Class notes Unit 3 - Understand Animal Anatomy and Physiology
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Evolution
Horse
Horses belong to the order of animals known as
Perissodactyla. Horses are known to have evolved
from creatures from around 50 million years ago.
The oldest horse ancestor known as the
Hyracotherium was known to be the size of a
creature we now recognise as a large dog such as a
‘Labrador’ at approx. 450mm tall and was found
within North America. This creature walked on four
legs, however had multi-toed feet (unlike horses
today) with only 4 digits on each foot rather than
the usual five digits found within a pentadactyl
limb. These multi-toed limbs had major bones that
were unfused, leaving them flexible and rotatable,
with the wrist and hock joints low to the ground.
This arrangement allowed them to run across the
open land with durability and stability, however,
did not allow for the speeds we see within horses
today at this current stage. It had a short head and
neck with a long skull, their eyes either side and
directly in the middle of the face. It also had a slightly arched, springy back. It had 44 low-
crowned teeth in the arrangement of an omnivorous mammal (however was mainly
herbivorous); three incisors, one canine, four premolars and three molars on each side of
the jaw, however, the characteristic horse-like ridges on the molars can be seen.
The next known ancestors both being discovered to have lived around 45-40 million years
ago, however, had no significant changes to the body. That is up until the Mesohippus was
introduced as a new species of horse ancestor whose bodies had changed to fit the climate
that had developed within North America. With the much drier climate and the earlier
grasses beginning to evolve, as well as some areas being covered in sand, the Mesohippus
was introduced around 35 million years ago and stood at around 610mm tall. It developed
tougher teeth and became slightly larger with longer legs allowing for increased running
speeds in open areas. The new and different textures upon the ground meant that the four
toes now turned to three with the second and fifth digit remaining but not being used
during walking. The fourth digit was diminished into a small bone at the side of the leg and
the third digit being stronger than the two other present digits. These changes allowed for a
more agile animal. It had a flatter back and a somewhat longer face, snout and neck. A slight
change within the skull was also developed, a slight depression within the skull called a
fossa was formed which can be seen as a much more detailed article in today’s horse
species. This specific species had developed six grinding teeth with a single premolar in
front, it also carried the sharp tooth crests of an ancestor, improving its ability to grind on
tough vegetation.
Up until the next notable adaptation, three new species were introduced, after these
species the Merychippus was introduced around 15 million years ago and measured around
1 meter tall. The two main adaptations within this species were the wider molar teeth,
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