Abstract

The previously monotypic bathyurid trilobite Aponileus Hu, 1963, was known only from poorly preserved material of its type species, A. latus, and was interpreted as a junior subjective synonym of Psephosthenaspis Whittington. New species from the upper Floian (Blackhillsian) Fillmore Formation of western Utah belong to Aponileus and help demonstrate that the genus is an entirely Lower Ordovician (upper Floian) clade phylogenetically separate from the Middle Ordovician (Dapingian) Psephosthenaspis. Species of either group have broadly similar morphology, but the species of Psephosthenaspis which most closely resembles those of Aponileus is the youngest and most derived member, and the similarities are convergent. Psephosthenaspis roots among a group of heavily tuberculate, mostly undescribed, upper Floian species, one of which is briefly illustrated for comparison. The species most resembling some species of Aponileus, P. glabrior, is illustrated on the basis of new silicified material which extends its range from the upper Juab Formation into the base of the Kanosh Formation. New Blackhillsian species are A. laikaae (Fillmore Formation; Presbynileus ibexensis Zone), A. aasei
(Fillmore Formation; probably Pseudocybele paranasuta Zone), A. belkaae, A. strelkaae, and A. ugolekae (all Wah Wah Formation; "Pseudocybele nasuta Zone"). Aponileus? veterokae n. sp., from high in the "Pseudocybele nasuta Zone" of the Wah Wah Formation, is tentatively assigned. Bolbocephalus glaber Poulsen, 1927, from the Nunatami Formation of northwest Greenland, is poorly known but is also a member of Aponileus, and is similar in morphology to A. latus and A.aasei; it is revised on the basis of reillustrated type material. Phylogenetic analysis indicates that A. laikaae is the basal species, followed by a sister pair of A. belkaae and A. strelkaae. These are sister to a pair of subclades, the sister pair of A. ugolekae and A.? veterokae, and an effaced group lacking genal spines including A. aasei, A. latus, and A. glaber.