Trilobites Trilobites were the most numerous and successful marine organisms of the early Paleozoic. These segmented "pill-bug" like arthropods ranged in length from several millimeters to 3 feet (90 cm). They had many legs, each with a set of gills, and most had sophisticated, multifaceted eyes. Like most arthropods, trilobites could grow only by shedding their old skins. Consequently, many of the trilobite fossils that are found are actually shed external skeletons and not the trilobite animals themselves. The name "trilobite" is derived from the three lobes into which each body segment is divided two lateral pleural lobes and an axial lobe. They may also be divided into three longitudinal sections, the cephalon (or head), the thorax (made up of variable numbers of hinged segments or somites), and the pygidium (or tail), made up of from one to 30 fused somites. The cephalon is formed from individual segments joined at facial sutures, which split apart during growth. The edges of the shell may be smooth or have a variety of spiny projections. Most trilobites lived in shallow ocean waters, and on reefs. It has been proposed, however, that some of the spiny trilobites may have been capable of swimming. Trilobite tracks and trails, reflecting a wide variety of their activities, have provided some of the best examples of the daily activities of these extinct organisms. Their skeletons are found in all types of sedimentary rock, from limestones to shales. Trilobites are relatively common in Cambrian, Ordovician, and Silurian rocks throughout the world. By the Devonian, they had declined to the point of being relatively rare finds. In the late Carboniferous and Permian, they are extremely rare. They are most likely to be found in limestones and shales. When preserved in shales, trilobites are usually flattened, and often the original shell has been destroyed, leaving only an external cast. In some shales, however, entire skeletons are preserved in excellent detail, allowing X-rays to be taken. Specimens in limestone are frequently preserved in three dimensions, with the skeletal material intact. Trilobites were gregarious animals and are frequently found in large numbers, either in deposits from storm activities or on bedding surfaces. It is not unusual for them to be locally abundant in Cambrian and Ordovician rocks as deposits or debris left by storms. It is also not unusual to find enrolled trilobites, curled up upon themselves, apparently a defensive behavior. The oldest known published scientific paper on trilobites dates back to 1698. Trilobites are particularly popular with amateur collectors, some of whom collect them exclusively.  Good specimens can be found in rock and fossil shops, but inspect carefully before you purchase. In some cases, large, complete specimens have been pieced together from parts collected separately. More than 1500 species of trilobite are known, and many are yet to be described.
See Also: Ammonite, Chondrichthyians (Shark Tooth) or Wooly Mammoth
Reproduced from "The Nature Company Guide to Rocks and Fossile" by Arthur B Busbey III, Robert R. Coenraads, David Roots, and Paula Williams, Consultant Editors David Roots and Paul Williams, published by Time Life Books available at (800) 227-1114 |
