Lost Giants and Long Survivors: A Zoo's Fossil Exhibit

Then and Now at the Prairie Ridge Zoo

1The Prairie Ridge Zoo breeds American bison and pronghorn for herds on restored grasslands, and its touch tank holds horseshoe crabs from a partner lab on the coast. This fall, eighth grader Leilani Forsberg joined the zoo's teen conservation team. Her first job is to help zoo educator Ezra Kowalczyk update a hallway exhibit called "Then and Now," which uses fossils to show visitors how life in the region has changed.

2Caves, sinkholes and old lake beds across the region hold the bones of large mammals, which the exhibit defines as animals whose adults weigh more than about 45 kg. Scientists date the bones with radiocarbon dating, which works on remains up to about 50,000 years old. For the graph, Mr. Kowalczyk sorted the dated sites by age and counted how many kinds of large mammals have been found at sites of each age. A kind is a group of closely related species, such as mammoths or camels. Table 1 follows five of those kinds through time.

3Mr. Kowalczyk reminded Leilani that the fossil record has gaps. Most animals that die are eaten, rot or are scattered before they can be buried, so only a small share ever become fossils. For that reason, the youngest bones found for a kind are usually a little older than the last animals of that kind. Mammoths also show how uneven the end of a group can be. They were gone from the mainland thousands of years earlier, yet a small group of woolly mammoths lived on a remote island in the Arctic Ocean until about 4,000 years ago.

4The second half of the exhibit is about groups with very long records, measured in millions of years ago (mya). Ginkgo trees line the zoo's front walk, and the horseshoe crabs in the touch tank have relatives in rocks more than 400 million years old. The coelacanth, a large fish with thick, lobe-shaped fins, has an odd history. Its youngest known fossils are about 66 million years old, so for decades scientists thought the group had died out. Then, in 1938, a fishing boat off the coast of South Africa pulled up a living coelacanth, and a second living species was found near Indonesia in the late 1990s.

5Table 2 lists these long-lived groups beside the mammoths. Before the new signs go up, Leilani's task is to check what the graph and tables really show and what they cannot show.