Student Worksheet
LSM 11.6-4
Additional Case Study: Evolution of the Horse
Background Information
The horse has evolved from Hyracotherium, a small creature standing less than
0.4 m tall, to the modern-day horse, a much taller animal standing approximately
1.6 m tall. Examine the figure below, which shows the evolution of the horse.
(a) Draw a graph showing changes in the height of the horse over time.
Time frame
Diagram
Description of horse
Equus
The modern-day horse
•
•
•
•
This horse lives in the grassy plains area.
It eats grasses and is classified as a grazer.
It is approximately 1.6 m tall.
It has a single visible digit.
Pliohippus
1.6 million years ago
• This horse lived in areas with shrubs
and on the grassy plains.
• It ate grasses and is classified as a grazer.
• It was approximately 1.0 m tall.
• It had a single visible digit.
Merychippus
5 million years ago
• This horse lived in areas with shrubs and on the
grassy plains.
• It ate leafy vegetation and grasses and is classified
as a grazer.
• It was approximately 1.0 m tall.
• It had three visible digits.
Mesohippus
24 million years ago
• This horse lived in less thickly wooded areas.
• It ate leafy vegetation and is classified as a browser.
• It was approximately the size of a small dog, about
0.6 m tall.
• It had four visible digits.
Hyracotherium
37 million years ago
• This horse lived in thickly wooded areas.
• It ate leafy vegetation and is classified as a browser.
• It was approximately the size of a small dog, about
0.4 m tall.
• It had four visible digits.
(continued)
Copyright © 2003 Nelson
Chapter 11 Origins of Evolutionary Science 333
LSM 11.6-4
(b) Describe changes in leg length and the width of the digits.
(c) Do horses have thumbs and fingers?
Case Study Application Questions
1. Hyracotherium lived in a dense forest area dominated by a rich undergrowth
of ferns. What do you think the advantages of small body stature might be in
this type of environment?
2. A change in vegetation to grasslands brought about changes in the shape of
the horse’s leg. What changes in vegetation could have caused the evolution of
the horse?
3. The teeth of the browsers, such as Mesohippus, were covered with a thick layer
of enamel. The teeth of the modern-day horse, by comparison, have less
enamel and appear to be much wider and flatter. Provide reasons for some of
the changes in teeth.
4. Why would changes in the environment have caused changes in the horses’
predators?
334 Chapter 11 Origins of Evolutionary Science
Copyright © 2003 Nelson