Reef Builders Along the Ledge Loop

Corals in the Limestone

1Itzel Marchetti runs cross-country for her middle school, and three mornings a week she trains on the Ledge Loop, a 6 km trail in a state park near her town. Halfway around, the trail climbs past gray limestone ledges. One morning, while she stretched her calves against the rock, Itzel noticed that it was packed with small tubes and horn-shaped fossils. Park ranger Delphine Okafor told her they were corals. The limestone formed on the floor of a warm, shallow sea that covered much of the region about 400 million years ago.

2After practice, Ranger Okafor showed Itzel the visitor center's exhibit, "Reefs Through Time." A reef is a large structure built by living things that make hard skeletons. Today most reefs are built by stony corals, but the exhibit uses fossils from sites around the world to show who built reefs in the past. Scientists date each fossil from the order of the rock layers and from dated beds of volcanic ash. Table 1 shows in which rock ages each group's fossils have been found.

3The exhibit points out two details. Horn corals and tabulate corals have their youngest fossils in rocks about 252 million years old, and stony corals have their oldest fossils in rocks about 240 million years old. Horn corals added the inner walls of their skeletons in groups of four, while stony corals add them in groups of six and build them from a different form of the mineral. Also, in rocks from a few million years after 252 million years ago, no reefs built by animals have been found anywhere on Earth.

4A second panel shows shells from fossil seafloor samples collected around the world, together with shells from today's seafloor (Table 2). Brachiopods are animals with two shells that cover the top and bottom of the body. Bivalves, such as clams and oysters, have two shells that cover the left and right sides. Both groups are still alive today.

5Ranger Okafor reminded Itzel that one park cannot tell the whole story. Many living things never become fossils, and each place holds rocks from only some times. Scientists decide that a group went extinct only when its fossils stop turning up in younger rocks at sites all over the world.