Triarthrus is an Ordovician age trilobite about 19 mm long which was widespread over North America. This arthropod lived as a scavenger or predator, and like a crab had segmented legs and a pair of pincers in front of the mouth and jaws which moved transverly. An example of a Triarthrus can also be seen in the environmental scene of the Billings Formation. A lot of these fossils that have been preserved in the Billings Formation shales are pyritized due to the anoxic conditions of the seas during preservation.

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