1. EVOLUTION OF THE HORSE: A LOOK AT TEETH
Kaelin Creange

2. The Question at Hand
How have horses’ teeth adapted to their more evolutionarily recent niche of grazers versus browsers?

3. 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.

4. “crown” is described as the enamel covered area above the gum line
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

5. The Mouth INCISORS Center Lateral Corner WOLF TOOTH (premolar 1)
MOLARS
(3)
PREMOLARS
CANINES
(premolars and molars are virtually the same size and shape)

6. 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)

7. 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)

8. A. Example of a brachydont (human) B. Examle of a hypsodont (horse)

9. 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.

10. Crown Height in Relation to Occurrence of Taxa

11. Which phylogenic tree? Black bar = trait of hypsodonty
Grey bar = appearance of the demand for feeding on grass

12. Stromberg’s 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? ?

13. Stromberg’s 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 ?

15. 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.

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