English: Elrathia kingii (Meek, 1870) - fossil trilobite from the Cambrian of Utah, USA. (rock is ~3.4 centimeters across at its widest)

Trilobites are extinct marine arthropods that occur in the Cambrian to Permian stratigraphic record. They are most common in Cambrian rocks. The group goes extinct at the Permian-Triassic mass extinction event (251 million years ago).

Trilobites have a tripartite skeleton both transversely and longitudinally. They have a head (cephalon), a body of many segments (thorax), and a tail (pygidium) (in the fossil shown here, the head is at the upper right & the tail is at lower left). During life, the trilobite animal had antennae and legs and gills and guts and other soft parts. The preserved portion of a trilobite fossil is the hard, calcitic exoskeleton that covered the soft anatomy. During molting or after death, the multipart exoskeleton separates into many pieces. Complete, articulated fossil trilobites are uncommon to rare.

The trilobite seen here is Elrathia kingii, one of the most abundantly preserved trilobites on Earth. It is affectionately called "Joe Trilobite" by trilobite researchers, in reference to its abundance and unremarkable skeletal morphology (i.e., no spines or unusual features).

Classification: Arthropoda, Trilobita, Polymerida, Alokistocaridae

Stratigraphy & age: upper Ptychagnostus atavus Interval-zone (= lower Bolaspidella Assemblage-zone), Wheeler Formation, upper Middle Cambrian

Locality: House Range, northern Millard County, western Utah, USA