Trilobite Morphology
The general body form, or morphology, of the class trilobita changed very little comparatively speaking during it's evolutionary history from the earliest Cambrian forms to the latest trilobites at the end of the Permian. All trilobites have three longitudinal divisions (lobes) that split the body into left, right and central sections. These lobes, named the left pleural, right pleural and axial (center) respectively, are what the name of the class "trilobite" is in reference to. Additionally, all trilobites have three main body parts: cephalon, thorax and the pygidium. The cephalon, or head of the trilobite, is composed of several segments that are fused together. Sense organs, in various forms of compound eyes, are found on the cephalon. Trilobite eyes are some of the earliest of all developed sensory systems that are found in the fossil record. They were composed of a series of lenses made of calcite, which aided in their preservation. Collectors of trilobites especially value specimens with well-preseved eyes, where many times lenses can be seen with the naked eye. The thorax, or body section, is composed of a number of "thoracic segments." Most trilobites are polymerids, meaning that they contain several thoracic segments, although some only have two or three. A pair of limbs (appendages) are attached underneath each segment, enabling the organism to move on the ground or through the water. The pygidium, or tail section of the animal also had segments fused in a single plate much like the trilobite cephalon. Early trilobites had a pygidium consisting of one segment, while later genera have multi-segmented pygidiums. Although trilobites evolved into a wilde array of body shapes and sizes, some with elaborite ornamentation and spines, all contained the same general body form including the cephalon, thorax and pygidium, as well as the three longitudinal lobes.
The trilobite general body form changed very little during its evolutionary history...
Trilobite pygidium - Carboniferous,
Mazon Creek Formation, Braidwood, Illinois
Phacops rana - Devonian, Silica Shale, Paulding, Ohio
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