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A bizarre dinosaur informs feather evolution

I have seen numerous feathered dinosaur fossils over the last two decades, but I was still shocked by the beautiful feather preservation of the Caihong juji type specimen when I first saw it in the Liaoning Paleontological Museum in Shenyang, China.

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Feb 02, 2018
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The paper in Nature Communications is here: http://go.nature.com/2DWAg4B

I have seen numerous feathered dinosaur fossils over the last two decades, but I was still shocked by the beautiful feather preservation of the Caihong juji type specimen when I first saw it in the Liaoning Paleontological Museum in Shenyang, China.

Feathers of various types cover the whole body of this pheasant-sized dinosaur, and at the first glance, this dinosaur appears to have nearly identical plumage to Anchiornis huxleyi, a similarly-sized dinosaur we reported a few years ago. For example, feathers cover the whole leg, even all of the toes, a feature rare in modern birds. However, close examination reveals a striking difference: Caihong juji has asymmetrical tail feathers i.e. with vanes of different widths on either side of the feather shaft, unlike the symmetrical ones in Anchiornis huxleyi.

Asymmetry is an important modification in feather evolution partly because its appearance has been suggested to be an indicator for the appearance of flight capability, though some flightless birds still have asymmetrical feathers. Prior to the discovery of Caihong juji, the earliest known dinosaur with asymmetrical feathers was Archaeopteryx, which lived about 145 million years ago. Thus, our new finding extends the fossil record of asymmetrical feathers by about 15 million years.

Does the presence of asymmetrical feathers suggest that Caihong juji had some flight capability? Caihong juji has proportionally short arms, which is against such a suggestion. So, the new discovery makes the situation more confusing: Caihong juji has asymmetrical feathers but relatively short arms, whilst its contemporary Anchiornis huxleyi has long arms but symmetrical feathers. One more bizarre feature of Caihong juji is that its tail feathers are asymmetrical but its wing feathers are not, while normally among dinosaurs including modern birds the asymmetry is similarly developed in the wing feathers and tail feathers. This suggests that flight control might have evolved first with tail feathers during the development of aerial locomotion.

Alternatively, the presence of asymmetry in Caihong juji is related to display or communication rather than flight, though personally i think it is less likely. However, recent evidence does suggest that signaling plays an important role in feather evolution. For example, Epidexipteryx hui, a bizarre dinosaur we reported a few years ago, possesses highly specialized ribbon-like tail feathers, which probably functioned as display structures. Although the signaling hypothesis for asymmetrical feathers is hard to test, there is micro-structural evidence from Caihong’s feathers supporting the signaling inference.

Using a scanning electronic microscope or SEM as well as other methods, we were able to observe fossil melanosomes — microscopic organelles containing pigments which are responsible for the colour of our skin and eyes — in Caihong’s feathers. Comparisons with melanosomes found in living birds revealed that they most closely resembled those in the iridescent, rainbow-colored feathers of hummingbirds. Iridescent coloration is normally linked to sexual selection and signaling, and therefore the new discovery provides further evidence for an important role of signaling in early feather evolution.

Figure 1. Photograph of Drs. Hu Dongyu and Xu Xing working at one of the feathered dinosaur fossil sites in northeastern China. The yellow-color rocks are rich in volcanic ashes, which are at least partly responsible for the preservation of feathers and other soft tissues in dinosaur fossils.
Figure 2 Photograph of the Caihong juji fossil. The dark matters surrounding the dinosaur skeleton represent feathers of various types.
Figure 3 Artist’s rendering of Caihong juji. The color reconstruction is based on our analysis of both fossil and living melanosomes. Illustration by Zhao Chuang.

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