SUBPHYLUM TRILOBITOMORPHA

The trilobites are the most primitive of all arthropod groups, and probably provide a good model for the primitive arthropod. Trilobites first appeared some 570 million years ago, and were abundant during the early part of the Paleozoic Era.

Examine the illustration and cast of a trilobite. Trilobites had three main body regions: a head, apparently composed of four fused segments, bearing a pair of antennae and, sometimes, compound eyes; a thorax, with a variable number of separate segments; and, an abdomen consisting of several fused segments. Each segment, even those in the head, bore a pair of jointed, unspecialized appendages. This basic body plan is the one which has been modified in the more advanced groups of arthropods. As you study more advanced groups try to keep in mind how each group has modified this basic arthropod plan.