Triarthrus is a genus of Upper Ordovician ptychopariid trilobite found in New York, Ohio, Kentucky, and Indiana, eastern and northern Canada, China and Scandinavia.

The specimens of T. eatoni that are found in the Beecher's Trilobite Bed, Rome, New York area are exquisitely preserved showing soft body parts in iron pyrite.

[4] Triarthrus is an average size trilobite (up to about 5 centimetres or 2.0 inches) and its moderately convex body is about twice as long as wide (excluding spines).

As the preservation of soft tissue in T. eatoni is excellent, it is probable that it lacked cerci, the most backward pair of extremities known from Olenoides serratus.

[5][6][7][8] Due to the existence of an excellently preserved deposit in the Frankfurt Shale near Rome, New York State, that suggests an entire population of Triarthrus eatoni was killed and quickly buried, it has been possible to make a plausible reconstruction of its life history.

After hatching from roughly spherical eggs averaging ~0.2mm in diameter, the small protaspid and early meraspid stages supposedly lived between the plankton in the water column.

This is derived from the fact that only exuviae of the early stages were found, making clear that they haven't died in the disaster that killed their elders.