DIETARY RECONSTRUCTION OF FOSSIL TURTLES: MODELING BEAK MORPHOLOGY OF EXTANT TAXA USING A MEDICAL CT SCANNER

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DIETARY RECONSTRUCTION OF FOSSIL TURTLES
MODELING BEAK MORPHOLOGY OF EXTANT TAXA USING A
MEDICAL CT SCANNER Nicholas E. Smith Department
of Biological Sciences, Marshall University,
Huntington, WV 25755
Materials Methods
Abstract Cranial and dental morphology of mammals
serve as dietary predictors, and paleontologists
use comparative morphology to reconstruct the
diet of extinct species. In turtles, however,
phylogeny has traditionally been the basis for
dietary predictions. The goal of this study was
to apply functional morphological techniques from
mammalian studies to the beaks of turtles, and
assess the use of a medical CT scanner for this
application. By establishing dietary cranial
characters among extant feeding strategists, it
may then be possible to more accurately predict
what fossil taxa may have eaten. Turtles lack
teeth, and therefore their shearing surfaces are
formed via a keratinized beak, the rhamphotheca.
Unfortunately, these keratinous sheaths do not
fossilize, and the difficulty in this experiment
was matching structures seen on the rhamphotheca
to those of the maxillae and mandible. To assess
this fit, a Computed Tomography (CT) scanner was
used to generate models of preserved specimens,
which were imported into a 3-D quality control
program. The taxa examined are endemic to West
Virginia, and were chosen based upon feeding
strategies Apalone s. spinifera (carnivore)
Pseudemys c. concinna (herbivore) Chelydra s.
serpentina (omnivore) Graptemys geographica,
(molluscivore) Graptemys p. ouachitensis
(insectivore). One representative adult male was
digitized per group, using a GE Lightspeed CT
scanner at Cabell Huntington Hospital (CHH). The
skulls were scanned at a slice thickness of 0.49
mm using a bone algorithm, and the DICOM file was
imported into 3D-Doctor for image processing. In
this program, the rhamphotheca were isolated from
the underlying bone and soft tissue using
differences in pixel intensity.
Three-dimensional surface files were then
exported into Geomagic Qualify, where the
rhamphotheca was superimposed with the cranium,
and a 3-D comparison was made. Morphometric
analyses were performed in 3D-Doctor, and bone
densities were taken using Voxtool 3.0.04v at
CHH. Not only did CT scanning provide a means of
acquiring data inaccessible via other forms of
microscopy, collaboration with CHH and the
Marshall University School of Medicine afforded
an opportunity to examine the feeding apparatuses
of these taxa without dissecting the specimens.
It was determined that the resolution of the
medical CT scanner was insufficient for modeling
microstructures, however, the internal structures
and density measurements proved invaluable. Even
on this small sample, trends in cranial
morphology are apparent among the dietary groups,
and warrant further investigation. Coupled with
a higher resolution instrument, like a Micro-CT
scanner, this technique yields tremendous
possibilities for functional morphological
applications.

• Five museum specimens were selected and scanned
  based on feeding strategy
• Apalone s. spinifera (carnivore)
• Chelydra s. serpentina (omnivore)
• Pseudemys c. concinna (herbivore)
• Graptemys geographica (carnivore)
• Graptemys p. ouachitensis (carnivore)
• GE Lightspeed Pro 16 CT Scanner (Cabell
  Huntington Hospital MUSOM)
  -slice thickness 0.625 mm -voxel
  volume 0.218 mm3 standard algorithm
• Slice data rendered into 3D surface models using
  3D-Doctor
• STL file imported into Geomagic Studio for 3D
  comparison
• Linear and area measurements taken in ImageJ

• Conclusions
• Even with low resolution, CT permits cranial
  analyses previously unattainable
• Rhamphotheca can be isolated and compared to
  underlying bone
• Ossified beak is a cast of rhamphotheca with
  little variation
• Gross cranial morphology is indicative of
  feeding strategy
• Future work with a High-Resolution Micro-CT
  scanner appears promising

• Objectives
• Generate 3D models of feeding apparatuses from
  turtle specimens using a CT scanner
• Isolate rhamphotheca from underlying bone
• Because keratin does not readily fossilize, it
  is necessary to assess variation between
  rhamphotheca and ossified beak
• If underlying bone adequately represents the
  beak, it is then possible to test for
  form/function relationship between feeding
  strategies and bony morphology

• Acknowledgments
• Suzanne Strait, David Neff, Michael Norton
  advise and review
• Thomas Pauley loaning museum specimens
• Pamela Haun and Carolyn Holschuh (CHH) CT
  scanning assistance