English: Olenelloid trilobite cephalon in greenish mudrock from the Cambrian of California, 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).

Classification: Animalia, Arthropoda, Trilobita, Polymerida, Redlichiida, Olenelloidea

Stratigraphy: ~25 meters above the base of the Echo Shale Member / Eagle Mountain Shale Member, lower Carrara Formation, a little above the FAD (first appearance datum) of Bristolia, upper Bonnia-Olenellus Zone, upper Dyeran Stage, upper Lower Cambrian

Locality: ~25 meter level of the Emigrant Pass West section (= stop 13, Laurentia 99, pp. 46-50), northern side of Old Spanish Trail Highway, just west of Emigrant Pass (= crest of Nopah Range), east of Tecopa, Inyo County, southern California, USA (vicinity of 35˚ 53.445' North latitude, 116˚ 04.238' West longitude)

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Reference cited:

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.