English: Triarthrus eatoni (Hall, 1838) - pyritized fossil trilobite with appendages in mudshale from the Ordovician of New York State, USA. (axial length of fossil ~1.2 centimeters)

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 remarkable trilobite shown above is from a lagerstätte - a fossil deposit that has preserved soft parts. One of the most famous lagerstätten in North America is Beecher's Trilobite Bed. It was discovered in the 1890s in dark shales of the Frankfort Shale (Upper Ordovician) northwest of Utica, New York State. Its most notable fossil component is pyritized Triarthrus trilobites with preserved appendages (legs and antennae).

Additional sites and horizons with Beecher's Bed Lagerstätte-type preservation have been identified in upstate New York (see Farrell et al., 2009). The pyritized Triarthrus trilobite shown here is from the Whetstone Gulf Formation (also Upper Ordovician), which consists of mixed marine siliciclastics, but is dominated by fine-grained rocks representing distal turbidites. Reported fossils from the Whetstone Gulf Formation lagerstätte include algae, conulariids, brachiopods, bryozoans, gastropods, ostracods, trilobites, eurypterids, crinoids, graptolites, trace fossils, and inferred coprolites.

Classification: Animalia, Arthropoda, Trilobita, Polymerida, Olenidae

Stratigraphy: Whetstone Gulf Formation, Lorraine Group, Maysvillian Stage, middle Cincinnatian Series, Upper Ordovician

Locality: unrecorded/undisclosed site in Lewis County (probably from the Martin Quarry), northern New York State, USA

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Some info. derived from:

Farrell et al. (2009) - Beyond Beecher's Trilobite Bed: widespread pyritization in the Late Ordovician Taconic foreland basin. Geology 37: 907-910.

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See info. at: en.wikipedia.org/wiki/Triarthrus and

en.wikipedia.org/wiki/Beecher%27s_Trilobite_Bed