
When many of us think of viewing things under a "
black light," we either think of those psychedelic posters from the 1960s or else the displays of fluorescent minerals that nearly every science museum has. It's also virtually mandatory to have a scene involving the use of "black light" in the popular CSI television programs - many bodily fluids show up nice and pretty under these conditions. "Black light," more properly known as "ultraviolet (UV) spectrum light", is just outside the visible light spectrum for us humans (past violet, hence the name). And, through some neat tricks of physics, many objects will brightly fluoresce under intense UV light when they wouldn't look like anything special under your standard sunlight or incandescent light bulb.
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.
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.
Citation
D. 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