Trilobites are an extinct group of arthropods occupying an important position in Lower Paleozoic times. They are entirely marine organisms often found in shallow sea environments, where they probably crawled over the sea bottom in search of food, or burrowed in the sediments. Some other species swam or floated in the water column.
Their body is covered by a hard chitinous-calcareous shell which is divided into three longitudinal lobes by two axial furrows. Three transverse lobes divide the shell into head shield (cephalon), thorax and pygidium (see above).
Some trilobites could protect themselves by rolling up like a modern woodlouse, and they are sometimes found in this state (see right).
The head bears a pair of jointed antennae; usually eyes which may be large and compound, and four pairs of two-branched jointed appendages on the ventral side. On the ventral side of the thorax and pygidium each segment bears a pair of two-branched appendages with a fringe of filaments probably serving as gills, and the other jointed branch serving a walking or swimming function.
Adult trilobites range in size from <5mm to 70cm. The largest specimens belong to the aptly named species Isotelus rex.