Middle Ordovician olenid trilobites (Triarthrus Green and Porterfieldia Cooper) from Jämtland, central Sweden

Abstract

AbstractTriarthrinid trilobites are described from the Middle Ordovician Andersön Shale and Örån Shale of Jämtland, central Sweden. The faunas include at least five species, belonging to two genera, Porterfieldia Cooper 1953 and Triarthrus Green 1832. Porterfieldia is represented by a single species, Porterfieldia humilis (Hadding 1913), while at least four species belong to Triarthrus: T. freji Thorslund, 1940, T. jemtlandicus Linnarsson, 1875, T. latissimus nom. nov. and T. linnarssoni Thorslund, 1940. P. humilis is found only in the lower Andersön Shale, equivalent to the Hustedograptus teretiusculus Zone. T. freji, T. jemtlandicus and T. latissimus occur frequently in the upper part of the Anderson Shale, which comprises the Nemagraptus gracilis Zone, and probably also the lower part of the Diplograptus multidens Zone. T. linnarssoni occurs throughout the Örån Shale (mainly Dicranograptus clingani Zone). T. jemtlandicus and T. aff. linnarssoni are also sporadically present in the lower and middle parts of the Oran Shale. Almost all growth stages of T. latissimus, from the anaprotaspid stage and onwards, are represented and the ontogeny is described. The early stages are, however, comparatively uncommon.