Feathered Dinosaur Tail Found Encased in Mid-Cretaceous Burmese Amber

An international team of paleontologists from China, Canada and the United Kingdom has discovered the tail of a non-avian theropod dinosaur (likely a coelurosaur) trapped in a piece of 99-million-year-old Burmese amber.

The feathered tail of a coelurosaur preserved in mid-Cretaceous amber from Kachin State, Myanmar. Image credit: Lida Xing et al, doi: 10.1016/j.cub.2016.10.008.

This remarkable specimen was bought at an amber market in Myitkyina, the capital of Kachin state in northern Myanmar, in 2015.

While this isn’t the first time feathers have been found in amber, earlier specimens have been difficult to link to their source animal.

“Amber pieces preserve tiny snapshots of ancient ecosystems, but they record microscopic details, three-dimensional arrangements, and labile tissues that are difficult to study in other settings,” said Dr. Ryan McKellar, adjunct professor at the Royal Saskatchewan Museum, Canada, and co-lead author of a paper describing the discovery in the journal Current Biology.

“This is a new source of information that is worth researching with intensity and protecting as a fossil resource.”

Photomicrographs and CT reconstructions of the specimen: (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. 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). Image credit: Lida Xing et al, doi: 10.1016/j.cub.2016.10.008.

“The new material preserves a tail consisting of 8 vertebrae from a juvenile. These are surrounded by feathers that are preserved in 3D and with microscopic detail,” Dr. McKellar added.

“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.”

“In other words, the feathers definitely are those of a dinosaur not a prehistoric bird.”

According to the team, the specimen represents the feathered tail of a juvenile coelurosaur.

This reconstruction depicts a small coelurosaur approaching a resin-coated branch on the forest floor. Image credit: Chung-tat Cheung.

While it was initially difficult to make out the details of the amber inclusion, Dr. McKellar and co-authors relied on CT scanning and microscopic observations to get a closer look.

“The tail is visible to the naked eye as an elongate and gently curved structure (length – 3.7 cm),” the researchers said.

“A dense covering of feathers protrudes from the tail, obscuring underlying details, so CT scanning was employed to examine concealed osteological and soft tissue features.”

The feathers suggest that the tail had a chestnut-brown upper surface and a pale or white underside.

The specimen also offers insight into feather evolution. The feathers lack a well-developed central shaft or rachis. Their structure suggests that the two finest tiers of branching in modern feathers, known as barbs and barbules, arose before a rachis formed.

Photomicrographs of coelurosaur feathers: (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). Image credit: Lida Xing et al, doi: 10.1016/j.cub.2016.10.008.

Dr. McKellar and his colleagues also examined the chemistry of the tail inclusion where it was exposed at the surface of the amber.

The analysis shows that the soft tissue layer around the bones retained traces of ferrous iron, a relic left over from hemoglobin that was also trapped in the sample.

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. Scale bars – 1 mm in (A) and (B). Image credit: Lida Xing et al, doi: 10.1016/j.cub.2016.10.008.

“Soft tissues — presumably muscles, ligaments, and skin — are visible sporadically through the plumage, clinging to the bones in a manner suggestive of the desiccation common to other vertebrate remains in amber,” the authors explained.

“These tissues have largely been reduced to a carbon film, retaining only traces of their original chemical composition.”

“Our calculations indicate that more than 80% of iron in the sample is ferrous (Fe²⁺). Similar measurements have been made on vessels preserved within Tyrannosaurus and Brachylophosaurus bones and have been interpreted as indicating the presence of goethite and biogenic iron oxides produced from hemoglobin decomposition.”

The researchers are now “eager to see how additional finds from this region will reshape our understanding of plumage and soft tissues in dinosaurs and other vertebrates.”

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Lida Xing et al. 2016. A Feathered Dinosaur Tail with Primitive Plumage Trapped in Mid-Cretaceous Amber. Current Biology 26: 1-9; doi: 10.1016/j.cub.2016.10.008