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The Evolution of Horses

Omar Moreno DEM 1F

Horses evolved from when they first spawned in North America !" million years a#o$

There are specific traits that chan#ed over time% such as their s&ull si'e% their lim( si'e and their

teeth si'e$ These chan#es were due to the chan#e in their environment) from thic& forest durin#

the time ofHyracotheriumto #rassy plains durin# the time ofEquusin the present day$ The

chan#e in their environment also led to chan#es in the horses* diets% thus leadin# to dental

chan#es$ Additionally the chan#es in the horses* environment also led to chan#e in their lim(

si'e% as they needed to (e taller to see over the #rass% rather than shorter to avoid predators$

+eein# as they were (i##er% the horses #ave up their fle,i(ility for endurance$ -./0A% 21"3

The earliest horse%Hyracotherium% lived in thic& forest with su(tropical climate$ This is

evidenced (y the miniature si'e of their lim(s) they were the si'e of house cats$ Their small si'e

was advanta#eous (ecause it #ave them increased mo(ility and a#ility$ They could easily hide or

clim( trees to evade predators$Hyracotheriumwere also di#iti#rade% meanin# that only the

middle phalan#e touched the #round% and the other two (ein# raised$ This also contri(uted to

their mo(ility% as it allowed them to move in different directions$ 0oo&in# at the s&ull% their eye

soc&ets were disproportionately (i# and aimed forward$ This allowed them to have (etter

eyesi#ht in the dar&ness of the forest% and allow them to see and avoid o(stacles directly in front

of them when runnin# away from predators$ Their diet in the forest would have consisted of

fruits and other foods loaded with nutrients$ Thus% their 4aw is smaller% as they can #et what they

need from little food$

5efore the Miocene% the diet of horses consisted mainly of fruits% (erries% nuts and other

nutrient rich foods$ Once #rasslands (e#an to develop% however% horses of the time%

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Merychippus% had to adapt and consume #rass$ 6rass has very little nutrients% is hard to di#est%

and leads to tooth dama#e very 7uic&ly$ The s&ulls of the horses need to (e (i##er to consume

more #rass to satisfy the horses* needs) (i##er s&ulls are more efficient$ Additionally%

Merychippuswere lophodunts% meanin# their teeth were flat% the ideal shape for chewin# on

#rass$ -./0A% 21"3

Hyracotherium*s lim(s were smaller and more a#ile% (ein# di#iti#rade allowed them to

move in different directions at a faster rate% very useful in forests$ The difference in lim( (etween

HyracotheriumandMesohippusis not very pronounced% as they lived in similar environments$

0im( shape (e#ins to really chan#e in the Miocene withMerychippus% where the middle

phalan#e% not only is it the only one to touch the #round% (ut is copiously (i##er than the other

two$ As the si'e of the horse also increased% the phalan#es were more prone to in4ury from

evadin# predators$ 8ith EarlyEquus% the hoof appears and solves the pro(lem of preventin#

in4ury due to the increased si'e of the horse) EarlyEquuswas di#iti#rade$ 5y the time Early

E7uus emer#ed% the forest was completely replaced (y #rassland% thus the EarlyEquuswas

definitely taller than the other horses that came (efore it$ This allowed it to see over the tall #rass

to spot predators very easily$ The modernEquushas very similar lim(s$ -./0A% 21"3

The transition fromHyracotheriumtoEquusled to increased (ody si'e and led to horses

#oin# from di#iti#rade to un#uli#rade$ 8ith this% the horse #ave up a#ility for endurance% as they

have trou(le chan#in# direction% (ut can move forward at hi#h speeds for lon#er periods of time$

Additionally% the hoof protects the phalan#es from dama#e due to prolon#ed use$ -./0A% 21"3

8ith this% they are a(le to outrun predators$

Horses* evolution% favorin# traits that lead to survival% such as tooth and lim( si'e% was

spurred (y the chan#e in environment and led to a trade!off (etween a#ility and endurance$

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9eferences

0ife +cience /ore% ./0A$ 22:!1"$Life Science 1 Evolution Ecology and BiodiversityDemonstration Manual$ Edition 12$1