Feathered dinosaur tail in amber!

For some time it’s been known (or at least mostly accepted, aside from a few outsiders) that birds evolved from theropod dinosaurs. This is known thanks to a lot of anatomical features, and a good transitional sequence leading to avians. However, for a long time it wasn’t really known how feathers fit into this. Since feathers are not boney, they rarely preserve. Sometimes you get amazing preservation of outlines in things like Archaeopteryx, but it’s only been within the last 20 years that we’ve started to find feathers on non-avian dinosaurs, starting with Sinosauropteryx in 1966.

Unfortunately, this has not been without controversy. Because of the preservation of these feathers, they often appear as a dark film around the dinosaurs. This has lead some to argue that rather than feathers, these are just collagen that has been degraded. Fortunately, amber has provided some examples of undeniable feathers from the Cretaceous of Canada, USA, and Myanmar, although it can be difficult to tell if these are bird or non-avian dinosaur feathers.

Today, an amazing new fossil was described from Myanmar that furthers this question. The fossil is a large piece of amber that comes from the mid-Cretaceous, and was found in an amber mine. It was actually discovered in a market where someone was selling the piece of amber in order to make jewellery, but one of the lead authors of the study, Lida Xing, saw it for sale and recognised it’s significance.

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Image of amber with theropod tail from Xing et al. 2016 with 3D reconstructions from CT scans.

The fossil consists of a tail of a non-avian dinosaur, a juvenile coelurosaur to be exact, with approximately 8 vertebrae and numerous feathers. The specimen was examined using a microscope, as well as CT scans and X-Ray. The morphology of the vertebrae and the tail as a whole suggest this was not from a bird, as birds did not possess a long, flexible tail with vertebrae of this shape. The structure of the feathers themselves are also consistent with non-avian coelurosaur dinosaurs since the feathers have only a weakly developed rachis (central shaft) and weakly developed barbs.

The feathers themselves also show evidence of countershading. The animal seemed to have darker coloured feathers on the top side of the tail, while the underside was lighter or white. Although no colour-bearing structures (melanosomes) were found, the amber appears to have preserved some evidence showing that some feathers were darker than others.

What happened to the rest of the animal? It seems that the animal may have got itself trapped in resin while it was still alive, and likely died as a result of not being able to escape. The amber showed chemical traces of ferrous iron, suggesting that organic material was trapped early without disturbances, unlikely if the animal had died and been lying on the surface for a long period before being encased in resin.

This new find reveals the importance and potential of amber as a tool to understand the evolution of feathers, and for palaeontology in general. Ryan McKellar of the Royal Saskatchewan Museum and co-author on this study has spent a lot of time studying amber and recognising it’s importance:

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

Paper:

Xing, L, McKellar, R, et al. 2016. A feathered dinosaur tail with primitive plumage trapped in mid-Cretaceous amber. Current Biology In Press.

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Artists reconstruction of the juvenile maniraptoran by Cheung Chung.
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