File:Agnostoid trilobites in limestone (Wheeler Formation, Middle Cambrian; southeastern corner of the Drum Mountains, Utah, USA).jpg

English: Agnostoid trilobites in lime mudstone from the Cambrian of Utah, 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).

The two small, dark-colored, dumbbell-shaped fossils seen above are agnostoid trilobites. Agnostoids differ from "ordinary" trilobites (= polymeroids) by having the following characeristics: two-segmented thorax; blind (no eyes); isopygous (the heads & tails are equal-sized). Agnostoids are usually quite small. Many modern arthropod phylogenetic studies conclude that agnostoids should be excluded from the trilobites.

Classification: Animalia, Arthropoda, Trilobita, Agnostida

Stratigraphy: Wheeler Formation, upper part of the Ptychagnostus gibbus Interval-zone, middle Middle Cambrian

Locality: northeast-southwest trending ridge crest in the SE1/4 of Section 17, 52 mm N, 33 mm E of the southwestern corner of the Drum Mountains Well 1:24,000 quadrangle map, southeastern end of the Drum Mountains, northern Millard County, western Utah, USA (= stop 2 in Laurentia 99 field trip guidebook) (vicinity of 39˚ 30' 46.60" North latitude, 112˚ 59' 24.05" 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.