File:Fallotaspidid trilobite (Montenegro Member, Campito Formation, Lower Cambrian; northwestern flanks of the Montezuma Range, Nevada, USA) 1.jpg

English: Fallotaspid 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 fallotaspidid trilobite cephalon from the Montezuma Range in Nevada. The specimen comes from a section that contains the base of the Montezuman Stage and has the oldest Laurentian trilobites of use. The rocks are silty shales and sandy shales of the lower Montenegro Member, Campito Formation. Fossils in the section include fallotaspidid trilobites, bradoriids, hyoliths, and trace fossils.

Classification: Animalia, Arthropoda, Trilobita, Redlichiida, Olenellina, Fallotaspididae

Stratigraphy: cf. Eofallotaspis sp. Range & Abundance Zone, just above the base of the Montenegro Member, upper Campito Formation, lower Montezuman Stage, Lower Cambrian

Locality: Montezuma West-3 South section, first north-south trending dry gully to the east of section 27's center, T2S, R41E, 55 mm E & 62 mm S of the northwestern corner of the Montezuma Peak 1:24,000 topographic quadrangle map, northwestern flanks of the Montezuma Range, eastern Esmeralda County, southwestern Nevada, USA (= stop 10 in Laurentia 99 field trip guidebook - MW-3S section) (vicinity of 37˚ 44.184' North latitude, 117˚ 21.664' West longitude)

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Reference:

International Subcommission on Cambrian Stratigraphy. 1999. Laurentia 99, V Field Conference of the Cambrian Stage Subdivision Working Group, Utah, Nevada, California, USA, 12-22 September 1999. 65 pp.