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Teeth for a Gritty Diet: Fossil Horses and Spreading Grasslands

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Teeth for a Gritty Diet: Fossil Horses and Spreading Grasslands

Use fossil teeth counts and a foal model to explain how a trait became common.

Part 1: Reading the layers

Read the passage and Table 1, then answer questions 1 to 5.

The Tooth Drawers

1On Saturdays, eighth grader Noa Feldstein volunteers in the collections room of a natural history museum, behind the public halls, where fossils are stored in rows of wooden drawers. Her project with the curator, Dr. Kwabena Ostrander, uses fossil horse teeth from rock layers in one region of the Great Plains. The layers formed one on top of another from about 24 million to 9 million years ago. The horses in them were related to today's horses, though most were much smaller.

2A horse grinds its food with its broad cheek teeth. Some fossil horses had low-crowned teeth, with a short crown. Others had high-crowned teeth, with a tall crown that sits mostly inside the jaw and slowly pushes up as its top wears away. The volunteers counted a tooth as high-crowned if its crown was more than twice as tall as it was wide. A crown's height is set while the tooth forms in a young horse. Chewing wears a crown down, but it cannot make the crown taller. In mammals, the size and shape of teeth are largely inherited from the parents.

3Grasses make tiny, hard bits of silica, a glassy mineral, inside their leaves. These bits, called phytoliths, last in rock for millions of years, so scientists count them to learn what plants grew in a place. Grasses also grow low to the ground, where wind-blown dust and grit settle on them. Scientists think the silica and the grit wear teeth down much faster than the soft leaves of shrubs and trees do. A horse whose teeth wear down to the roots can no longer chew well, and it may starve.

4Table 1 shows simplified data like the data scientists collect from layers like these. The grass share is the percent of all plant phytoliths in a layer that came from grasses. Some layers held more horse teeth than others, so the volunteers did not measure the same number of teeth in every layer.

5Horses usually begin to breed at about 3 years old, and a mare has at most one foal a year. Dr. Ostrander told Noa that a new generation of horses came along about every 8 years.

Table 1: Fossil Horse Teeth and Grass Phytoliths in Six Layers
Layer	Age (million years)	Grass share	Teeth measured	High-crowned teeth
A (oldest)	24	10%	40	2
B	21	35%	50	3
C	18	60%	40	6
D	15	70%	40	18
E	12	75%	50	36
F (youngest)	9	80%	40	36

1.

Use Table 1. Find the percent of the teeth measured in each layer that were high-crowned.

Layer C: % high-crowned. Layer F: % high-crowned.

2.

Use paragraph 4. Why did the volunteers compare percents of high-crowned teeth instead of the numbers of high-crowned teeth?

A. The numbers of teeth include the grass phytoliths, so they are too large.B. Percents show how old each tooth is, and the plain numbers of teeth do not.C. The layers had different numbers of teeth measured, so percents are fairer.D. Percents leave out the teeth that broke while the layers were dug up.

3.

Use Table 1 and your answer for Layer C. Layers C and D formed 3 million years apart.

From Layer C to Layer D, the share of high-crowned teeth rose by percentage points. That is about percentage points per million years.

4.

Use Table 1 and your answers so far. Which statement best describes how the two shares changed from Layer A to Layer F?

A. The grass share rose first, and high-crowned teeth rose most after grass passed 50%.B. The high-crowned share rose fastest from Layer A to Layer B, as grass first spread.C. Both shares rose by about the same number of points between each pair of layers in turn.D. High-crowned teeth became common first, and the grass share rose after that.

5.

Use paragraphs 2 and 3. Which explanation best fits why high-crowned teeth became common after grasses spread?

A. Gritty grass wore teeth down faster, so horses with taller crowns could chew to an older age.B. Grass leaves are softer than shrub leaves, so horses with low crowns had trouble gripping them.C. Chewing grass made each horse's crowns taller, and its foals were born with the taller crowns.D. Horses with high crowns ate less grass than the others, so their teeth wore down more slowly.

Part 2: A model of many generations

Use the model, Table 1 and the passage to answer questions 6 to 11.

Foal model: A herd on a gritty grass diet has 1,000 mares, and 100 of them have high-crowned teeth. Because their teeth last longer, the high-crowned mares raise 5 foals each in their lives, while the low-crowned mares raise 4 foals each. Each foal has the same kind of teeth as its mother.

6.

Use the foal model. Round the percent to the nearest whole number.

Total foals raised by all 100 high-crowned mares: foals. Total foals raised by all 900 low-crowned mares: foals. Share of all the foals with high-crowned teeth: %

7.

Use paragraph 5. Layers C and D formed 3 million years apart. Use 8 years per generation and do not round.

Between Layer C and Layer D, about generations of horses were born.

8.

In the model, the share of high-crowned foals rose by only about 2 percentage points in one generation. Why could an advantage this small still explain the change from Layer C to Layer D?

A. The grass share rose 10 points from C to D, which raised the teeth share 10 points.B. Low-crowned mares grew new, taller crowns on their teeth once the grass spread.C. The model's advantage doubles the share of high-crowned foals in each generation.D. The small gain repeated over hundreds of thousands of generations, so it added up.

9.

Sort each statement by whether it fits the evidence on this page.

10.

Suppose a younger layer showed that the region had turned back into forest, where horses ate soft leaves with little grit. What would most likely happen to the share of high-crowned teeth over the generations after that?

A. It would drop to 0% in one generation, since the horses would stop forming tall crowns.B. It would likely stop rising, since tall crowns would no longer help mares raise more foals.C. Each horse would start forming low crowns, since it would no longer need its tall ones.D. It would rise faster, since horses would need taller crowns to reach the leaves of trees.

11.

Explain how natural selection changed the share of high-crowned teeth among the horses from Layer A to Layer F. Write a claim, use at least two calculations from this page as evidence, and explain your reasoning. Include the foal model in your answer.