Laser-Stimulated Fluorescence in Paleontology

On Wednesday, October 7 at 1 pm ET, PLOS Paleo hosted our second redditscience ‘Ask Me Anything’ on  laser-stimulated fluorescence (LSF) in paleontology. [Here’s a link to the completed AMA.]

Our featured paleontologist for this AMA was PLOS author Thomas Kaye, a Research Associate at the Burke Museum in Seattle, Washington. Kaye and co-authors (Amanda R. Falk, Michael Pittman, Paul C. Sereno, the late Larry D. Martin, David A. Burnham, Enpu Gong, Xing Xu, and Yinan Wang) have developed a pretty impressive, non-invasive technique to examine fossils in ways we might not have really considered, using equipment that is readily accessible to any paleontologist. Let’s discuss…

For decades paleontologists have used and manipulated different modes of lighting to examine fossils. Something as simple as moving a flexible arm lamp so that it casts light across a specimen laterally rather that directly down upon it can make surface features of a fossil “pop” — and pretty common when, like me, you work on flattened specimens like fish fossils and are desperate for some surface topography. Paleontologists also use fluorescence to examine specimens by casting UV light upon a fossil, causing some minerals, such as hydroxyapatite or fluorite, to absorb UV wavelengths and emit light in a different wavelength. This produces those lovely bright colors that make the fluorescing fossil stand out from the darker rock matrix, and thus allowing the scientist to recognize morphological characters that are otherwise unseen in normal light with little contrast against the matrix.

Still these traditional methods are limited for some specimens, particularly if you have fossils that don’t fluoresce under standard UV light or your are trying to examine soft-tissues or details that are fairly close in color to the matrix or obscured by matrix. Kaye et al.’s paper describes a next-generation technique using stronger laser tools to look for characteristics in a fossils that would have otherwise remained unseen using older techniques. As for the specifics of LSF, I am not an expert and I urge you to read the paper yourself, but let’s talk about some of the great case studies that this paper provided, giving paleontologists and paleoanthropologists all kinds ideas for using LSF.

I spoke with one of the paper’s co-authors, David Burnham, the Fossil Preparation Lab Manager at the University of Kansas (KU), and he gave me a little backstory about his involvement with LSF. He and Larry Martin, the late Curator of Vertebrate Paleontology at KU, had found a mystery fossil on a slab containing a Microraptor, but the visible bones were small and identification proved difficult.

Martin had known Kaye and the techniques he had been developing, so they sent the slab to Kaye to analyze, and using this newly-developed LSF technique, they were able to identify the specimen as a fish, and even were able to recognize teeth, scales, and other bones that would have remained invisible to Martin and Burnham without the use of lasers.

Likewise, Amanda Falk, a Visiting Assistant Professor of Biology at Centre College in Danville, Kentucky and co-author on the paper, needed a better way to decipher primitive bird plumage on a fossil and began working with Kaye. Other techniques, such as scanning electron microscopy (SEM) really only detail surface topography, and to better see the barbs and barbules of the feather, Kaye and Falk use LSF in a different way by shining the laser on the fluorescent carbonaceous matrix, thus ‘backlighting’ the feathers.

“I think it [LSF] has excellent utility in a variety of lab situations,” Burnham said, “I have used the laser on rock samples as well with very good luck. The laser illuminated the volcanic constituents in the sediment really well so it was helpful to illustrate the amount of that material and how it was distributed in the rock.” LSF can also be used to sort microfossils, and this technique is laid out wonderfully in the paper. Kaye and co-authors were even able to automate the process with success.

LSF is not limited to geochemical and paleontological studies. Paleoanthropoligsts can take advantage of the LSF technique to examine sensitive specimens. The Kaye et al. paper outlines a case where a mid-Holocene female skeleton had a bracelet around her upper arm, but removing the bracelet was impossible without destruction to the skeleton.

A laser was scanned over the artifact, and revealed details that allowed the scientists to determine that the bracelet was made from a hippopotamus tusk.

Interested in this technique for your own specimens? Lucky for you, the equipment and lasers are relatively inexpensive and available. A cheap, everyday laser pointer won’t cut it, though. “We found that the power of the laser was important, so everyday laser pointers were not enough,” Burnham told me.

“I used [a] 400 mW laser pointer,” Falk added, “The trouble with the laser pointer is that it can only be used for about five minutes before you need to turn it off and cool it down. If you want continuous use of the laser then you need a lab-grade laser. We bought a 300 mw one, which [cost] about $500 and not that bad. These are really affordable…in short you can get a working [LSF] system in your lab for less than a thousand dollars. The more powerful the laser, the brighter the fluorescence. Just remember to buy eye protection, too!”

Burnham echoed his sentiment about lab safety, “…One should use caution as the lasers can damage your eyes, and correct protection (safety glasses matched to laser type) must be used. I think the only other drawback is getting the specimen in a room with no light.”

Want to talk to Kaye directly about the technique? Bring your questions to our Reddit AMA on Wednesday, Oct. 7, and 1 pm ET! Check the PLOS Paleo Twitter for updates and links to the conversation.