Dig Camp Fossils: Teeth, Toes and the Horse Family

Teeth, Toes and Grass at Dig Camp

1Each July, Matteo Pryce and about twenty other teens spend two weeks at a fossil dig camp in badlands of the western United States. Badlands are dry hills where wind and rain have worn away the soil and left layers of rock exposed. The layers here lie flat, one on top of another, so lower layers formed earlier than the layers above them. Thin beds of volcanic ash between some layers let scientists measure their ages, which are given in millions of years ago (mya). Camp leader Dr. Rhea Wrenfield, a paleontologist, tells campers that the hills hold one of the richest fossil records of the horse family in the world.

2On the first day, Dr. Wrenfield showed the campers five skeletons from the camp museum, labeled A to E. Each one is a member of the horse family from rocks of a different age, and E is a living horse. Table 1 compares their size, their front feet and their cheek teeth, the large flat teeth at the back of the mouth that grind food. Some cheek teeth have short crowns, the part of a tooth above the gum. Others have tall crowns that keep growing up out of the jaw as the top wears down.

3Grass is tough to chew. It contains tiny hard grains of silica, and grass that grows close to the ground is often coated with grit, so it wears teeth down much faster than soft leaves do. To find out when grasses became common in the area, the camp team studies tiny fossil plant bits that stay in the rock long after the plants rot. They work out what share of these bits came from grasses in each layer, shown in Table 2.

4Last, Dr. Wrenfield showed a graph from a museum study. It shows about how many different kinds of animals in the horse family are known from fossils of each age in North America. Today, one kind is left, which includes all horses, zebras and donkeys. Matteo noticed that many skeletons in the camp museum do not appear in Table 1, and he asked whether every kind on the graph was the parent of the next one.