File:Phacops rana fossil trilobite (Silica Formation, Middle Devonian; quarry in Sylvania, northwestern Ohio, USA) 1 (15324284186).jpg

Phacops rana (Green, 1832) fossil trilobite from the Devonian of Ohio, USA. (public display, CMNH 4570, Cleveland Museum of Natural History, Cleveland, Ohio, USA)

Trilobites are extinct marine arthropods. They first appear in Lower Cambrian rocks and the entire group went extinct at the end of the Permian. Trilobites had a calcitic exoskeleton and nonmineralizing parts underneath (legs, gills, gut, etc.). The calcite skeleton is most commonly preserved in the fossil record, although soft-part preservation is known in some trilobites (Ex: Burgess Shale and Hunsruck Slate). Trilobites had a head (cephalon), a body of many segments (thorax), and a tail (pygidium). Molts and carcasses usually fell apart quickly - most trilobite fossils are isolated parts of the head (cranidium and free cheeks), individual thoracic segments, or isolated pygidia. The name "trilobite" was introduced in 1771 by Johann Ernst Immanuel Walch and refers to the tripartite division of the trilobite body - it has a central axial lobe that runs longitudinally from the head to the tail, plus two side lobes (pleural lobes).

Shown above is a famous trilobite species whose remains are relatively common in the Middle Devonian-aged Silica Formation of northeastern Ohio. This is Phacops rana (Green, 1832) (some place the species in "Eldredgeops", an unnecessary name based on insufficiently justified genus-level taxonomic splitting). Phacops trilobite fossils occur with other typical Middle Paleozoic shallow marine invertebrates: brachiopods, bryozoans, crinoids, and corals.

Classification: Animalia, Arthropoda, Trilobita, Polymerida, Phacopidae

Stratigraphy: Silica Formation (Silica Shale), Middle Devonian