PHYLUM ARTHROPODA, CLASS TRILOBITA
The most common Recent arthropods are insects, but insect fossils are fairly rare. The most common arthropod fossils are trilobites, but there are no Recent trilobites. They have been extinct since the end of the Paleozoic.
Trilobites are the only common invertebrate fossils in which the head and eyes (if present) are preserved, and so should be very easy to recognize. The first picture shows a common Middle Cambrian species Elrathia kingi from Utah. The head (cephalon) and eyes are obvious near the top, there is a small 'tail' (pygidium) at the bottom of the photo, and the segmented central region is called the thorax. the specimen is about 3cm long. (Photo by B. Carter.)
Like all other arthropods, trilobites grew by shedding their exoskeletons and growing new ones. The next photograph shows another specimen of E. kingi, also from Utah, but notice that the head has been broken. the front part came off, the animal crawled out of its old skeleton, and wandered away to grow and make a new one. The scale bar is 1cm. (Photo by B. Carter.)
Not all trilobites had eyes. One common group of Cambrian trilobites were very small, had only two thoracic segments, and lacked eyes. It is difficult to tell which end is which on these species. The next photo (Peronopsis interstrictus from Middle cambrian rocks at Antelope Springs, UT) is an example. The scale bar is 1cm. (Photo by B. Carter.)
Like certain other arthropods some trilobites were able to enroll their shells, presumably to protect themselves. The final photo shows two enrolled specimens of Flexicalymene meeki from Upper Ordovician rocks near Cincinnati, OH. The scale bar is 1cm. (Photo by B. Carter.)
