Evolution of horses

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Questions (answer on a separate page: use complete sentences- TYPED + double-spaced)

  1. What changes occurred in the environments of horses from Hyracotherium to Equus?

  1. As their environments changed, what happened to the teeth of the horse? (Describe carefully. TEETH are not on the charts above; look at your graph)

  1. What changes (plural=more than 1) occurred in the size of the horse from Hyracotherium to Equus?

  1. Describe the overall changes in foot length, number of toes, and size of toes in the horse over time (that 3 different features).

  1. How might natural selection have caused changes in the size, feet, and teeth of the horse?

Part B: Comparison of Horse Fossils


Members of the early genus Hyracotherium are the oldest known animals of the horse family, Equidae. Many fossil remains of this family have been found in rocks of the late Eocene epoch and of succeeding epochs. Scientists have classified the animals represented by these fossils into 17 different groups (Table 3). Classification is based on comparison of many kinds of structural evidence. In this investigation you will study only one of the many physical characteristics that scientists have used, and determine whether this characteristic is useful in developing an evolutionary scheme for the modern horse and its ancestors.

In horses, the grinding teeth are in the back of the mouth, separated from the front teeth by a toothless space. On each side of the jaw, the grinding teeth (cheek teeth) consist of 3 premolars and 3 molars. The span (size) of the cheek teeth has been measured in many horse fossils. An increase in the size of cheek teeth indicates an increase in the size of the entire horse skull. It is logical to assume that as the size of the skull of an animal increases, the size of the entire animal increases as well. An average of the data of the size of cheek teeth is presented in Table 3.
Table 3: Average span (size) of Horse Fossil Teeth Over Time

Genus of Horse Fossil

Age (Millions of Years Ago)

Size of Cheek Teeth (cm)

1. Hyracotherium



2. Orohippus



3. Epihippus



4. Mesohippus



5. Miohippus



6. Parahippus



7. Anchitherium



8. Archaeohippus



9. Merychippus



10. Hypohippus



11. Megahippus



12. Pliohippus



13. Nannippus



14. Calippus



15. Neohipparian



16. Astrohippus



17. Equus




    • Draw and label the axes for a graph comparing time (in Millions of Years Ago) to cheek teeth span (in cm). Give your graph a descriptive TITLE at the top.

    • Using a midpoint value for the age, draw in each of the points shown in Table 3. Next to each dot, place the number of the genus as shown in Table 3. For example, plot a point for Hyracotherium at about 52.5 million years ago and 4.3 cm. Next to the point write the number 1.

    • (Be very careful when you draw connecting lines; double-check which horses go with which lines)
      Connect the first FIVE points only (Hyracotherium, Orohippus, Epihippus, Mesohippus and Miohippus) with a line.


Questions (continued):

  1. What seems to be the simple trend in the evolution of cheek teeth for the first 5 genera only (that is, what is happening to the size of the teeth as evolution occurs)?

  2. It is evolutionarily correct to connect the first five points in a single line (this is NOT a ?). What does this tell you about the early evolution of horses (that is, how many horse species seem to exist at any one time)?

Now, look closely at the diagram at the end of this lab handout. Notice that, beginning in the late Eocene, there is no longer only one type of horse fossil found. From the fossil record, it seems that several types of horses lived on Earth at the same time!!! How did evolution proceed in each of those different types of horses? To answer this, we can try drawing some additional lines starting from Miohippus (5). Draw one line connecting Miohippus (5) through Anchitherium (7), Hypohippus

(10) and ending at Megahippus (11). This is one evolutionary lineage of horses that continued the ancestral type of horse lifestyle: browsing for mixed vegetation.

Now draw some additional lines coming from Miohippus (5): Draw one line from Miohippus (5) to Archaeohippus (8); a separate line from Miohippus (5) to Merychippus (9); and a third line from Miohippus (5) to Parahippus (6). Paleontologists aren’t certain of the actually phylogeny (since these animals are extinct), but many think that Miohippus was the ancestor to the horses of the Miocene.

By the middle of the Miocene, the different kinds of horses really started to spread around the world. To see what happened in the Americas where horses began to make a shift towards grazing lifestyles, draw separate lines from Parahippus (6) to Pliohippus (12), from Parahippus (6) to Calippus (14), and from Parahippus (6) to Equus (17), which is the modern horse. Next, draw a line from Pliohippus (12) to Astrohippus (16). Last, go back to Merychippus (9) and draw a line to Neohipparian (15), and then a separate line from Merchyippus (9) to Nannippus (13). Whew! Who knew that there were so many kinds of horses? 

Questions (continued--again, type/double-space your answers):

  1. What was the biggest horse as indicated by the fossils? What do you think this name means?

  2. Examine the data for total change in size of cheek teeth from Hyracotherium (1) to Miohippus (5).

  1. What is the total age difference between these two genera? (include UNITS in your answer)

  2. What is the total change in the size of teeth between these two genera? (with units)

c. What is the average change in size of teeth over time (divide answer b by answer a)? Your answer is a RATE of change: X cm/y time units. Don’t forget to include the units for your answers!

d. In examining your answer for 3c, does this indicate that the change in horses over time was rapid or slow?

  1. Examine the pattern shown by the entire graph.

  1. What happened to the skull size over time? (be specific: discuss 1-5 separately from 5-17)

  2. Was there one general direction of evolution, or did multiple branches of evolution occur?

  3. Are there any examples of genera (plural) in which the teeth and skulls became smaller than their predecessors? (In other words, the horses were smaller.) If so, name (All of) them.

N. America

Old World

Mostly grazers

Mixed feeders

Mostly browsers


  1. How many years have passed between the first horses and modern horses?

  2. How many genera of horses currently exist according to the data?

  3. How many genera of horses have become extinct?

  4. Why do you think these species became extinct?

  5. Attach your graph and page 2 (charts) to the typed answers to these questions.

  6. Why do you think these species became extinct?

S. America











Million years ago

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