Transcript EVOLUTION OF THE HORSE: A LOOK AT TEETH Kaelin Creange

EVOLUTION OF THE HORSE:
A LOOK AT TEETH
Kaelin Creange
The Question at Hand
• How have horses’ teeth adapted to their more
evolutionarily recent niche of grazers versus
browsers?
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.
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.
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)
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 23mm/yr)
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)
A. Example of a brachydont (human)
B. Examle of a hypsodont (horse)
Caroline A. E. Stromberg
• Examined the hypothesis that hypsodonty in equids
evolved as an adaptation to the emergence of grassdominated 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 grassdominated habitats in the Great Plains of North
America.
Crown Height in Relation to
Occurrence of Taxa
Which phylogenic tree?
Black bar = trait of hypsodonty
Grey bar = appearance of the demand for feeding on grass
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?
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 ?
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.
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.
•
Dacre, I.T. 2006. Physiology of mastication. American Association of Equine
Practitioners
(reproduced in the IVIS website).
•
Kainer, R.A., McCracken, T.O. 1998. Horse anatomy, a coloring atlas. Loveland,
CO: Alpine
Publications, Inc.
•
Matthew, W.D. 1926. The evolution of the horse: a record and its interpretation.
The Quarterly Review
of Biology 1: 139-185.
•
McKenna, M.C. 1993. Review: the horse tree. Science 260: 1156-1157.
•
Strömberg, C.A.E. 2006. Evolution of hypsodonty in equids: testing a hypothesis
of adaptation.
Paleobiology 32: 236-258.