Efficient extraction of oxygen from ambient waters played a critical role in the development of early arthropods. Maximizing gill surface area enhanced oxygen uptake ability but, with gills necessarily exposed to the external environment, also presented the issue of gill contamination. Here we document setae inserted on the dorsal surface of walking legs of the benthic-dwelling middle Cambrian Olenoides serratus and on the gill shaft of the Late Ordovician Triarthrus eatoni. Based on their physical positions relative to gill filaments, we interpret these setae to have been used to groom the gills, removing particles trapped among the filaments. The coordination between setae and gill filaments is comparable to that seen among modern crustaceans, which use a diverse set of setae-bearing appendages to penetrate between gill filaments when grooming. Grooming is known relatively early in trilobite evolutionary history and would have enhanced gill efficiency by maximizing the surface area for oxygen uptake.

Following Fortey and Owens (1987), the Ordovician trilobite taxon Ellipsotaphrinae is established as valid, but is argued to have full family status within the Cyclopygoidea. It encompasses the existing genera Ellipsotaphrus, Girvanopyge Gamops and Circulocrania together with two new genera proposed herein, Arisemolobes and Synaptotaphrus. Typical ellipsotaphrid genera have a totally circumscribed ‘foreglabella’, incorporating extended S1 furrows and a portion of the occipital furrow. The known range of the family is Floian to Katian. Genera are conservative in form throughout their ranges and are widespread. All occur only in deeper water sediments with palaeooceanic access. Ellipsotaphrus monophthalmus and Ellipsotaphrus infaustus are reassessed and Ellipsotaphrus zhongguoensis, from the Katian of China, is regarded as a junior synonym of the Katian Girvan species Ellipsotaphrus pumilio. Girvanopyge [ = Cremastoglottos; Nanlingia; Waldminia] is demonstrated, partly on the basis of new material from the Katian of Girvan, to be an ellipsotaphrid cyclopygoid and not to have a close affinity to the remopleuridids, as had been clained. Girvanopyge barrandei, from the Katian of the Czech Republic, is synonymised with Girvanopyge caudata from China. Gamops is revived for forms showing a relationship to both Girvanopyge and Ellipsotaphrus. It encompasses three Czech species including the Dapingian Gamops triangulatus, which probably also occurs in correlative strata in South Wales. The systematic treatment is supported by new material from the Upper Ordovician of the Girvan district, and the relevant geology of this area is described in detail. New species proposed are: Arisemolobes zhouzhiyii, Synaptotaphrus oarion and Circulocrania ? dichaulax.

Trilobites are an iconic group of extinct arthropods that lived in Palaeozoic oceans for c. 270 Ma, before their demise at the end of the Permian Period. Despite their considerable diversity (> 22 000 species) and abundance, particularly in Cambrian and Ordovician rocks, as well as intensive study for well over 200 years, trilobite systematics remains in a state of flux. In this contribution, the complex history of trilobite classification over the last century is briefly reviewed, including the seminal scheme published by Henry Swinnerton in 1915. The cryptogenesis problem, which relates to the supposedly obscure phylogenetic links between major post-Cambrian trilobite clades and their Cambrian sister taxa, is also discussed. Previous studies have suggested that the cryptogenesis problem is largely a taxonomic artefact, but the Cambrian origins of some post-Cambrian groups, such as the orders Proetida and Odontopleurida, are still unclear. Future directions for research on trilobite systematics are outlined, from taxonomic studies involving comprehensive documentation and extensive illustration of morphology at the species level, through to broad-scale phylogenetic analyses that initiate or test hypotheses about relationships across the major groups. Other ongoing issues to be addressed include identifying the sister group of Trilobita, and determining whether certain taxa, such as the suborder Agnostina and Cambrian family Burlingiidae, represent trilobites.

The ontogeny of the pelturine olenid trilobite Leptoplastides salteri (Callaway, 1877) from the Shineton Shales, Shropshire, England, was first described in 1925 by Frank Raw. Since that time, scanning electron microscopy and other new technologies have revealed many more details of structure, of early developmental stages in particular, than were available to Raw. Whereas protaspides are not preserved and the state of preservation is less than perfect for the smallest meraspides, we have established that the latter had an array of delicate, long thoracic and pygidial spines, as well as paired procranidial spines, which disappear by meraspid degree 8. Raw's reconstructions of early meraspides, and his measurements of the early stages in development, are here amended in the light of new information. Dorsal spines in the adult are much more highly developed than have been documented in any other olenid. The hypostome is preserved in place in several specimens. Initially conterminant (attached to the doublure), it becomes natant (free) in late meraspid to early holaspid stages of development, with its anterior contour fitting exactly to that of the glabella. The ecology of the widespread Leptoplastides is best known from very extensive sections in South America, which provide a useful basis for comparison. It was well adapted to a range of environments, both oxygenated and dysoxic, and is usually the dominant taxon in the biofacies in which it is found.

Two hundred and eighty specimens of paradoxidids from two localities in the Holy Cross Mountains (Poland) have been reanalysed using morphometric techniques. Revision of the dataset provided evidence for the presence of two endemic taxa: Acadoparadoxides kozlowskii (Orłowski, 1959) and Acadoparadoxides samsonowiczi (Orłowski, 1959), both belonging to the earliest group of Acadoparadoxides, initially considered to be present only in Gondwanan successions. Thus, this is the first description of the members of this group outside West Gondwana. The taxonomic revision, and the presence of representatives of the earliest acadoparadoxidines coupled with the absence of taxa typical of Scandinavia that were previously described from this locality have resulted in the modification of the biostratigraphic scheme hitherto used in the Holy Cross Mountains. The newly established A. samsonowiczi – A. kozlowskii Assemblage Zone is correlated with the Morocconus notabilis Zone of Morocco, and thus represents the Cambrian Series 2 and 3 boundary interval. Links with West Gondwana challenge the existing palaeogeographic interpretations for the southern part of the Holy Cross Mountains and point to an urgent need to revise the position of the Małopolska Block during the middle Cambrian.

Trilobites are important elements of the Devonian macrobenthos; some of them were collected in the Chefar el Ahmar Formation, from two sections located near Béni Abbès in the Saoura Valley (Ougarta Basin, Saharan Algeria). This formation is characterized by alternations of claystones and limestones, and it is considered to be late Emsian to early Frasnian in age. Only the lower part of this formation has yielded trilobites so far; their presence has been known for a long time. Phacopines clearly dominate the trilobite assemblages, with Austerops, Barrandeops, Chotecops and Phacops s.l. as the main genera. Two new species are described (Austerops salamandaroides sp. nov. and Phacops ouarouroutensis sp. nov.), while some other taxa are presented in open nomenclature. Comparisons are made with closely allied species. These new trilobite occurrences have been analysed in terms of their intra- and interspecific variability and biodiversity. The occurrence of Struveaspis maroccanica, previously known from the Saoura Valley, provides an early Eifelian age, which is also confirmed by the presence of trilobites Thysanopeltis and Koneprusites, and ostracods Bairdiocypris devonica and Bufina ?subovalis.

The early stages in ontogenetic development are described in the co-occurring Sphaerophthalmus alatus (Boeck, 1838) and Ctenopyge (Mesoctenopyge) tumida Westergård, 1922, using fragmentary but otherwise well-preserved material from Bornholm, Denmark. The former species is a senior synonym of Ctenopyge (Eoctenopyge) angusta Westergård, 1922, as has been recently proposed, but the early stages of S. alatus from Bornholm are appreciably more spiny than those found of the same species in other places in Scandinavia, and spinosity is retained until later in development. Increasing convexity with development and other morphological changes are noted. Pygidia were previously unknown in C. (M.) tumida. Here, they are represented by early pygidia. These are shield-shaped, with a very spiny margin and a strong axial spine. The distinction between Sphaerophthalmus and Ctenopyge is discussed.

The Iranian territory is composed of a mosaic of tectonic units, several of which underwent in the Permian and Triassic periods a migration from northern Gondwana to southern Laurussia associated with the opening of the Neo-Tethys Ocean. Although this broad outline of Permo-Triassic palaeogeographical evolution of Iranian microplates is now widely accepted, the individual timing of migration of these blocks, and their biogeographical relationships, remain insufficiently known. Here I review the Late Palaeozoic record of trilobites in Iran and Armenia, and discuss their palaeobiogeographical affinities in an attempt to shed light on the Permian palaeogeographical evolution of Iranian and Armenian terranes. Seven Iranian or Armenian localities, representative of five tectonic units, have yielded Carboniferous and Permian trilobites. Ten species are recognized, including two new taxa, Persia praecox gen. nov. sp. nov. and Pseudophillipsia (s.l.) parvizii sp. nov. P. praecox is the only Carboniferous (Tournaisian) species. The others are Wordian to Wuchiapingian in age and can be separated into three morphological groups, probably representing clades. One is composed of representatives of Acropyge, while the two others (armenica-group and paffenholzi-group) comprise species of Pseudophillipsia. Only P. (s.l.) parvizii sp. nov. from the Zagros Mountains (Arabian Plate) is not attributed to one of these groups. The distribution of trilobites in Iran and Armenia strongly suggests that the Alborz, Central Iran and Transcaucasia microplates represented a single biogeographical unit in Middle and Late Permian times. Special relationships of this biochore with South China can also be stressed.

Recent investigation of the historical Mont d’Haurs section near Givet yielded numerous trilobites from Middle Devonian carbonate deposits. The trilobite fauna described in the present paper comprises a number of taxa from the basal Givetian, including two new ones: Dechenella givetensis sp. n.; Dechenella calxensis sp. n.; Dechenella ziegleri Struve, 1992; Goldius cf. capitellum (Archinal, 1994); and Nyterops cf. hollandi van Viersen, 2007a. This fauna confirms the previously recognised taxonomic diversity pattern and the Dechenella and Nyterops Association from the middle Middle Devonian in the Ardenne area.

A revision of the Lower Devonian dalmanitid trilobites of the Prague Basin (Czech Republic) is presented. The subfamily Odontochilinae Šnajdr is considered a synonym of Dalmanitidae Vogdes. Twenty-one previously and five newly described (three in open nomenclature) species and subspecies occur in the Prague Basin from the lowermost Pragian (one problematic specimen possibly comes from the uppermost Lochkovian) to the Lower Emsian; the last questionable record is from the Upper Emsian. The species have been assigned to four genera and subgenera: Odontochile Hawle & Corda, Reussiana Šnajdr, Zlichovaspis (Zlichovaspis) Přibyl & Vaněk and Zlichovaspis (Devonodontochile) Šnajdr. These trilobites are considered as scavengers or opportunistic predators, living most of their lives as vagrant benthos burying in the muddy substrate to find organic remains. The first undoubted adult-like meraspid specimens are described.

A new otarionine trilobite Otarionella rastrum sp. nov., from the late Emsian Mont Peyroux Formation (Montagne Noire, France), is described. Silicified remains, recovered from the underlying Bissounel Formation (early to late Emsian), are also attributed to this new species. These isolated silicified sclerites represent metaprotaspid to young holaspid growth stages, which enables the almost complete ontogeny of an otarionine trilobite with a spinose adult morphology to be described for the first time. Comparison with associated larval and juvenile growth stages of Cyphaspis reveal that the pattern of juvenile cranidial spine distribution in Otarionella rastrum sp. nov. differs from all patterns described so far in the Otarioninae, in particular that characterizing the tribe Otarionini. A second species, Otarionella lkomalii sp. nov., known only from a complete articulated specimen discovered in the early Eifelian of southern Morocco, is also described. Like the middle Eifelian Otarionella chamaeleo (Basse, 1997), this new species has only ten thoracic segments, with the fourth and the sixth segments each bearing a long axial spine. In the light of the new elements provided by the ontogenetic sequence of O. rastrum sp. nov. and the adult specimens of this species and O. lkomalii sp. nov., the putative synonymy of Otarionella and Otarion is rejected and a restricted concept of the genus Otarionella is defined.

A revision of the trilobite fauna from Brzechów in the Holy Cross Mountains (Poland) shows that the assemblage consists of Palaeolenus medius (Bednarczyk), Issafeniella orlowinensis (Samsonowicz), Kingaspidoides sanctacrucensis (Czarnocki), Paradoxides (Acadoparadoxides) oelandicus Sjögren, P. (A.) cf. mureroensis Sdzuy and P. (A.) sp. It is earliest Middle Cambrian in age according to the West Gondwana standard of Geyer & Landing (2004) and can be correlated with the Cephalopyge notabilis Zone of Morocco. Moreover, the co-occurrence of the ranges of P. (A.) oelandicus with P. (A.) cf. mureroensis might suggest that P. (A.) oelandicus appears earlier in the Brzechów sandstones than in Scandinavia, thus the strata exposed here do not find their correlative in the Scandinavian succession. As a result, the Banian/Agdzian boundary (Lower–Middle Cambrian boundary according to the West Gondwana standard) probably lies within the Kamieniec Formation, and also below the Brzechów sandstones, in the upper part of the Ociesęki Formation. Close similarity of the discussed assemblage with contemporaneous assemblages from Morocco, Jordan or New Brunswick indicates that the fauna is generally of peri-Gondwanan and Gondwanan aspect.

Numerous silicified and calcareous sclerites of various sizes, recovered from the latest Famennian of Thuringia (Germany), allow the description of the first complete growth series of a blind proetoid trilobite: Drevermannia richteri. In addition, the partial ontogenetic development of Drevermannia antecurvata sp. nov. and undetermined species, Drevermannia sp. 1, are described. The proetoid anaprotaspides, associated with D. richteri, illustrate that a marked increase in larval size occurred prior to the terminal Devonian extinction event. Considering the homogeneity of larval size in older Devonian proetoids, it is interpreted as evidence that the developmental strategy of these trilobites was significantly modified. Though largely speculative, two alternative hypotheses are proposed to explain this modification. Finally, all three ontogenetic sequences show that ocular structures never develop externally in Drevermannia, but also illustrate that the development of optical nerves is not completely lost in this group. This suggests that blindness in the Drevermannia lineage followed a centripetal mode of eye reduction.

A new aglaspidid arthropod, Chlupacaris dubia gen. et sp. nov., is described from the Pusgillian (lower Ashgill, Upper Ordovician) Upper Tiouririne Formation near Erfoud, southeastern Morocco. Although disarticulated, careful documenting of the tergites allows a reconstruction of the exoskeleton to be made. Although somewhat trilobite-like in appearance, the lack of facial sutures, a well-defined axis with articulating half-rings and a pygidium clearly prove Chlupacaris gen. nov. is not a trilobite. An interesting feature is the presence of a hypostome in this non-trilobite arthropod. In contrast to other aglaspidids usually considered to be carnivorous, a filter-feeding mode of life is proposed for Chlupacaris gen. nov., based on the strongly vaulted cephalon, subvertical orientation of the hypostome and less strongly vaulted trunk. Chlupacaris gen. nov. is probably most closely related to the atypical aglaspidid Tremaglaspis unite from the Tremadoc (Lower Ordovician) of the U.K., but it can also be tentatively linked to the problematic Lower Cambrian arthropods Kodymirus vagans and Kockurus grandis from the Czech Republic. The relevance and validity of previous definitions and of possibly significant characters used for identifying aglaspidids are evaluated, and as a result, a new combination of characters diagnosing Aglaspidida is proposed. Contrary to previous reports, it is suggested that aglaspidids are probably more closely related to trilobites than they are to chelicerates. This notion may be supported by the shared possession of a mineralised cuticle, a possibly similar number of cephalic appendages, and the presence of a hypostome in some forms, although this last character may alternatively be homoplastic.

The Notch Peak Formation (Late Cambrian, Sunwaptan) of western Utah yields diverse silicified trilobite faunas that provide new information on the anatomy of many taxa. The family Ptychaspididae Raymond, 1924, is represented by species of Keithiella Rasetti, 1944; Idiomesus Raymond, 1924; Euptychaspis Ulrich in Bridge, 1931; and Macronoda Lochman, 1964. At least four species are new, of which E. lawsonensis and M. notchpeakensis are named formally. Much previous work on Late Cambrian trilobites has emphasized biostratigraphic utility and the recognition of geographically widespread species. Data from new silicified collections indicate that this approach is difficult to justify because many putative ‘index species’ actually represent a plexus of closely related species whose biostratigraphic significance has yet to be determined. One such plexus is represented by E. kirki Kobayashi, 1935, whose previously reported occurrences in Texas, Oklahoma, Utah, Nevada and northern Canada record at least four distinct species. Similarly, Macronoda can now be shown to consist of at least five late Sunwaptan species in south-central and western North America.

A new Late Cambrian trilobite–brachiopod fauna from the Kurchavinskaya Formation, Severnaya Zemlya, northern Siberia, allows correlation of the Ketyi Horizon of the NW Siberian succession with the praecursor Zone of the Baltic olenid zonation. The presence on Severnaya Zemlya of the typically Siberian trilobite Kujandaspis ketiensis indicates that even if Severnaya Zemlya lay on a separate plate, whether Kara or Arctida as postulated by other authors, then it was still probably not far from Siberia. However, the associated brachiopods are partly endemic to Severnaya Zemlya, thus giving some support to the independent palaeomagnetic evidence for their origin on a plate separate from Siberia.

This present revision of Bohemian Ordovician illaenid trilobites of the Prague Basin (Czech Republic) follows the recent evaluation of their systematic position. This revision contains redescriptions and illustrations of the type species of several illaenid genera, the most up-to-date references for those genera, and an assessment of their variability and also of the palaeobiogeographic distribution of some illaenid genera. Bohemian illaenids in several cases include the type species of globally distributed taxa, whose distinction has often been uncertain and controversial. The validity of the genera Zbirovia Šnajdr, 1956 and Zdicella Šnajdr, 1956 is discussed and confirmed. The genus Alceste Hawle et Corda, 1847 stays in open nomenclature. In Ectillaenus katzeri katzeri (Barrande, 1856) allometric growth is described and Vysocania Vaněk & Vokáč, 1997 is considered to be a junior synonym of Stenopareia Holm, 1886.

Dionidid and raphiophorid trilobites are described from the middle Ordovician (Viruan Series) of Jämtland, central Sweden. The family Dionididae is represented by one genus, Dionide Barrande, 1847, and the family Raphiophoridae by five genera, Ampyx Dalman, 1827, Ampyxella Dean, 1960, Ampyxoides Whittington, 1965, Cnemidopyge Whittard, 1955 and Lonchodomas Angelin, 1854. Seven species are described and discussed. Three of these are new: Dionide jemtlandica sp. nov., Ampyxoides minor sp. nov. and Lonchodomas striolatus sp. nov.. The majority of the trilobites described are from the Andersön Shale, equivalent to the H. teretiusculus, N. gracilis and lowermost D. multidens Zones (upper Llanvirn and lower Caradoc), and the Örån Shale, equivalent to the lower D. dingani and the uppermost D. multidens Zones (middle Caradoc). Meraspid and early holaspid material of D. jemtlandica is described.

The Lower Cambrian Sirius Passet fauna from Peary Land, North Greenland, is a rich repository of soft-bodied and poorly-sclerotised fossils. A new arthropod from the fauna, Kleptothule rasmusseni, is described. The animal is broadly trilobite-like, possessing a trilobed exoskeleton which is divided into distinct cephalic, thoracic and caudal regions. However, it is unusual in that it possesses a large number of segments, and demonstrates pronounced cephalic segmentation, and a very narrow cephalon and thorax. There is some evidence that the exoskeleton was lightly mineralised.

Kleptothule is compared to some of the olenellimd trilobites, especially those taxa that possess a many-segmented ‘opisthothorax’. Its morphology raises some issues discussed by Lauterbach (1983) in his assignment of some olenellids to the stem-group of the chelicerates. However, it is not considered herein that such a model can be supported. A complete analysis of basal trilobites and the stem-group leading to them must await a fuller description of key taxa from China and Greenland.