National Geographic : 1988 Mar
D Trilobites, long-term rulers of the planet nce upon a time, and a very long time it was, creatures like these dominated the planet. They first appeared 570 million years ago, and by the time they vanished some 340 million years later, they had pro liferated into about 10,000 different species. Their onset marks the beginning and their demise the end of the Paleozoic era. Numerous and well-known, they are an aid to paleontologists in dating other fossils. Scientists call them trilobites for the three ridges, or lobes, that run the length of their bodies. Like other members of the phylum Arthropoda, these invertebrates had an external skeleton supporting the soft organs. A primitive armor, it also protected them as they crawled along the sea bottom in search of food. The exoskeletons they shed and the animals themselves became the fossils we find today in rock formations that have survived undisturbed from early times, as David Jeffery reported in the August 1985 NATIONAL GEOGRAPHIC. Best known of the North American trilobites, Phacops rana, seen here about life size, is often found beautifully preserved in Ohio. Another genus, found in rocks near Boston, grew to 45 centimeters (18 inches) and weighed 4.5 kilograms (10 pounds). Trilobites were the first creatures to have jointed legs. One segment was used for walking, the other for respiration. Why did trilobites become extinct after surviving a third of a billion years? Jawed fishes evolved and could have preyed on them. For whatever reasons, they are-like the dinosaurs gone forever with only their fossils to tell us of their long existence on earth.