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Ceratarges spinosus fossil trilobite (AM Limestone, Middle Devonian; southern Morrocco) | by James St. John
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Ceratarges spinosus fossil trilobite (AM Limestone, Middle Devonian; southern Morrocco)

Ceratarges spinosus fossil trilobite from the Devonian of Morocco. (public display, FMNH PE 60826, Field Museum of Natural History, Chicago, Illinois, 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).

 

Some trilobites evolved extremely spinose bodies, especially odontopleurids and some lichids. Shown above is an example of a spinose lichid trilobite.

 

This complete exoskeleton has eight prominent spines. It has two pairs of long, curving cephalic (head) spines - one pair of genal (cheek) spines and one pair extending from the glabella. The eyes are perched atop a pair of stalks that extend posteriorly upward. The pygidium (tail) also has two pairs of long, curving spines - one pair from the anterolateral corners and one pair extending from the posterior pygidial margin, straddling the axis. Series of smaller, straight spines are present between the larger pygidial spine pairs.

 

Classification: Animalia, Arthropoda, Trilobita, Polymerida, Lichidae, Trochurinae

 

Provenance: AM Limestone, Middle Devonian; southern Morocco.

 

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Taken on June 11, 2010