Oddly enough, many fossils fluoresce under UV light (certain minerals in fossils, including phosphates, are behind this phenomenon). Thus, this technique has been used to look for otherwise hidden features of some exceptionally well-preserved fossils. Historically, it's been the domain of invertebrate paleontologists (looking at crustaceans from the Jurassic of Germany, for instance), but vertebrate paleontologists have used the technique to identify forged fossils (like Archaeoraptor), study Archaeopteryx, and much more. What might be a very subtle or invisible structure under regular light (such as a feather shaft, or antenna, or soft tissue outlines) sometimes shines nicely under UV light.
Thus, Beijing paleontologist Dave Hone and colleagues applied the UV light technique to some of the spectacular fossils coming out of the Cretaceous-aged beds of China. In particular, they were interested in a little critter called Microraptor. A dromaeosaur (part of the same group including Velociraptor), Microraptor is relatively well-known as the "four-winged dinosaur." Spectacular fossils with feather impressions show the standard pair of bird-like wings on the arms and a second set of wings on the hind limbs. This suggests to some researchers that birds went through a four-winged flight phase early in their evolution, and the two-winged flight with which we are familiar only happened later.
Cast of the type specimen of Microraptor gui, from the Wikimedia Commons, reproduced under a Creative Commons Attribution-Share Alike 2.5 Generic license.
Although the fossil looks spectacular, many paleontologists speculated that appearances might be deceiving. Were the feathers on the legs actually in place, near their life position? Or had they gotten moved around from somewhere else on the body? A pale halo of sediment (probably from the decomposition process) obscured the contact of the feathers with the bones, so the issue remained unresolved. Either way, it had major implications for avian evolution.
Hone and colleagues wondered if the full anatomy was obscured under visible light. So, they turned a UV light source against the specimen. It turns out that the feather structures fluoresce quite nicely - and can be traced right through the "halo" and up to the very edge of the leg bones. So, the feathers really are in place. Problem solved! [image, showing full skeleton, modified from Figure 2 in Hone et al. 2010]
Now that we're more confident that Microraptor really was four-winged (and not just an accident of fossilization), the conversation can move forward. And, this is a great rallying cry for other researchers - who knows what structures we might discover with UV light!
Close-up of hind legs of Microraptor under UV light, with arrows indicating feathers. The yellow stripes leading up to the leg bones are portions of the feathers visible only under UV. Modified from Figure 3 in Hone et al. 2010
Read the full paper in the freely-available, open access journal PLoS ONE (full disclosure: I was the editor who handled this manuscript). You can post comments or ratings for the article there, too! In the blogosphere, check out Dave Hone's posting on his article and this follow-up, Adam Yates' write-up, as well as ReBecca Hunt's interview with Dave.
CitationD. W. E. Hone, H. Tischlinger, X. Xu, & F. Zhang (2010) The extent of the preserved feathers on the four-winged dinosaur Microraptor gui under ultraviolet light PLoS ONE 5 (2) : 10.1371/journal.pone.0009223