Triarthrus

Triarthrus is an Upper Ordovician trilobite found in New York, Ohio, Kentucky, and Indiana, eastern and northern Canada, and Scandinavia. It is the last of the Olenid trilobites, a group which flourished in the Cambrian period. 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. Pyrite preservation has given scientists a rare opportunity to examine the gills, walking legs, antennae and digestive systems of trilobites, which are rarely preserved. Triarthrus is therefore commonly used in science texts to illustrate trilobite physiology.

Distribution

 * T. beckii Upper Caradoc and Ashgill, N.Y. and Kentucky.
 * T. billingsi Ashgill?, Quebec
 * T. canadensis is known from the Upper Ordovician of Canada (Katian, lower Member of the Whitby Formation, Craigleith vicinity, Georgian Bay area; middle Member of the Whitby Formation, Whitby, Rogue River and Pickering, all Lake Simcoe area, Ontario)[1]
 * T. eatoni Upper Caradoc-Ashgill, N.Y., is known from the Upper Ordovician of Canada (Ashgill, lower Member of the Whitby Formation, Craigleith vicinity, Georgian Bay area, Lake Simcoe area, Ontario;[1] and Quebec) and the United States (New York)
 * T. glaber Ashgill, Quebec
 * T. huguesensis Ashgill, Quebec
 * T. latissimus Sweden
 * T. linnarssoni Norway and Sweden
 * T. rougensis Ashgill, Ontario
 * T. sichuansis China
 * T. spinosus is known from the Upper Ordovician of Canada (Katian, middle Member of the Whitby Formation, Rogue River and Pickering, both Lake Simcoe area, Ontario[1] and Quebec), and the United States (New York).

Taxonomy
T. beckii and T. eatoni have long been considered closely related and possibly synonymous. Recent comparative analysis showed that there is no sharp distinction between the two, but that they seem to represent opposing morphotypes. T. beckii dominates earlier in the distribution and in shallow water, while T. eatoni does so later and in deeper waters. It is probable that the transgression of the sea level aided the eventual disappearance of the T. beckii morph. The occurrence of the supposed pluriform species continued for more than two million years.

Species previously assigned to Triarthrus

 * T. angelini = Bienvillia angelini
 * T. belli = Parabolinella sp.
 * T. caenigenus = Porterfieldia caenigena
 * T. convergens = Porterfieldia convergens
 * T. fisheri = Porterfieldia fisheri
 * T. goldwyerensis = Porterfieldia goldwyerensis
 * T. humilis = Porterfieldia humilis
 * T. jachalensis = Porterfieldia jachalensis
 * T. pacificus = Porterfieldia pacifica
 * T. papilosus = Porterfieldia papilosa
 * T. parapunctatus = Porterfieldia parapunctata
 * T. parchaensis = Bienvillia parchaensis
 * T. punctatus = Porterfieldia punctata
 * T. rectifrons = Bienvillia rectifrons
 * T. reedi = Porterfieldia caenigena
 * T. shinetonensis = Bienvillia shinetonensis
 * T. sinensis = Porterfieldia sinensis
 * T. tetragonalis = Bienvillia tetragonalis
 * T. thor = Porterfieldia thor
 * T. variscorum = Parabolina frequens