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Roger Weller, geology instructor

by Detric Miles
Physical Geology
Spring 2010







          Trilobites are a  fossil group of extinct marine arthropods that form the class Trilobita. Trilobites first appear in the fossil record during the Early Cambrian period (540 million years ago) and flourished throughout the lower Paleozoic era before beginning a drawn-out decline to extinction when, during the Devonian, all trilobite orders, with the sole exception of Proetida, died out. Trilobites finally disappeared in the mass extinction at the end of the Permian about 250 million years ago.


          Trilobites had many life styles; some moved over the sea-bed as predators, scavengers or filter feeders and some swam, feeding on plankton. Most life styles expected of modern marine arthropods are seen in Trilobites, except for parasitism. Some trilobites (particularly the family Olenida) are even thought to have evolved a symbiotic relationship with sulfur-eating bacteria from which they derived food.


Physical Description


          When trilobites are found, only the exoskeleton is preserved (often in an incomplete state) in all but a handful of locations. A few locations preserve identifiable soft body parts (legs, gills, musculature & digestive tract) and enigmatic traces of other structures (e.g. fine details of eye structure) as well as the exoskeleton.

Trilobites range in length from 1 millimeter (0.039 in) to 72 centimeters (28 in), with a typical size range of 310 cm (1.23.9 in). The world's largest trilobite, Isotelus Rex, was found in 1998 by Canadian scientists in Ordovician rocks on the shores of Hudson Bay






          The exoskeleton is composed of calcite and calcium phosphate minerals in a protein lattice of chitin that covers the upper surface (dorsal) of the trilobite and curled round the lower edge to produce a small fringe called the doublure. Three distinctive tagmata (sections) are present: cephalon (head); thorax (body) and pygidium (tail).

          During molting, the exoskeleton generally split between the head and thorax, which is why so many trilobite fossils are missing one or the other. In most groups facial sutures on the cephalon helped facilitate molting. Similar to lobsters & crabs, trilobites would have physically "grown" between the molt stage and the hardening of the new exoskeleton.






Soft body parts

          Only 21 or so species are described from which soft body parts are preserved, so some features (e.g. the posterior antenniform cerci preserved only in Olenoides serratus)remain difficult to assess in the wider picture.



          Trilobites had a single pair of preoral antennae and otherwise undifferentiated biramous ). Each exopodite (walking leg) had 6 or 7 segments, homologous to other early arthropods. Expodites are attached to the coxa which also bore a feather-like epipodite, or gill branch, which was used for respiration and, in some species, swimming. The base of the coxa, the gnathobase, sometimes have heavy, spiny adaptations which were used to tear at the tissues of prey. The last expodite segment usually had claws or spines. Many examples of hairs on the legs suggest adaptations for feeding (as for the gnathobases) or sensory organs to help with walking.


Digestive tract

          The toothless mouth of trilobites was situated on the rear edge of the hypostome (facing backwards), in front of the legs attached to the cephalon. The mouth is linked by a small esophagus to the stomach that lay forward of the mouth, below the glabella. The "intestine" led backwards from there to the pygidium. The "feeding limbs" attached to the cephalon are thought to have fed food into the mouth, possibly "slicing" the food on the hypostome and/or gnathobases first. Alternative lifestyles are suggested, with the cephalic legs used to disturb the sediment to make food available. A large glabella, (implying a large stomach), coupled with an impendent hypostome has been used as evidence of more complex food sources, i.e. possibly a carnivorous lifestyle.


Internal organs

          While there is direct and implied evidence for the presence and location of the mouth, stomach and digestive tract  the presence of heart, brain and liver are only implied  with little direct geological evidence.



          Although rarely preserved, long lateral muscles extended from the cephalon to midway down the pygidium, attaching to the axial rings allowing enrollment while separate muscles on the legs tucked them out of the way.








"Life History and Ecology of Trilobites." UCMP - University of California Museum of Paleontology. Web. 30 Apr. 2010. <>.

"Trilobite Fossils - Ohio History Central - A Product of the Ohio Historical Society." Ohio History Central - An Online Encyclopedia of Ohio History - Ohio Historical Society. Web. 30 Apr. 2010. <>.


"Trilobites: The Greatest Survivors in Earth's History - Trilobites - Io9." Io9. We Come from the Future. Web. 30 Apr. 2010. <>.