A new odontopleurid trilobite, Meadowtownella serrata, is described from a new Llanvirnian (Darriwilian; Didymograptus murchisoni Biozone) locality in the Builth Inlier, Mid-Wales. This unusually spinose species displays a remarkable array of pygidial spines and extends the stratigraphic range of the genus back into the Abereiddian. Benthic faunas are rare in the predominantly ‘anoxic’ D. murchisoni Biozone of the Welsh Basin, and these beds are an unusual habitat for odontopleurids. Environmental analyses (framboidal pyrite and associated total organic carbon) in this study, together with the distribution of benthic faunas (described and figured) in the section, indicate a likely oxic water column and episodically oxic bottom waters. A new diagnosis for Meadowtownella is presented with a review of the genus, as a clarification of previous work. The greater spinosity of earlier members of the lineage may reflect either changing environmental preferences, or the derivation of Meadowtownella from an original more spinose lineage.

Biostratigraphical ranges and palaeogeographical distribution of mid-Givetian to end-Frasnian odontopleurids are investigated. The discovery of Leonaspis rhenohercynica sp. nov. in mid-Givetian strata extends this genus unexpectedly up to the late Middle Devonian. New material of Radiaspis radiata (Goldfuss, 1843) and the first koneprusiine in Britain, Koneprusia? sp., are described from the famous Lummaton shell-bed, Torquay, Devon. New taxa of Koneprusia, K. serrensis, K. aboussalamae, K. brevispina, and K. sp. A and K. sp. B are defined. Ceratocephala (Leonaspis) harborti Richter & Richter, 1926, is revised and reassigned to Gondwanaspis Feist, 2002. Two new species of Gondwanaspis, G. dracula and G. spinosa, plus three others left in open nomenclature, are described from the late Frasnian of Western Australia. A further species of Gondwanaspis, G. prisca, is described from the early Frasnian of Montagne Noire. Species of Gondwanaspis are shown to possess a number of paedomorphic features. A functional analysis suggests that, unlike other odontopleurids, Gondwanaspis actively fed and rested with the same cephalic orientation. The sole odontopleurid survivors of the severe terminal mid-Givetian biocrisis (‘Taghanic Event’) belong to the koneprusiine Koneprusia in the late Givetian and Frasnian, and, of cryptogenic origin, the acidaspidine Gondwanaspis in the Frasnian. Whereas the former became extinct in the late Frasnian at the Lower Kellwasser Event, the latter disappeared, and with it the entire Odontopleuroidea, at the terminal Frasnian Upper Kellwasser global biocrisis.

The subfamily Koneprusiinae is revised as part of a larger study of the Odontopleuridae, and Laethoprusia gen. nov. is described. The type species L. salax sp. nov. is from the Wenlock of Gotland, Sweden, and the genus is known also from the Wenlock of the Montagne Noire in south France, and from the Lower Devonian of Australia. The type species is most unusual among trilobites in possessing large fenestrae perforating the exoskeleton on segmental boundaries from the cephalic posterior margin to within the pygidium. The fenestrae were probably covered with soft tissues. Their function is enigmatic. Laethoprusia gen. nov. is the sister taxon to Isoprusia and/or Koneprusia. These genera are revised, and a review of all species assigned to them is given. A cladistic analysis of the best known taxa of Laethoprusia, Isoprusia, and Koneprusia is presented. Laethoprusia is the most derived genus of the subfamily, although stratigraphically it is the earliest.

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 taxonomie 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.