CLASS CRUSTAEA
SUBCLASS TRILOBITA

Trilobita: Morphology

Sometimes features of dark fossils on dark rocks are hard to see. As a representative of the Trilobita, we present the well-known trilobite Elrathia kingi, from the Middle Cambrian of Utah, near the town of Delta.
This is a normal picture of the specimen.	This picture is in reversed black and white, which makes it easier to see the features.
TRILOBITES ("three lobes") Trilobites derive their name from the fact that their bodies are divided into a central axial lobe in the middle and two pleural lobes on the sides. The head, or cephalon, is typically crescent-shaped and often bears two compond eyes (although some trilobites were blind). The swelling in the middle of the cephalon is the glabella; to the sides are the eyes and the facial sutures, which separate the free cheeks, or librigenae, from the rest of the cephalon. The free cheeks on the specimen above have been slightly detached. The pleural lobes are made up of overlapping pleurae, which covered the delicate legs. Trilobite legs were biramous, bearing both walking branches and feathery gills; food was passed forward to the mouth by the expanded basal segments of the legs, called gnathobases. At the posterior of the body, the pleurae were fused into a flattened tail-like segment, or pygidium. In some trilobites the pygidium is quite small; in others it is large and ornamented with spines.	Illistration from the  Kansas Geological Survey During the Ordovician Geological Period, some species reached over 30 inches in length, but by the Pennsylvanian Geological Period most were extinct and the few left were small.  Trilobites became extinct at the end of  the next geological period, the Permian.

The above information from University of California, Museum of Paleontology and the Kansas State Geological Survey.

 

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