The phylogenetic affinity of the Ordovician trilobites Agerina, Forteyaspis gen. nov., and related genera, with new and revised species from Canada and the United States

Abstract

A phylogenetically related group of variably well-known Ordovician trilobites have in the past been assigned to six separate genera and a number of families, and their taxonomic cohesiveness has not always been recognized. When most of them have been understood to form an evolutionarily related group, they have historically most often been assigned to the genus Agerina Tjernvik, 1956, and assigned initially to Bathyuridae Walcott, 1886, and more recently to Leiostegiidae Bradley, 1925. The systematics of this group is revised, and its 25 formally named species are assigned to four potentially monophyletic genera. Ontogenetic information from a new silicified species, together with some adult morphology, demonstrates that the group is unambiguously related to the Ordovician family Phillipsinellidae Whittington, 1950, to which it is assigned as a suite of less derived genera. The new genus Forteyaspis (type species Forteyaspis idoli sp. nov. from the Darriwilian of western Newfoundland, Canada) is proposed for four Laurentian species, and other species are assigned to the genera Agerina, Brackebuschia Harrington and Leanza, 1957, and Hexianella Zhang in Qiu et al. Other new species are Agerina boygeorgei (Floian, Nevada, USA) and Forteyaspis adamanti (Floian, western Newfoundland, Canada). Forteyaspis norrisi (Ludvigsen, 1980) is photographically revised.