Imaging of the Cretaceous Shark

     Cretoxyrhina mantelli.  87 million years ago, this extinct
species of shark roamed the seas covering Kansas.  Modern research
into this species has been very limited.  Upon returning from a
paleontological convention, Kenshu Shimada, a senior geology
student at Fort Hays State University, contacted me about a
research project he planned to work on.  He had decided to research
Cretoxyrhina mantelli, and felt that radiographs of a specimen from
the Sternberg Museum on the campus of Fort Hays State University
would provide valuable insight into the anatomy of both this
species and other closely related sharks from the Cretaceous
period.  The radiographs would assist Shimada in locating
suspected, but covered, structures and help to identify certain
ambiguous structures present.  I agreed to help him with this
unusual project and arranged to use the radiology laboratory at
Fort Hays State University.
     The specimen consisted of three large plates and numerous
vertebrae.  One large plate represented the head portion containing
jaws and numerous teeth, while the other two plates contained
multiple vertebrae, some teeth, and other skeletal elements
embedded in the rock.  Because of the large, sharp teeth present,
paleontologists consider Cretoxyrhina mantelli to be a large
carnivorous shark, similar to the mako or great white shark present
in modern seas.  The results from this study will aid Shimada and
other colleagues to make a more definitive determination of the
classification, body structure, and other paleontological
considerations with respect to Cretoxyrhina mantelli in the future.
     The head region of the specimen was radiographed in the
ventrodorsal projection with the top of the head on the table. 
This projection was needed to prevent damage to the teeth of the
specimen.  It also placed the small teeth closer to the film for
better imaging.  The plate representing the head measured
approximately 55 cm (21.7 inches) from the tip to the base, and
approximately 48 cm (18.9 inches) from side to side.  It varied in
thickness from 2 cm (0.79 inches) at the tip to 5 cm (2 inches) at
the base of the head.  The techniques used were 1.6 mAs and 90 kV
for the upper jaw region, and 3.2 mAs and 90 kV for the thicker
lower jaw region.  The optimal visualization of the upper jaw
region was most critical to this study.
     Multiple small teeth appeared within the upper jaw region. 
These teeth were not visible on the specimen due to overlaying rock
material.  By examining these teeth, paleontologists can
reconstruct the dentition of the species and possibly determine the
lineage of the shark.  Shimada, using the radiographs for
localization purposes, plans to expose the buried teeth, allowing
the different teeth present to be counted and examined.  In the
lower jaw, numerous larger teeth covered by fossil elements
appeared on the radiographs.  These teeth will be counted but not
uncovered.
     Radiographs were taken of the two other large plates
containing numerous vertebrae and other items of interest.  Both
plates were approximately 5 cm (2 inches) in thickness; therefore
3.2 mAs and 90 kV was used to image them.  The plate nearest the
head region contained vertebrae and teeth, including one section of
fused vertebrae laying partially obliqued and partially covered
with sediments and fossil elements.  The other plate, laying
posterior to the previous two plates, contained mostly vertebrae. 
Radiographs provided a better view of the vertebrae and allowed an
accurate count to be made.
     Three vertebrae from varying regions were radiographed for
this study.  These vertebrae came fom another specimen of
Cretoxyrhina mantelli, since most of the vertebrae from the first
specimen were distorted, compressed, and fused together.  The three
vertebrae imaged had diameters of 7 cm, 6.5 cm, and 6 cm (2.8
inches, 2.6 inches, 2.4 inches), and corresponding thicknesses of
2.5 cm, 2 cm, and 1.5 cm (1.0 inches, 0.8 inches, 0.6 inches).  Two
sets of images were obtained, using 0.83 mAs and 90 kV on the
first, and 1.7 mAs and 78 kV for the second.
     Shark vertebrae are thin disk-like structures, with foramen
around the edges extending into the vertebrae.  When radiographed,
the neural canals within a vertebra create a low-density pattern.
Shimada will report the results from these radiographs, along with
other findings, in other scientific journals.  The patterns formed
by the neural canals can provide valuable assistance in classifying
the shark group the vertebra belongs to by ruling out a large
number of other sharks whose neural canal patterns are different. 
Related species of sharks tend to have similar patterns within
their vertebral structures, thus assisting paleontologists in
determining a shark's relationship to other sharks. 
     The radiographs taken of this specimen provide important
information regarding the species of this shark and its possible
lineage.  The small teeth visualized in the head region were the
most important findings of this exam.  A more accurate count of the
number of vertebrae and teeth present was also achieved.  Two
ambiguous structures on the second large plate were imaged with
inconclusive results.  The films of the individual vertebrae and
the neural canal patterns within will be useful for future
reference.  Studies such as this emphasize the advantage of
interdisciplinary work in the sciences as a whole.