EVOLUTION%20OF%20THE%20HORSE:%20A%20LOOK%20AT%20TEETH

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EVOLUTION OF THE HORSE A LOOK AT TEETH

• Kaelin Creange

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The Question at Hand

• How have horses teeth adapted to their more
  evolutionarily recent niche of grazers versus
  browsers?

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The Modern Equine Skull

• Long face to accommodate large crown reserves of
  grinding teeth, and set of cropping teeth in the
  front
• Deep mandible, allowing for large masseter
  muscles that enable grinding type mastication.

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Tooth Structure

• Horse teeth have evolved hypsodonty, which means
  they have a high crown.
• This allows them more tooth to wear down, thus
  being able to utilize a tougher food source
• Presence of cementum increases the hardness of
  the tooth.
• Also creates ridges on the occlusal surface of
  the tooth, which aids in more effective grinding
• Born with full set of fully developed adult teeth
• Milk teeth precede the eruption of the adult
  teeth, which will continuously erupt throughout
  their lifetime.

crown is described as the enamel covered area
above the gum line
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The Mouth
INCISORS Center Lateral Corner WOLF TOOTH
(premolar 1) MOLARS (3) PREMOLARS
(3) CANINES (premolars and molars are
virtually the same size and shape)
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Mastication

• Grinding, circular motion
• Aids in eating grasses, which are an abrasive,
  fibrous food source
• Condemns the teeth to a great deal of wear (avg
  2-3mm/yr)

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Looking at the Evolution

• Early horses (Eohippus, Orohippus, Mesohippus)
• Short crowned
• No cement on the outside of tooth
• Premolars smaller than molars (4 premolars, 3
  molars)
• Later horses (Merychippus, Pliohippus, Equus)
• High crowned (hypsodonty)
• Teeth covered in cement
• Premolars resemble molars (except pm1 very small)

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A. Example of a brachydont (human)B. Examle of a
hypsodont (horse)
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Caroline A. E. Stromberg

• Examined the hypothesis that hypsodonty in equids
  evolved as an adaptation to the emergence of
  grass-dominated habitats
• Compared the timing of the emergence of the taxa
  first displaying hypsodonty (along with sister
  taxa having mesodont and brachydont teeth) with
  the emergence of grass-dominated habitats in the
  Great Plains of North America.

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Crown Height in Relation to Occurrence of Taxa
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Which phylogenic tree?
Black bar trait of hypsodonty Grey bar
appearance of the demand for feeding on grass
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Strombergs Results

• Hypsodonty fully present in members of Equinae
  (18mya)
• Equinae emerged at least 4my after the earliest
  open, grass-dominated habitats.
• With this 4 million year gap, does this result
  support hypsodonty as an adaptation to increasing
  dominance of open grasslands?

?
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Strombergs Results cnt.

• Parahippus possible intermediate taxa between
  brachydont and hypsodont taxa?
• Were mixed feeders (grazed and browsed)
• Increased crown height (mesodonts)
• Emerged almost same time as open-grasslands
• Species within taxa showed a thicker, and
  (modestly) modified radial enamel that are
  similar to some characteristics of the hypsodont
  horses of Equinae
• This suggests that small changes were made over
  much longer time periods than initially thought
• supports ?

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Future Research

• How genetically similar are Parahippus and
  Equinae taxa?
• This could shed light on if parahippus is in fact
  very closely related to equinae, or if there is
  another intermediate taxa between them.

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REFERENCES

• Cerling, T.E., Harris, J.M., MacFadden, B.J., and
  Prado, J. 1999. Ancient latitudinal gradients
  of C3/C4 grasses interpreted from stable
  isotopes of new world pleistocene horse (Equus)
  teeth. Global Ecology and Biogeography 8
  137-149.
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• Dacre, I.T. 2006. Physiology of mastication.
  American Association of Equine Practitioners
  (reproduced in the IVIS website).
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• Kainer, R.A., McCracken, T.O. 1998. Horse
  anatomy, a coloring atlas. Loveland, CO Alpine
  Publications, Inc.
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• Matthew, W.D. 1926. The evolution of the horse
  a record and its interpretation. The Quarterly
  Review of Biology 1 139-185.
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• McKenna, M.C. 1993. Review the horse tree.
  Science 260 1156-1157.
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• Strömberg, C.A.E. 2006. Evolution of hypsodonty
  in equids testing a hypothesis of adaptation.
  Paleobiology 32 236-258.