Abstract

A phylogenetic analysis is performed on all species previously assigned to the odontopleurid trilobite genus Leonaspis. The PAUP analysis shows this genus, as conventionally defined, to be polyphyletic, composed of four groups of equal taxonomic rank. Leonaspis (s.s.)
is here restricted to one of these groups only, a monophyletic set of species characterised by 'four-spined' pygidia (i.e. with four spines between the major border spines) and a thorax of nine segments. Kettneraspis is recognised for the largest of these four groups, and is composed of 'two-spined' species with nine thoracic segments. Leonaspis and Kettneraspis belong to the Odontopleurinae. A third group of previous 'Leonaspis' species constitute a new genus of the Acidaspidinae. Its species are characterised by being 'four-spined' and having ten thoracic segments. For these the new genus Exallaspis is erected, with type species E. bufo. A fourth group of 'Leonaspis' species, being 'four-spined' with nine thoracic segments, belongs to Eoleonaspis, an Ordovician odontopleurine genus. Leonaspis and Exallaspis are temporally and spatially non-overlapping, Leonaspis being exclusively Gondwanan post-Wenlock, whereas Exallaspis is restricted to areas north of the Rheic Ocean and ranges from basal Llandovery to Ludlow. Kettneraspis is pandemic. The condition of five epiborder spines and ten border spines on the free cheek is proposed as plesiomorphic for odontopleurids, and the border spines are shown to originate as two separate rows with alternating spines. A shift in position of the facial suture is shown to transfer the genal spine from the cranidium to the free cheek in Kettneraspis meraspides of degree 1 or 2, and the previously suggested co-occurrence of a fixigenal and librigenal spine in these stages is refuted. Earlier proposed dimorphism is rejected, and two mechanisms for reduction in the number of pygidial border spines are proposed. Kettneraspis reetae sp. nov. is described.