Non-Invasive
Many of the tools and techniques used by today’s preparators have changed little since the origins of the field over 100 years ago.
However, in recent years a number of new technologies that have been applied to preparation are opening up exciting possibilities for paleontological research and preparation.
Three tools of particular utility are:
- High-resolution X-ray computed tomographic (HRXCT) scanning
- 3-D laser surface scanning
- 3-D printing
The use of these technologies is unlikely to replace traditional preparation methods anytime soon, but for some research projects and for certain specimens they can provide information that would be otherwise unobtainable.
High-resolution x-ray computed tomographic scanning (HRXCT)
HRXCT, also known as computed axial tomographic (CAT) scanning, is a non-destructive imaging technique in which the inside of an object can be visualized by capturing a series of two-dimensional X-ray images as the object rotates between an X-ray source and a detector. The resulting images, or ‘slices,’ are reassembled digitally to create three-dimensional renderings of the object.
While medical CT scanners typically employ X-ray energies lower than 140 kV, industrial scanners such as those housed at The University of Texas High-Resolution X-ray CT Facility are capable of generating X-ray energies more than three times as powerful (up to 450 kV or more). This permits penetration of denser materials, like meteorites and fossilized bone, than is possible with medical CT.
Extensive information about the development, application, and physics of HRXCT is available on the UTCT website.
Reasons to use HRXCT scanning
HRXCT scanning is not appropriate for all types of specimens. It is dependent on having contrast between matrix and bone – which varies greatly. However, in certain applications it can eliminate the need for traditional preparation techniques, thereby potentially saving time and money. Scanning can also aid in specimen conservation as it minimizes the need to handle specimens, both in preparation and in research.
Scanning before traditional preparation
Scanning an unprepared slab may allow researchers/preparators to determine the distribution of elements within that slab. A quick-and-dirty “scout” scan can easily produce this level of information at reasonable cost. This can be particularly useful if postcranial material is scattered within a block, allowing the preparator to locate elements without expending unnecessary time and energy.
Scanning to reduce or eliminate the need for traditional preparation
In some cases, the differentiation of matrix and bone and the anatomical detail seen in HRXCT data are so good that a researcher may be able to obtain the morphological and taphonomic information needed without any physical preparation of the specimen.
Scanning advantages over preparation
Where HRXCT offers a unique advantage is the ability to study internal structures without destructive preparation techniques. An example is the morphology of the braincase. This can sometimes be studied with a good cast, but this requires a degree of manipulation during the preparation and molding process and can result in damage to the specimen. Not only is the cranial cavity easily studied from HRXCT data, but its volume can be easily quantified. Another example of the advantage of non-destructively inspecting interior volumes is the examination of developmental morphology of embryos in intact fossil eggs. See an example on the digimorph.org website.
Selecting appropriate specimens
How well a fossil will scan depends on its geometry and its composition. HRXCT scanning geometry is cylindrical, so the more equant a specimen is in the scanning plane, the more scanning artifacts will be minimized. A slab’s geometry is, unfortunately, quite problematic: the X-rays must travel through much more material on one axis than on the other. The composition of the specimen will determine the degree of contrast between the bone and the matrix.
Generally, specimens in a clastic silt or sandstone matrix will scan more successfully than specimens preserved in limestone, because in the latter the calcium phosphate in bone has a similar effect on X-rays as the calcium carbonate in the limestone. Specimens rich in iron do not scan well, because the iron stops a lot of the X-rays. Two specimens from the same locality/formation can scan quite differently due to localized taphonomic regimes. Specimens from some formations, such as the White River, scan particularly well. Again, a quick “scout scan” will indicate whether scanning is a worthwhile investment.
For more information on HRXCT scanning:
- Visit UTCT Scanning Services FAQ page.
- To see examples of HRXCT imagery of fossil specimens visit the Digital Morphology library website.
- Click here to see a Quicktime HRXCT image of Interatheriidae (Notungulata, Mammalia). Upeo Fauna (middle Cenozoic, ?Oligocene), Abanico Formation, Andes Mountains of central Chile. Specimen number SGOPV 3774 (courtesy of vertebrate paleontology collections, Museo Nacional de Historia Natural, Santiago, Chile; scans courtesy of Center for Quantitative Imaging , Pennsylvania State University; animation copyright J. Flynn/AMNH).
- Clark, Sandy and Ian Morrison. 1994. CT scan of fossils. Vertebrate paleontological techniques Volume 1. Patrick Leiggi and Peter May eds. New York: Cambridge University Press.
- “Osteological Description of an Embryonic Skeleton of the Extinct Elephant Bird Aepyornis (Palaeognathae:Ratitae)”, by Amy M. Balanoff and Timothy Rowe, Journal of Vertebrate Paleontology 27:4, December 2007 pp. 1-53.
3-D laser surface scanning
Three dimensional laser surface scanning is another non-contact, non-destructive technology that can provide information that may be difficult to obtain using traditional preparation techniques. In 3-D laser scanning the specimen is placed on a digitizing platform (often a turntable) and an optically safe laser beam is projected over the surface while cameras record the measurements. The resulting data points are captured by a computer which can then be merged into a 3-D representation of the specimen. An increasingly popular alternative to laser-scanning is structured (or “white”) light scanning, which uses photogrammetry to produce similar surface representations.
3-D laser scanning is particularly useful when you are only interested in surface detail and when extremely accurate measurements of complicated shapes must be obtained (measurements are can be accurate to ±.0005). Based on a laser scan, missing portions of a bilaterally symmetrical specimen can be reproduced (e.g. creating a full skull when half remains in the matrix, is lost or distorted). And new software is allowing for “retro-deformation” where the scan can be manipulated to allow researchers to correct for deformities in a misshapen specimen.
3-D printing
In 3-D Printing, data files from either a 3-D surface scan or from a High Resolution X-Ray Computed Tomography scan are fed into a rapid-prototyping machine where a three-dimensional model is created, typically by building up layers of thermoplastic resin. These models can often be molded and cast repeatedly if necessary.
The ability to create molds from a 3-D printout means that preparation or modeling of the smallest or most delicate specimens may not be necessary. The technique is particularly useful for extremely small specimens, as the models can be easily enlarged to allow for easier viewing of the smallest morphological features. When the technique uses HRXCT files, internal anatomy can be illustrated by printing; for example, using just one half of a skull, or printing an endocast of the cranial cavity. Data files for the rapid-prototyping machine can be shared digitally allowing others with the technology to create their own cast models, thus increasing research access.
The thermoplastic resins used to create the printed model may not be stable in the long-term, so it is recommended that this original cast be modeled and re-cast in more archival media if longevity and multiple copies are necessary. At this point in the development of the technology the casts created are good, but some resolution is lost and, with the exception of small specimens, a 3-D printed model will not have quite as much detail as a well-made silicone mold.
For more information on 3-D Printing, viisit the University of Texas at Austin Digital Morphology library 3-D printing page.
