File:Trilobite in mudrock (Montenegro Member, Campito Formation, Lower Cambrian; northwestern flanks of the Montezuma Range, Nevada, USA) 3.jpg

English: Trilobite in mudrock from the Cambrian of Nevada, 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).

Seen here is a trilobite cephalon from the Montezuma Range in Nevada. The specimen comes from a succession of silty shales and sandy shales of the Montenegro Member, Campito Formation.

Classification: Animalia, Arthropoda, Trilobita

Stratigraphy: "Nevadella" Zone, ~212 meters above the base of the Montenegro Member, upper Campito Formation, upper Montezuman Stage, middle Lower Cambrian

Locality: Montezuma South section (Ed Fowler discovery quarry), southern side of small east-west trending ridge just to the west of the crest of the Montezuma Range (just west of Indian Springs Canyon), NE1/4 of section 34, Montezuma Peak 1:24,000 topographic quadrangle map, ~3.6 kilometers north-northeast of Montezuma peak, eastern Esmeralda County, southwestern Nevada, USA (vicinity of 37˚ 43.718' North latitude, 117˚ 21.196' West longitude)