Phylum Arthropoda (2)



Class Trilobita

Trilobites are abundant fossils in lower Paleozoic rocks but became extinct at the end of the Paleozoic. The name trilobite comes from the division of the body into three longitudinal lobes (the central or axial lobe and two pleural lobes) and into three transverse lobes, the cephalon (head), thorax, and pygidium, or tail. Since trilobites molted their skeletons in order to grow, many head, thorax, and tail sections are often found disarticulated in rocks. To each thoracic segment was attached a pair of legs, which are seldom preserved. The head and pygidium were often composed of fused segments.


Trilobites possessed compound eyes on either side of the glabella. The mouth is on the ventral side below the glabella. The appendages were biramous (two-branched) with one branch consisting of a walking leg, and the other branch having a gill. Definite trilobite trails and burrows are found in the fossil record.


With a ventral mouth and walking/swimming appendages, trilobites are thought to have been bottom-dwelling occasional swimmers that grazed or scavenged the seafloor, and some may have burrowed. They ranged in size from less than an inch to nearly 2 feet. Some trilobites rolled the pygidium under the cephalon for protection, as modern sowbugs do. Trilobite species were relatively short-lived and are useful as index fossils for the lower Paleozoic. Their geologic range as a group is lower Cambrian to Permian.