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Trilobites

© Beverly Eschberger

When you visit a museum with a natural history exhibit, you will probably see several examples of Trilobites on display. They are members of the Phylum Arthropoda, the same group that insects, spiders, and crustaceans belong to. (See my article "Biological Nomenclature" for more information about how biologists divide animals into groups.) These invertebrates (animals without backbones) were the dominant animals of the Lower Paleozoic seas. In fact, they are so abundant in Cambrian marine sediments, that the Cambrian Period (570 to 500 million years ago) is sometimes called "The Age of Trilobites."

Trilobites survived until the Middle Permian Period (about 265 million years ago). The reason for their extinction is unknown, but many paleontologists believe that it was due to competition with fishes and the spread of mollusks. Because their development has been so well-studied, and they had a worldwide distribution, paleontologists can use trilobites to determine how old sediments and other fossils are, using relative dating.

The name "trilobite" means "three-lobed." Can you spot the three lobes? The first is the head shield, at the top of the animal shown in the diagram, and contains the trilobite's eyes and mouth. The second lobe is the segmented middle area. The segmentation allowed trilobites to roll up on themselves for protection, the same way that a "pillbug" or "rolly polly" does. Trilobites had paired legs on most, if not all, of their body segments. The third lobe is seen at the bottom of the diagram, and sometimes had a tail, or "telson", attached.

Trilobites had a calcified cuticle exoskeleton to protect their bodies, similar to the cuticle found on our modern Horseshoe crabs. (See my article "Horseshoe Crabs" for more information about these interesting animals.) In fact, the horseshoe crab goes through a time in its larval development in which it has a tri-lobed stage, leading some scientists to believe that the animals might be related.

Because trilobites have a cuticle, they would have had to shed this cuticle in order to grow larger. Quite often, paleontologists will find several trilobite shells of different sizes all in one area. For a long time, they thought that the trilobites would travel together in groups, but further study revealed that many of the trilobites were actually only empty shells left behind by one animal as it shed its cuticle.

Paleontologists have determined that trilobites had very sophisticated compound eyes. This, and the fact that trilobites were probably very good swimmers, has led paleontologists to believe that at least some trilobites were predatory. Some trilobites had quite enormous eyes, and scientists believe that they were planktonic, meaning that they swam freely near the surface of the sea the way that plankton does. Other trilobites had greatly reduced eyes, and may have even been blind, these are believed to have been bottom feeders.

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