November 2, 2017
By Sven Vogel
Paleontologists studying the skull of a 74-million-year-old tyrannosaur fossil colloquially called the Bisti Beast from northwest New Mexico faced a dilemma. How could they peer inside the skull to determine the size of this bone-crusher’s brain and the layout of other features – details that would flesh out the dinosaur’s place in the evolutionary line culminating in the fearsome Tyrannosaurus rex – without damaging this rare, stunning and toothy treasure?
The New Mexico Museum of Natural History and Science had excavated the Bisti Beast, or Bistahieversor sealeyi (“Sealey’s Destroyer of the Bistis”), in 1998 in the Bisti/De-Na-Zin Wilderness Area near Farmington. The magnificent fossil is a show-stopper, but its internal structure had remained a stubborn mystery. To examine the innards of this 40-inch chunk of solid rock the size of a horse’s skull, you can’t simply a plunk it down on the local imaging clinic’s X-ray machine. The skull is way too big to fit in the viewing area and regular X-rays are too weak to penetrate the dense fossil stone.
A solution appeared from 90 miles up the road at Los Alamos National Laboratory. Our microtron accelerator creates very high-energy X-rays whose penetrating power far exceeds what you’d find at your dentist’s office. Beyond that, the Los Alamos Neutron Science Center (LANSCE) – one of the country’s most powerful particle accelerators – creates a beam of high-energy neutrons, particles from an atom’s nucleus. Most people are familiar with the medical applications of X-ray images, but neutron scans are more exotic. Neutrons lack an electric charge and don’t interact with the electrons of an atom the way X-rays do. Instead, neutrons are scattered by the atom’s nuclei, which makes them more penetrating and gives different information about the sample being scanned.
The lab is one of just a few places in the world that can perform neutron computer tomography (CT) scanning and one of only two that has the ability to do high-energy (or “fast”) neutron CT, combine it with high-energy X-ray scans, and do both on samples ranging from small to large. Combining neutron CT – 3-D imaging – with X-ray CT yields unique insights into dense objects, more than can be gleaned by either method alone.
With these techniques, we create 3-D images and animations of various materials and components, inside and out. Typically, scanning work supports the Lab’s primary mission of ensuring the safety, security and reliability of the nation’s nuclear stockpile, for which the medical X-ray variety also does not work, but as a user facility with unique imaging capability, LANSCE is also available to outside researchers for a variety of projects.
With all these world-class scanners, why not aim them at a skull to illuminate the rocky innards of one of New Mexico’s iconic fossils while developing a new method to characterize objects bigger than what fits on a single picture/radiograph? The lab started a collaboration with Tom Williamson, paleontology curator of the New Mexico Museum of Natural History and Science. They carefully packed the skull inside a padded barrel and shipped it up to Los Alamos.
Right away, we had a size problem they were hoping for. The skull was so big they had to figure out new ways of positioning it in the field of view at both the X-ray and neutron beam facilities. As it turned out, though, they would immediately put these new techniques to use for the lab’s mission work.
For the paleontologists, the results were worth the effort, too. We produced the highest-resolution scan of a tyrannosaur skull ever done. The X-ray images show un-erupted teeth, the brain cavity, the internal structure in some bones, sinus cavities, pathways of some nerves and blood vessels, and other anatomical structures. The neutron images are still being processed, but preliminary work shows details that can’t be seen in the X-rays, validating this dual method of scanning fossils. In the future, methods need to be developed that merge the neutron and X-ray data sets together rather than viewing them separately, hopefully unraveling details that neither of the two shows. Engineers and mathematicians who are specialists in computer tomography already were excited to hear about this data set and are eager to get to work – once again expanding Los Alamos’ unique capabilities.
Our study illuminates the Bisti Beast’s place in tyrannosaur ancestry and adds important new pieces to the puzzle of how the bone-crushing top predators evolved over millions of years before reaching their food-chain apogee in the T. rex. At the same time, the work advanced the state of the art in imaging capabilities at the laboratory on stockpile stewardship projects for the lab’s primary mission, which makes the world a safer place. We’ve all come a long way.
Sven Vogel is a technical staff member in Los Alamos National Laboratory’s Materials Science and Technology Division and works in material characterization with neutrons at the Los Alamos Neutron Science Center.