The California Collection/TheCaliforniaCollection.cdr/his065/fostooth.arj/FOSTOOTH.DOC

Text File | 1991-01-13 | 14KB | 256 lines

   AN INFORMAL REPORT ON A SCANNING ELECTRON MICROSCOPE STUDY
   OF A HUMAN-LIKE INCISOR FROM CRETACEOUS STRATA IN GLEN ROSE

                     David N. Menton, Ph.D.
                  St.Louis, MO  September 1987


      A  recently  discovered glossy black fossil  tooth  in  the
cretaceous strata near Glen Rose Texas called FSCM bears striking
resemblance  to a human central maxillary incisor.   If this were
found to actually be a correct identification for this tooth,  it
would  be  difficult  to  reconcile  with  current   evolutionary
scenarios   for  hominid  evolution.    Paleontologists  at   the
Smithsonian  Institute were understandably critical of the  human
status  of the fossil tooth and concluded after examination  that
it  was  a  tooth  from a class of fossil ganoid  fish  known  as
pycnodonts,  possibly Sargodon.   Several practicing dentists, on
the other hand, have concluded on the basis of a gross anatomical
and  stereo microscopical examination of the fossil that it  does
indeed bear resemblance to a cross fractured anatomical crown  of
a human deciduous central incisor.

     The  fossil  was  loaned  to  me  for  three  weeks  for  my
evaluation  by its discoverer Dr.  Carl Baugh of Glen Rose Texas.
I am a histologist and not qualified to identify the species of a
tooth on the basis of its gross morphology.   I understand that I
was approached by Dr.  Baugh because he believed that I would  be
unbiased  in  seeking an identification of the  tooth.   I  first
showed  the tooth to a dental histologist who concurred with  the
opinion  of  other  dentists that the tooth grossly  resembles  a
human  deciduous  maxillary  incisor  though  he  felt  that  the
curvature  of  the cingulum was unusually prominent  compared  to
that of most human teeth.   I then showed the tooth to Dr.  Frank
Winter a physiologist and paleontologist at Washington University
School of Dental Medicine who in addition to his familiarity with
human  teeth  has  a particular interest  in  fossil  fish  teeth
(Bradyodonts).  Dr. Winter was given no advance information about
the tooth other than that it was believed to be a fossil.  Winter
noticed  the  black  translucent color and  architecture  of  the
"enamel" and suggested that it was probably a modified dentine or
durodentine  of a type found on the surface of the crown of  fish
teeth.   He  concluded  that  the  morphology of  the  tooth  was
consistent  with the marginal incisorform teeth of  certain  fish
(both  living  and  fossil) which are  specialized  for  crushing
shelled  invertebrates or coral but suggested no specific  taxon.
According  to  Winter,  most paleoichtheologists  largely  ignore
teeth  in  their  descriptions and classification of  fish  which
might  make  classification of FSCM difficult.   Winter  made  no
comment  regarding the specimens human appearance until  I  asked
whether  it  might possibly be human.   He replied that this  was
very unlikely but that if a ground section of the tooth could  be
examined  in the polarization microscope its relationship to fish
or man could be more accurately evaluated.   This would obviously
be a destructive procedure and as of now, permission has not been
granted to do this.

     I  was  given  permission  by  Dr.  Baugh  to  engage  in  a
relatively noninvasive examination of the surface microtopography
of his fossil in an effort to ascertain whether it is  consistent
with  a  human tooth.   My specific interest was to  examine  the
tooth  in the scanning electron microscope (SEM) for the presence
of  enamel prisms which are found in nearly all mammals  and  are
relatively  distinctive  for  a group  including  man,  primates,
carnivores and some rodents.  I felt that the presence or absence
of enamel prisms in the fossil could at least confirm whether  it
is  likely to be mammalian or not.   Since the enamel of teeth is
easily the hardest biological tissue known,  it seemed reasonable
to  expect  that  it  would be  preserved  in  the  fossil.   The
stability  of  enamel prisms has been confirmed in  fossil  teeth
from a wide range of putative ages.

     Closely packed "key hole" shaped enamel prisms (as they  are
viewed  on  end at the tooth crown surface) are one of  the  most
distinctive  features of mammalian teeth and are found in only  a
few  other vertebrates including a few reptiles (both living  and
fossil  but  interestingly,  not in the "mammal-like"  reptiles).
Enamel  prisms  are comprised of  many  organized  hydroxyapatite
crystals.    Each  prism  measures  approximately  5  microns  in
diameter  and  extends in length through the entire thickness  of
the enamel following a rather sinuous course.   In other  classes
of  vertebrates  such  as  bony fish,  the  enamel  or  enameloid
material,   where  present,   is  not  prismatic  but  rather  is
comprised  of  large hydroxyapatite crystals that  are  uniformly
arranged near the surface of the tooth and are often more  random
in architecture in the deeper tissue.   Pycnodont teeth are known
to  lack enamel on or near their incisal surface and instead have
a  superficial  layer of modified dentine (Peyer 1968)  which  is
both  apatitic  and  fibrous and  thus  should  be  distinctively
different in its appearance than enamel.

     Since  FSCM  is said to closely resemble a  human  deciduous
maxillary  central  incisor  by  dentists  and  to  resemble  the
incisorform   marginal  teeth  of  a  fish  like  a  Sargodon  by
paleontologists,  the  purpose of this study was to  compare  the
fine  structure of the superficial hard tissue of FSCM with  that
of  an appropriate tooth from both man and fish.   An incisorform
tooth  from  a sheepshead fish of the genus Sargus was  used  for
comparison  as no fossil tooth of Sargodon was  available.   This
seemed  useful,  none the less,  as Sargodon was named after  the
living  genus Sargus because of the similarity of both the  gross
and histological structure of its teeth.   No attempt was made to
actually classify the taxon of FSCM but merely to see whether the
enamel of FSCM is more like that of Homo or Sargus.


Preparation of tooth specimens for scanning electron microscopy:

     The specimens examined in this study consisted of the fossil
tooth  FSCM found in cretaceous strata near Glen  Rose  Texas,  a
modern  deciduous human maxillary central incisor (not a  fossil)
and  an incisorform fish tooth from the living genus Sargus known
as  a  salt water sheepshead (courtesy of  Dr.  Frank  Winter  of
Washington University School of Dental Medicine).   All specimens
were  etched  in  2% hydrochloric acid for 2 minutes  except  the
Sargus tooth was etched for 1 minute.   The tooth fossil was also
scrubbed  on its incisal surface with a cotton applicator  dipped
in  toluene  prior  to etching in an effort  to  remove  a  heavy
acrylic  coating and was lightly ground on a small portion of its
incisal  surface with a fine carborundum  cloth.   All  specimens
were  then  glued  to specimen studs and stored in  a  desicator.
Prior  to examination in the scanning electron  microscope  (SEM)
each  specimen was lightly ion etched for 30 seconds at 5 amps in
a  Denton  Desk-1  triode  sputter unit  to  remove  any  surface
volatiles  followed  by  a sputter coating  of  approximately  30
angstroms  of gold to produce an electrical  conductive  surface.
The  specimens  were  examined  in  a Philips  501  SEM  and  all
photographs were taken on Polaroid type 52 directpositive film at
a magnification of 1,250 X.   All specimens were photographed  on
or  near the incisal surface at an angle of about 40{o} as viewed
from the medial surface of the tooth,  the labial surface of  the
tooth  is in each case to the left of the photographed field  and
the lingual surface to the right.

Observations:

Figure 1250/10   Human maxillary central incisor (deciduous)-
The  incisal  surface  seen here reveals a mosaic field  of  "key
hole"  shaped  enamel  prisms measuring about  5  micrometers  in
diameter  (note  10 micrometer bars at  bottom  of  photo).   The
prisms  are  separated  by a more  deeply  etched  interprismatic
region.   The  prisms  have the staggered arrangement typical  of
human  teeth  near the enamel surface.   The  enamel  prisms  are
comprised  of  hydroxyapatite  crystals which  measure  about  40
nanometers  in  width and over 150 nanometers in length  and  are
below the resolution of these micrographs.   X 1,250,   Bars = 10
micrometers

Figure 1251/10    Fossil tooth FSCM -
No  trace  of a mosaic appearance or enamel prisms are  found  in
this  specimen.   The tooth appears to be comprised of randomized
apatitic  crystals  and  fibrous  elements,   possibly   collagen
fibrils.   This  is  consistent  with the modified  dentine  that
covers  much  of the crown of pycnodonts  such  as  Sargodon.   X
1,250,  Bars = 10 micrometers

Figure 1252/10    Fossil tooth FSCM -
This  is  from an area that was lightly ground  with  carborundum
cloth.   The architecture here is not greatly different from that
of  the  previous  photo.   Neither  of these  photos  bears  any
resemblance to that of the human enamel in figure 1250/10  above.
X 1,250,  Bars = 10 micrometers

Figure 1259/10   Tooth from sheepshead,  Sargus (species unknown)
The  woven and apparently fibrous nature of this modified dentine
is  similar  to that of the fossil tooth  FSCM  (though  somewhat
coarser  in appearance) but quite unlike that of the human tooth.
This  field  is  on the labial surface adjacent  to  the  incisal
surface.  X 1,250,  Bars = 10 micrometers


Conclusions:

     The lack of enamel prisms in FSCM does not support the human
status  of this fossil tooth but neither does it give us a  clear
indication  of its specific taxon.   It is highly  unlikely  that
FSCM  had  a  prismatic  enamel architecture in  life  which  was
subsequently  lost  by the fossilization process in view  of  the
hardness  of  enamel crystals and the excellent  preservation  of
detailed  morphology in the fossil specimen.   Gantt  (1982)  has
described   enamel  as  the  "only  tissue  which  is   virtually
fossilized  before  death."   The  architecture  of  the  incisal
surface  hard tissue of FSCM has a woven fibrous appearance  like
that  of  Sargus but the latter is of a  coarser  structure.   It
should  be  emphasized  that whereas enamel  is  almost  entirely
mineral in structure, dentine has a significant organic component
much  of  which  is the fibrous protein  collagen.   The  fibrous
component of FSCM then is entirely incompatible with true  enamel
quite apart from its lack of enamel prisms.

     While  these  observations seem to exclude  the  possibility
that  FSCM is human they do not necessarily allow us to  conclude
with  complete  confidence that it is a pycnodont  fish.   Still,
other than among certain fish,  there are no other teeth found in
nature  that  are  so similar in gross morphology  to  the  human
incisor  and yet have modified dentine on their incisal  surface.
The  "enamel"  (modified dentine?) of FSCM  while  not  virtually
identical  to  Sargus is similar in several important   respects.
It  would,  of  course,  be  interesting to examine  one  of  the
existing fossil marginal teeth of Sargodon itself in the SEM  but
to  my  knowledge  this  has never  been  done.   The  cretaceous
formation  in  GlenRose is believed to be marine and  this  would
further support the possibility of this specimen being the  tooth
of a ganoid fish.  Moreover, if FSCM is a fish, there should be a
high  probability of finding more specimens which would permit  a
more destructive analysis of its chemistry and microarchitecture.
Human deciduous teeth, on the other hand, are literally temporary
teeth as anyone knows who has tried to save one for any number of
years, and thus are rarely found as fossils.

     Finally,  the  marginal  (incisor-like tooth) of the  fossil
fish  Sargodon  tomicus  has  been  studied  in  some  detail  by
Plieninger  (see Payer 1937).   The marginal tooth shown in  text
figure 25 of this book bears a striking similarity to FSCM.  More
importantly,  Plieninger's description (in German) provides  some
very interesting observations.  Plieninger describes the tooth as
being comprised of two distinct parts,  a mat surfaced base and a
glossy  black  (!) crown.   He commented that the crown is  often
fractured from the base in fossil specimens.  He even stated that
the  crown  of  the tooth has a strong similarity  to  the  upper
deciduous  incisor  of  man!   He also described  facets  on  the
lingual surface of the teeth near the incisal edge but these were
more punctuate in his specimen.

     I  must conclude that it is now inappropriate to claim  that
fossil FSCM is a human incisor.  It is an unusual and interesting
specimen  and  merits  further  study  as  to  the  organism   it
represents.

References cited:

Gantt, David G., 1982 Neogene Hominoid evolution A tooth's inside
view.   In:Teeth:  Form,  Function,  and Evolution,  Bjorn Kurten
editor,  Columbia University Press,  New York pp 95-107

Peyer,  Bernhard,  1937,  Die Triasfauna der Tessiner  Kalkalpen,
Kommissionsverlag von E. Birkhauser & Cie, Basel pp 29-33.

Peyer,  Bernhard, 1968, Comparative Odontology, The University of
Chicago Press, Chicago.


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