Revision of the type species of the Early Ordovician (Tulean, late Tremadocian) bathyurid trilobite Licnocephala Ross, 1951 demonstrates that it has significantly different morphology than that ascribed to it in the earlier literature, which was based largely on species now assigned to a different genus. In addition to the type species, L. bicornuta Ross, 1951, which is fully revised on the basis of new material, four species, all apparently new, have been recovered, two of which, L. ngi n. sp. and L. bradleyi n. sp., are well enough known to formally name. The overall phylogenetic structure of bathyurids is yet to be determined, but several apparent clades can now be recognized and are discussed. Among these is what is termed the “Chapmanopyge group,” including Chapmanopyge Fortey and Bruton, 2013, Punka Fortey, 1979, Uromystrum Whittington, 1953, and Licnocephala. These genera are united in the occurrence of much of the anterior cephalic border on the librigenal anterior projection, with most of the anterior margin of the cranidium representing the suture, the possession of very short (exsag.) strap-like posterior cranidial projections, and extremely narrow visual surfaces. A fifth genus of the group, Ibexocephala n. gen., is represented by two new species, I. lossoae (type species) and I. dekosterae. The taxon features a remarkable cranidial morphology involving a strong deflection of the posteriormost part of the cranidium from the anterior part in sagittal profile.

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Successions in Oklahoma and Nevada record trilobite extinction and replacement near the Steptoean–Sunwaptan boundary in inner-shelf and outer-shelf settings, respectively. Prior to the extinctions, different trilobite biofacies occupied these environments, but faunas became similar in composition across the environmental gradient in the overlying I. “major” and Taenicephalus zones. Faunal changes in the outer shelf at the I. “major” Zone begin at a drowning unconformity that brought dark, laminated calcisiltite and silty lime mudstone above a subtidal carbonate succession. In contrast, Oklahoma shows facies continuity in a succession of tidally influenced bioclastic carbonates. Loss of genera and a dramatic abundance “spike” of Irvingella are features of the I. “major” Zone in both regions. Turnover of biofacies occurred in the succeeding Taenicephalus Zone, with both the inner and outer shelf dominated by Orygmaspis (Parabolinoides). Blooms of orthid brachiopods in shallow water settings are underappreciated signals of faunal change in the extinction interval. Although absent from the outer shelf in Nevada, orthids became abundant enough in Oklahoma to form shell beds in the lower Taenicephalus Zone, but became rare in overlying strata. Carbon isotope stratigraphy includes a modest positive δ13C excursion that peaks in the extinction interval at 1.4‰ (Oklahoma) and 2.2‰ (Nevada), which is congruent with previous reports from Utah and Wyoming. Although consistent with regional upwelling of dysoxic waters, the absence of sedimentary evidence for significant environmental change over much of the shelf is problematic. This suggests that physical environmental change acted primarily as a catalyst for cascading ecological and biogeographic effects.

The tiny Ordovician trilobite Oenonella Fortey, 1980 has previously been known only from cranidia and pygidia of its Middle Ordovician type species. Two new species, Oenonella wasisnamei and O. otherfellersorum, from the Darriwilian Table Cove Formation, western Newfoundland, Canada, are known from silicified material that provides new information on the librigena, ventral morphology, and intraspecific variation within the group. The stratigraphic range of the genus is extended downward by an occurrence in the Floian of the Shallow Bay Formation, western Newfoundland. New knowledge of Oenonella confirms its likely phylogenetic relationship with Proscharyia Peng, 1990, known from the Tremadocian of South China. In addition, the overlooked Amechilus Ross, 1951, seems to belong to the same group. Both Oenonella and Proscharyia have in the past been assigned to the aulacopleuride family Scharyiidae Osmólska, 1957. New silicified material effectively rules out this possibility. The broader affinity of the group remains obscure, but together the three genera are assigned to the new family Oenonellidae.

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The Early Ordovician (late Tremadocian; Stairsian) trilobite Gonioteloides Kobayashi has long been known from a small number of pygidia assigned to a single formally named species, and its affinities have not been assessed. Silicified material from western Utah and southeastern Idaho includes six distinct species assigned to the genus, one of which is the type species. Two others (G. moffitti and G. pankowskii) are new and formally named. An additional three species that are clearly new but known from sparse material are described in open nomenclature. Gonioteloides has a stratigraphic distribution through five consecutive trilobite zones in the mid-Stairsian Stage (upper Tremadocian). Although exoskeletal morphology of three species is almost completely known, the phylogenetic affinity of the taxon remains difficult to determine. It is tentatively assigned to Dimeropygidae Hupé.

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In order to maximize the utility of future studies of trilobite ontogeny, we propose a set of standard practices that relate to the collection, nomenclature, description, depiction, and interpretation of ontogenetic series inferred from articulated specimens belonging to individual species. In some cases, these suggestions may also apply to ontogenetic studies of other fossilized taxa.

Hystricurus millardensis Hintze, 1953 has been recorded widely in Laurentia. Revision on the basis of abundant new silicified topotype material indicates that most of these records are of morphologically distinct, though related, taxa and that a plexus of related species is involved. The new genus Millardicurus, with H. millardensis as type, is proposed for this clade, which also includes two well-documented new species, M. housensis and M. paramillardensis, from the House Formation of western Utah, several named species from Greenland, and several unnamed new species from various parts of Laurentia that have in the past been referred to M. millardensis. Millardicurus is not apparently closely related to younger Skullrockian-Stairsian hystricurids from Laurentia, but may be related to poorly known coeval Siberian Platform species classified in the genus Nyaya Rozova, 1963. Landmark-based geometric morphometric methods comprising principal component analysis of partial warp scores and Goodall's F test of pairwise means indicate significant shape differences among cranidia, librigenae, and pygidia of the House Formation species of Millardicurus.

Lower Ordovician sections in the type Ibexian area of western Utah contain a considerably more diverse trilobite fauna than has previously been reported. Reinvestigation of these faunas, based on new field sampling, allows a reassessment of the dimeropygid genera Ischyrotoma Raymond, 1925, and Dimeropygiella Ross, 1951. These taxa have been considered synonyms, but parsimony analysis indicates each is a well supported clade, and they are best recognized as sister genera. The number of species known from Ibex has been doubled, from four to eight, and morphological information is now available for most parts of the exoskeleton. New species include Ischyrotoma juabensis (Juab Formation), I. wahwahensis (Wah Wah Formation), Dimeropygiella fillmorensis (Fillmore Formation), and D. mccormicki (Fillmore Formation). The previously named species Dimeropygiella caudanodosa, D. blanda, and D. ovata are fully revised on the basis of abundant new material. Pseudohystricurus is a paraphyletic group, with species distributed as a basal grade of the Ischyrotoma/Dimeropygiella group.

The first records of the upper Cambrian agnostoid genera Kormagnostella, E. Romanenko, in Romanenko and Romanenko, 1967, and Biciragnostus F. Ergaliev, in Eraliev and Ergaliev, 2001, in Laurentian North America are from a narrow stratigraphic interval in the Steptoean–Sunwaptan boundary interval (Furongian, Jiangshanian) of Nevada and Utah. In Nevada, both genera occur in a condensed bioclastic lag below a major flooding surface, and Kormagnostella also appears in a transgressive interval in Utah. Immigration of these genera is associated with sea level rise, and also with faunal turnover. Biciragnostus is confined to the latest Elvinia Zone, immediately below the onset of a trilobite and agnostoid extinction event at the base of the Irvingella major Zone (basal Sunwaptan). Kormagnostella is present in the latest Elvinia Zone, and has its highest occurrence in the I. major Zone. Stratigraphic data from the Karatau-Naryn Terrane, Kazakhstan indicate that both genera disappear near the local extinction of Irvingella, suggesting that faunal turnover in that region may have been broadly correlative with the more profound extinction in Laurentia. New species are Kormagnostella advena, K. insolita and Biciragnostus viator.

Alcymene n. gen. comprises a monophyletic group of calymenids known from the Ludlow Series of Sweden and the British Isles. The sister species Alcymene alveus and A. lindstroemi n. gen. and spp. (Hemse Beds, Gotland, Sweden) are most closely related to the type species A. neointermedia (Richter and Richter), which occurs in overlying and underlying strata at nearby localities. Structural modification of A. alveus and A. lindstroemi, especially the development of a greatly broadened and troughed librigenal border, can be interpreted as “snowshoe” adaptations for increased stability on an offshore mud substrate.

Five species of aulacopleurid trilobites occur in rich, silicified trilobite faunas from the Upper Ordovician of Virginia: Harpidella triloba (Hu, 1975a), Strasburgaspis cona (Hu, 1971), and Strasburgaspis? n. sp. A, all from the Turinian Edinburg Formation, Harpidella whittingtoni new species, from the overlying Turinian Oranda Formation, and Harpidella evitti new species, from the Chatfieldian Martinsburg Formation. The species of Harpidella, similar to other sets of congeneric taxa known from the formations, are subtly but pervasively differentiated. In addition to qualitative differentia such as the expression of the eye socle and of pygidial tubercle rows, the species are shown to differ in cranidial and librigenal shape via landmark-based geometric morphometric analysis. The genus Strasburgaspis (type species S. cona) is new. It is assigned to Aulacopleuridae on the basis of its micropygous morphology, but it shares potential apomorphies with Brachymetopidae and could prove to be the most plesiomorphic representative of that family.

The genus Cyphaspis Burmeister, 1843, is first known from the northern Laurentian Sheinwoodian. By the late Homerian, the genus had appeared in England and Baltica, and from the Ludlow through the Lower and Middle Devonian it had an essentially cosmopolitan distribution. The Mississippian Dixiphopyge Brezinski, 1988, may represent a relict distribution of Cyphaspis, and if so should be considered a junior subjective synonym. Cyphaspis is considered the sister taxon of Otarion Zenker, 1833. The oldest, Sheinwoodian, species of either genus are very similar, but the clades evolved to the point of gross morphological disparity in the Devonian.

New species from the Wenlock and probably Ludlow of northwestern Canada are Cyphaspis lowei, C. munii, C. buchbergeri, and C. mactavishi. New ontogenetic material of the Zlichovian C. dabrowni (Chatterton, 1971) further demonstrates the pervasiveness of the basic juvenile morphology of the tribe Otarionini.

Odontopleurids are a diverse component of rich silicified trilobite faunas recovered from the Wenlock and Ludlow of the Cape Phillips Formation, central Canadian Arctic. Odontopleurinae and Acidaspidinae are common, but Ceratocephalinae and Koneprusiinae are also represented. This work treats all of the Odontopleurinae, with the exception of the genus Acanthalomina Prantl and Přibyl, 1949.

New species of Kettneraspis Prantl and Přibyl, 1949, include the upper Sheinwoodian K. wrightae, the lower Homerian K. lindoei, and the Gorstian K. caldwelli. Rare specimens assigned to Odontopleura Emmrich, 1839, and Radiaspis Richter and Richter, 1917, occur in the Sheinwoodian of the central Arctic.

Edgecombeaspis (type species E. johansonae new species) is proposed for an odontopleurine clade endemic to Laurentia, and in the Silurian restricted to northern Laurentia. Cladistic analysis yields a hypothesis of ingroup structure that is in general calibrated with stratigraphic sequence. An exception is a group of Telychian species from the Mackenzie Mountains, whose stratigraphic sequence was used to support a previous hypothesis of an ancestral-descendant lineage. The cladistic result indicates that the stratigraphic pattern is the inverse of the phylogenetic pattern: the stratigraphically lowest species in the proposed lineage is the most derived, and the highest is most primitive. In addition to the type, new species of Edgecombeaspis include the mid-Sheinwoodian E. jahansi and the lower Homerian E. soehni. Edgecombeaspis apparently became extinct in the Homerian.

The species Kettneraspis lenzi (Chatterton and Perry, 1983) and Radiaspis cf. R. norfordi (Chatterton and Perry, 1983) occur in the central Arctic, and further strengthen previous correlations with strata in the central Mackenzie Mountains, Northwest Territories.

Maurotarion Alberti, 1969, is a diverse otarionine clade that can be readily distinguished from Harpidella M'Coy, 1849. Both genera appear in the Upper Ordovician and range until the Middle Devonian. A new diagnosis for each taxon is given. New species of the genera from the Ordovician and Silurian of the central Mackenzie Mountains, Northwest Territories, Canada, include Harpidella kurrii (Ashgill), H. tikkaneni (Llandovery), H. greggi (Wenlock), and Maurotarion messieri (Llandovery). Harpidella megalops (M'Coy, 1846), H. triloba (Hu, 1975), H. spinafrons (Williams in Cooper and Williams, 1935), Maurotarion struszi (Chatterton, 1971), and M. instita (Whittington and Campbell, 1967) are revised.

The first major international trilobite conference (which also dealt with related arthropod groups) was convened by David Bruton in Oslo in 1973. That meeting resulted in a 467 page proceedings volume (Martinsson, 1975) that has become an indispensible reference on arthropod paleobiology. With its abundance of high-quality empirical work and influential ideas, the ongoing utility of “Fossils and Strata 4” is assured, yet it is also a fascinating microcosm of the themes and concerns of paleontology as a whole a quarter of a century ago. Paleoecology blossomed in the late 1960s and early 1970s. Autecology, and particularly functional morphology, dominates the volume, accounting for about one-third of the contributions. Plate tectonics was a relatively recent development in 1973 and, following the lead of Whittington and Hughes (1972), several papers tackled trilobite paleobiogeography. They vary from a bold attempt to determine oceanic circulation through the Ordovician by Ross to more conventional studies of biogeographic distribution that were aimed at providing further support for concepts of continental drift. There was a burgeoning interest in community paleoecology, and Richard Fortey's (1975) now-classic paper on the Early Ordovician faunas of Spitsbergen was the first to define trilobite “communities.” The volume was rounded out by diverse papers on trilobite anatomy based on material from the Burgess Shale and Beecher's Trilobite Bed, the microstructure of the trilobite cuticle, trilobites eyes, high level classification of trilobites, and various evolutionary themes (character displacement in agnostoids; adaptive radiation in Cambrian trilobites).

With this issue we mark publication of the 75th volume of the Journal of Paleontology, and celebrate the occasion with a series of review articles on the systematics of major groups of fossils. Instructions to authors were very broad: we suggested consideration of the history of study, current problems, and future directions, but otherwise left authors to focus their reviews as they saw fit. We hoped in this way, with a mix of traditions and approaches, to fashion a general overview of the systematics of fossil organisms as practiced today. With the enthusiastic efforts of the contributors, I think we've been successful. The papers in this issue comprise authoritative reviews of the state of the art in various branches of paleontology. But even if one is not concerned with the details of particular groups, the contributions provide a fascinating sense of where the discipline is, and where it might be going. Although concerned mainly with systematic history, they nevertheless provide a flavor of the kinds of concerns we have as a community for the future development of our science.

Borealarges n. gen. (type species B. reedi n. gen. and sp.) is a group of trochurine trilobites of mainly northern Laurentian distribution, closely related to both Richterarges Phleger and Terranovia Maximova. Richterarges is restricted to a Laurentian Ludlow-Přídolí clade of unambiguous monophyly. Taken together, the genera form a monophyletic subdivision of Trochurinae. Phylogenetic relationships within this clade are poorly resolved. The precise relationships of some members of the group are difficult to determine at present, and these species are referred to Borealarges sensu lato. Members of Borealarges are ubiquitous elements of silicified Wenlock–Ludlow trilobite faunas contained in debris flows of the Cape Phillips Formation of the central Canadian Arctic. At present, known post-Wenlock diversity of Borealarges is exclusively northern Laurentian, although this may be a function of lack of study in other areas. New species include B. reedi, B. morrisoni, and B. tuckerae, and B. s.l. B. calei. Borealarges mikulicorum (Perry and Chatterton) is revised.

Diverse silicified trilobite faunas from the lower Wenlock to lower Ludlow of the Cape Phillips Formation, central Canadian Arctic, have been the subject of works by Perry and Chatterton (1977), Chatterton and Perry (1979), Adrain (1994), and Adrain and Edgecombe (1995, and in press). The present work describes a very minor component of these faunas, the family Phacopidae, which is nevertheless of considerable biogeographic interest.

Very different questions are involved when we attempt to assess modern versus ancient global biodiversity. Because of the megabiases of taphonomy, eustasy, and tectonics, our estimates of paleobiodiversity can never be absolute; whether or not we can accurately estimate total diversity in the modern world is an open question, but we certainly cannot in the fossil record. The issues are whether we have any way of studying relative change in biodiversity through time, and how best this might be accomplished. That is, can we meaningfully estimate the shape of a global temporal diversity curve?

In a recent Matters of the Record, Vermeij (1999) identifies what he regards as a fundamental problem with cladistic analysis. “The phylogenetic method,” he states (p. 431), “derives phylogeny in one part of the tree by relying on character-taxon relationships in another part of the tree, in direct violation of the principle that clades, once they have diverged from each other, are independent of one another.” Vermeij fails to appreciate the distinction between pattern-based cladograms, which are hierarchies founded on homology hypotheses, and logically contingent, process-based trees, which are depictions of temporal evolution. As a result, he treats cladograms as literal and direct estimates of evolution and thus confuses the treatment of characters and taxa in parsimony analyses with biological processes and interactions.