipedal dinosaur that lived about 99 million years ago.

“Since Jurassic Park came out, paleontologists have joked about finding dinosaurs in amber, since it would contain so much extra information. And now we have a piece of one,” says Thomas Holtz, a vertebrate paleontologist at the University of Maryland in College Park who was not involved in the study.

Researchers aren’t using ancient blood from the belly of preserved mosquitos to recreate dinosaurs, as in the movies. But the finding does reveal a feathered dinosaur tail in 3D for the first time, and offers a unique glimpse into the early evolution of feathers. Amber is a uniquely useful fossilizer, notes Michael Engel, a paleontologist and entomologist at the University of Kansas in Lawrence who was also not involved in the study. “It preserves things in lifelike fidelity.” Although it’s rare to find larger animals preserved in the sticky flow, researchers have found everything from frogs to lizards to ancient bird wings, likely entombed after death.

The amber deposits of northern Myanmar harbor one of the most diverse arrays of animals from the Cretaceous period. Paleontologist Lida Xing of China University of Geosciences in Beijing was hunting through an amber market in Myanmar for lizard and insect specimens when a particular chunk caught his eye: Along with the usual scattering of insects, it contained a 3.6-centimeter-long section of a flexible, finely feathered tail. Right away, he knew he had something special.

<p>A feathered dinosaur's tail, frozen in amber.</p>

A feathered dinosaur's tail, frozen in amber.

Royal Saskatchewan Museum (RSM/ R.C. McKellar)

Xing contacted paleontologist Ryan McKellar of the Royal Saskatchewan Museum in Regina, Canada, and the team used photographs taken through microscopes and computerized tomography scanning (computer-processed combinations of images taken by x-rays at different angles to reveal interior details of the fossil) to study the eight preserved vertebrae and their feathers.

Unlike Archaeopteryx (a 150-million-year-old creature thought by many researchers to be among the very earliest birds) or modern birds, the vertebrae were not fused into a solid rod at the tip of the tail. Instead, the tail in amber is whiplike and flexible, bending in several places at once. That, the researchers report online today in Current Biology, suggests that its owner was not a bird but in fact a dinosaur, and likely a member of a group of small two-legged dinosaurs called coelurosaurs. (Jurassic Park fans, take note: Compsognathus—nicknamed “compys” in the movies—are a member of this group.)

Plumage pigments preserved in the amber suggest the theropod was colored chestnut-brown along its dorsal side (the top of the tail), and lighter on its underside. The amber also allowed the researchers to study the structure of the animal’s plumage in 3D. Many well-known feathered dinosaur fossils—such as those of the “Jehol Biota,” a fossil deposit in northeastern China dating to about 130 million years ago—are “squashed flat, so that we have to deconstruct what the original shape of the feathers was,” Holtz says. “Here we can see them in the round, and this gives a better sense of some of the shapes.”

The feather of the bird you see out your window today has a central shaft, or “rachis,” that branches out into a series of barbs that branch again into fine barbules. In the new specimen, the rachis is relatively thin and flexible compared with the thick, rigid central rachis of modern birds; however, the structure of barbules is complex, with fine tiers of branching as in modern feathers, distributed evenly across the length of the feathers. In all, the structure of the feathers suggests that the animal wasn’t capable of flight, although “it may have been a glider,” McKellar says.

That combination of features—weak rachis and evenly spaced barbules—has not previously been directly observable in the flattened 2D fossils, Holtz says. He agrees that the bird likely couldn’t fly with this configuration—and notes that the discovery thus further reinforces the idea that feathers “evolved in a context other than flight,” such as for warmth or for mating.