Abstract

Study of type and new material of the pterygometopine Achatella Delo, 1935 demonstrates the presence of four species in Upper Ordovician (Katian) strata of Laurentian North America, A . achates (Billings, 1860) from the northeastern United States and the St. Lawrence lowlands of Canada, A . carleyi (Meek, 1872) from the Cincinnati region, Ohio and Kentucky, A . katharina (Bradley, 1930), from Missouri and Oklahoma, and A . clivosa Lespérance and Weissenberger, 1998 from the Gaspé Peninsula, Quebec. Perhaps as many as five additional species are present in Sandbian–Katian strata of the Laurentian terranes of Scotland and Northern Ireland, although only three of these are known well enough to code for phylogenetic analysis. The oldest pterygometopines, including species of Achatella , are known from Middle Ordovician strata of Baltica. Phylogenetic analysis supports a single migration event from Baltica from Laurentia, followed by a modest diversification in the latter region.