A new phylogeny of cyclopygid trilobites (Superfamily Cyclopygacea Raymond, 1925) is presented, based on an analysis of 47 genera and 63 characters. The majority-rule consensus supports a paraphyletic Taihungshaniidae Sun, 1931, basal to two clades united by the loss of pygidial spines: Nileidae Angelin, 1854, united by a hypostomal notch in the cephalic doublure, a sub-ocular socle and a pointed posterior margin of the hypostome, excluding Borthaspidella Rasetti 1954, Lakaspis Kobayashi, 1937, and Petrbokia P & rcaron;ibyl and Va & ncaron;ek, 1965; and Cyclopygidae Raymond 1925, united by hypertrophied eyes and the loss of a pestle-shaped glabella, including Illaenopsis Salter, 1866a, a supposed nileid. Nileidae contains two clades, Parabarrandiinae Hup & eacute;, 1955 and Nileinae Angelin, 1854. Character transformations within cyclopygids indicate increased adaptations to a pelagic life habit and deeper depth of life habit, including increased eye size and their migration to the lateral margin of the cephalon, improving the field of vision, reduction in the number of thoracic segments, decrease in body size, overall improved streamlining, an increased length/width ratio and changes in the glabellar shape.

The affinities of the Tully monster (Tullimonstrum gregarium), from the late Carboniferous Mazon Creek Lagerstätte (~309 Ma) of Illinois, have been debated since its discovery. Tullimonstrum is up to ~35 cm long with a long proboscis ending in pincers and an elongate tapering body with caudal dorsal and ventral fins. This iconic evolutionary enigma or ‘weird wonder’ has been suggested to relate to arthropods, various ‘worms’, tunicates, conodonts, lancelets, vetulicolians and even vertebrates, but it may be a mollusc (Caenogastropoda) such as a pterotracheid (heteropod) pelagic gastropod, similar to Pterotrachea coronata (the ‘sea elephant’).

Carcinosomatoidea (Chelicerata: Eurypterida) are regarded as active benthic predators. Megalograptidae, Carcinosomatidae and Mixopteridae have long, spinose raptorial prosomal appendages for capturing and holding prey. Previous models for their mode of life are assessed, based on a review of their trace fossils, functional morphology and a new analysis of their biotic associations. They are here interpreted as slow-swimming, ambush predators and mud grubbers, using their spinose anterior appendages to capture epifaunal and infaunal prey, and their coxal gnathobases and metastoma to process it. Megalograptids tend to associate with, and likely specialised on, invertebrates such as trilobites. Carcinosomatids tend to associate with lightly armoured phyllocarid crustaceans and lingulid brachiopods. Mixopterids tend to associate with more heavily armoured prey, such as thelodonts, osteostracans and pteraspids, although their trackways also indicate a mud-grubbing mode of life. The prosomal appendages (number of podomeres), opisthosomal differentiation and pretelson cercal blades support the hypothesis that megalograptids were more basal than carcinosomatids and mixopterids, which has implications for the (overestimated) diversity of Ordovician eurypterids.

The diversity, ichnology, ecology and evolution of the euthycarcinoids, an enigmatic group of Cambrian to Triassic aquatic and amphibious mandibulate arthropods, is reviewed. Euthycarcinoids are stem myriapods and possibly evolved from juvenile fuxianhuiids via neotony; spawning in tidal pools, individuals acquiring sexual maturity early had a selective advantage. Euthycarcinoids first came onto land either to feed on early land plants, tidal infauna, algae or microbial mats or to scavenge dead animals washed up by the tide. Alternatively, it was to protect their eggs from aquatic predators or because they were left stranded on the tidal flats and migrated from pool to pool to maintain moisture. A new family, Apankuridae nov. is proposed for Cambrian taxa with a barrel-shaped preabdomen and six postabdominal segments. A Middle Cambrian putative euthycarcinoid from shallow lagoon or intertidal facies in Russia is reinterpreted as a phyllocarid crustacean. Cambrian and Siluro-Devonian euthycarcinoids average ca. 8 cm long, but taxa halve in size as they pass into the Carboniferous, probably due to competition. Potential synapomorphies of taxa include a middorsal crest or carina, different length tergites, axial and pleural lobes, preabdominal epimera, postabdominal epimera, setae on the abdominal appendages, smaller posterior appendages, a longer telson and terminal moveable telson spine. Sottyxerxids are interpreted as more closely related to myriapods than euthycarcinids, due to their multi-segmented trunk and homopodous legs; a long ghost range (Silurian to Carboniferous) is predicted by their relatively poor fossil record. Kampecarids are interpreted as intermediate between sottyxerxids and myriapods, based on their legless collum and tail segments.

Abstract. Palmichnium gallowayi (Sharpe, 1932) new combination from the Middle Ordovician Martinsburg Formation (proximal deltaic facies) of Rondout, near Kingston, New York State, is redescribed. It consists of opposing series of five tracks, the outer two large and pear-shaped, the inner three smaller and elliptical, arranged in a chevron converging in the direction of travel, on either side of a wide medial impression. It is attributed to a medium-sized stylonurid eurypterid using a decapodous gait, crawling onto the shoreline, traversing the intertidal zone, a behavior interpreted as part of its reproductive life cycle. This provides the earliest ichnological evidence for the ‘mass-molt-mate’ hypothesis, which proposes that eurypterids migrated en masse into nearshore environments to molt and mate.

An early form of eurypterids (Chelicerata), Archopterus anjiensis n. gen. n. sp., is described from the uppermost Ordovician Wenchang Formation of Anji County, Zhejiang Province, South China. It is the earliest record of eurypterids in China and likely the oldest representative of the Adelophthalmidae. The species, represented by a single specimen, is diagnosed by a ventrally preserved prosoma with a parabolic carapace, Hughmilleria-type prosomal appendages, a short vase-shaped metastoma, and a three-segmented type A (female) genital appendage. The specimen is preserved, together with diverse sponges, graptolites and occasional nautiloids, in a 10 m thick shale of marine deep-water setting. This finding extends the stratigraphic range of adelophthalmids from the previously recorded early Silurian into the Late Ordovician (some 10 million years older) and supports an earlier cryptic phase of eurypterid evolution in Gondwana.UUID: http://zoobank.org/a2d090e5-1a15-40b6-8e2a-dda45b01e94d

An unusual worm, previously interpreted as the earliest leech, is described from the early Silurian (Llandovery, Telychian) Brandon Bridge Formation Lagerstatte (Waukesha Biota) of Wisconsin (similar to 437 Ma). Lacking preserved internal organs, it is up to similar to 16 cm long, 8.2 mm wide, with similar to 250 annulations and a circular structure at one end, interpreted here as the broken end of a molt. It is therefore referred to Cycloneuralia incertae sedis.

A new arthropod resting trace, attributed to a phyllocarid crustacean, is described from the middle Cambrian (upper Miaolingian) Hickory Sandstone Member of the Riley Formation (Moore di igen. et isp. nov. is preserved as small, isolated, bilaterally symmetrical hypichnia comprising two regions. The wider anterior region shows imprints of antennae, a rostrum, five thoracopods (walking legs) and at least five pleopods. The narrower posterior region terminates in a bibbed (furca) imprint and terminal (telson) imprint. Three specimens of M. shieldi occur on the base of a sandstone slab, deposited in a tidal channel or tidal flat environment, associated with a juvenile bivalved phyllocarid carapace and worm burrow, interpreted as either the same individual repositioning itself or as seperate animals crowded together, trapped in a tidal pool. These fossils were previously interpreted as the ventral surface of a chasmataspidid (Euchelicerata), representing the oldest record of this clade; the oldest unequivocal euchelicerates are now Ordovician horseshoe crabs.

The Waukesha Biota (Brandon Bridge Formation Lagerstätte) is a diverse assemblage of early Silurian (437 Ma) shallow marine fossils from Wisconsin, USA. Their exceptional preservation, in finely laminated dolomitic mudstones, was mediated by tidally‐influenced conditions and/or microbial entombment. Shelly biomineralized animals are rare, but soft‐bodied organisms are common. Arthropods dominate, including trilobites, phyllocarids, ostracodes, a thylacocephalan, a synziphosurine and various enigmatic groups. The fauna also includes evolutionary ‘holdovers’, such as palaeoscolecids and a lobopodian.

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Pterygotid eurypterids (Chelicerata) were all regarded as active apex nektonic predators, particularly in the Silurian. The chelicerae and lateral eyes of Acutiramus cummingsi were then interpreted as being inconsistent with a predatory lifestyle. Jaekelopterus, Pterygotus and Erettopterus were interpreted as having more acute vision than Acutiramus, but their chelicerae implied they had different ecologies, with not all taxa as top predators. The chelicerae of Acutiramus are here interpreted as being much more robust than was previously assumed, and their eyes as adapted to hunting at depth, at night or in murky water, so neither precludes a predatory lifestyle. Previous models of the mode of life of pterygotid eurypterids are assessed, based on a review of their trace fossils, functional morphology and a new analysis of their biotic associations. All pterygotid taxa are here interpreted as slow swimming vagrant and ambush predators, using their chelicerae to rapidly capture prey, and their more robust coxal gnathobases and metastoma to process it. Slimonia, and Acutiramus with more gracile cutting chelicerae, tend to associate with, and likely specialised on, lightly -armoured phyllocarid crustaceans. Erettopterus and Pterygotus, with more robust chelicerae, tend to associate with more heavily armoured thelodonts and osteostracans, respectively. Jaekelopterus tend to associate with osteicthyans, placoderms and pteraspids. Praedichnia (predation traces) on the pteraspids Lechriaspis and Larnovaspis, eurypterids Eurypterus and Acutiramus and trilobite Spinisscutellum, and their coprolites, provide further evidence for such predatory interactions. The cheliceral morphology, visual acuity, associations and fossil record all support the hypothesis that Acutiramus is actually more basal to Jaekelopterus and Pterygotus, the largest ever arthropods, with Jaekelopterus now estimated at 2.59 m long. center dot Key words: Arthropoda, Chelicerata, Eurypterida, Chelicerae, Palaeozoic, predation, gigantism.

ABSTRACT Hibbertopterus lamsdelli sp. nov., from the Late Carboniferous Kinney Quarry Lagerstätte of New Mexico (USA), is a large (ca. 1.1 m long) stylonurid eurypterid (sea-scorpion; Chelicerata), similar to H. scouleri from Scotland but with less serrate segment margins, a wider pretelson, shorter telson (tail-spine), and more parallel ventral keels. It is only the fourth, yet most reliable record of an American hibbertopterid. A taxonomic reassessment of Hibbertopterus regards Dunsopterus and Vernonopterus (but not Cyrtoctenus) as synonyms. Hibbertopterids were aquatic (benthic) scavengers and microphagous sweep-feeders, but their trackways indicate that they were capable of brief terrestrial, seasonal nuptial walks, despite their large size; Hibbertopterus had walking legs with spinose extensions at the base (Laden) to spread their load, and the ventral keels on their telson functioned like sled rails to reduce body drag. Hibbertopterids were interpreted as moving into freshwater during the Late Palaeozoic, but a trackway from the middle Permian Collingham Formation (Ecca Group) of South Africa may be from a marine setting, though further analysis is needed to fully evaluate this possibility.

Recent discoveries of trackways and trails on ancient tidal flats at Blackberry Hill, Wisconsin, USA, have transformed our understanding of the initial invasion of land, 500 Ma. Huge slug‐like molluscs grazed on microbial mats. Euthycarcinoid (stem myriapod) death traces (mortichnia) suggest that they did not come onto land to feed or breed, but simply to survive; Moon was closer to Earth then, and massive tides stranded animals in tidal pools that gradually dried up.

A critical review of the sea scorpion (eurypterid) fossil record (c. 250 species) indicates that c. 8 % of co- occurring species (and some genera) may be ontogenetic (juvenile) or preservational related synonyms. Eurypterella ornata MATTHEW, 1889 from the Late Carboniferous (Pennsylvanian: Langsettian) "Fern Ledges" locality of New Brunswick, Canada, is interpreted as a juvenile (c. 2 cm long) Adelophthalmus brasdorensis BELL, 1922, known from the Pictou Formation of nearby Campbelltown, as it preserves a median abdominal appendage, with lateral spatulae, and ten tubercles along the posterior margin of the carapace and tergites. A formula describing the relationship between a carapace with an apparent curved posterior margin (deflection from mid- line, D), its `observed' length (O), its actual length (L) and its height (H), relative to the angle (A??) that it was orientated on the substrate, is presented: Cos A = 0 / L; H = D / Sin A.

Parioscorpio venator Wendruff et al., 2020a from the early Silurian Waukesha biota of Wisconsin, USA, interpreted as the earliest scorpion, then a basal euarthropod, is reinterpreted here as a cheloniellid-like arthropod with large raptorial appendages. The diversity of Cheloniellida Broili, 1932 is reviewed. Drabovaspis complexa Chlupac, 1963, from the Upper Ordovician Letna Formation of Czechia, interpreted as an aglaspidid, then a xiphosuran, is also reinterpreted here as a cheloniellid.

The stylonurid eurypterid Leiopterella tetliei Lamsdell, Braddy, Loeffler, and Dineley, 2010 (Chelicerata: Eurypterida: Rhenopteridae) from the Early Devonian (Lochkovian) of Nunavut in Arctic Canada is redescribed. Restudy of the holotype under polarized light revealed a labrum, epistomal sutures, prosomal appendage III, and deltoid plates anterior to the genital appendage. An additional new specimen preserves the distal podomeres of appendage VI and gradually tapering opisthosomal tergites. The characters resolved here support the hypothesis that L. tetliei was relatively basal within the wider Stylonurina clade, with its tapering postabdomen supporting a more basal position within Rhenopteridae than previously suggested.

A new specimen of the eurypterid Adelophthalmus luceroensis KUES & KIETZKE, 1981, is described from the middle Missourian (Kasimovian; Late Pennsylvanian; late carboniferous) of the Kinney Quarry Lagerstatte of central New Mexico, U.S.A.. It reveals the enigmatic antero-medial triangular process (so-called 'doublure lock'), a possible labrum, chelicerae, and curled-up walking legs, enabling an emended diagnosis of the species. The taxonomy, respiratory palaeobiology and ecology of Adelophthalmus is reviewed: this longest-lived (Early Devonian to early Permian) and most diverse eurypterid genus is likely taxonomically oversplit and in need of revision; their paddles indicate that they were primarily aquatic (nektonic) predators and scavengers, but they were capable of brief terrestrial excursions to breed.

Examination of material held at the Palaeontological Institute, Moscow, enables the identification of two novel chasmataspidid species: Nahlyostaspis bergstroemi gen. et sp. nov. and Skrytyaspis andersoni gen. et sp. nov. ‘Eurypterus’ stoermeri and ‘Tylopterella’ menneri are both redescribed as chasmataspidids, having previously been assigned to Eurypterida. ‘T’. menneri is transferred to the new genus Dvulikiaspis gen. nov. An identical prosomal structure is identified in ‘Eurypterus’ stoermeri and Heteroaspis novojilovi from the Devonian of Germany and the two species are synonymized, with ‘E’ stoermeri having priority. The previous synonymy of H. novojilovi with Diploaspis casteri is rejected. The presence of ophthalmic ridges is confirmed within Diploaspididae, and new structural characteristics of their bucklers are identified.

Cope's rule defines lineages that trend towards an increase in body size through geological time. The trilobite family Asaphidae is one of the most diverse of the class Trilobita and ranges from the Upper Cambrian through to the Upper Ordovician. The group is one trilobite clades that displays a large size range and contains several of the largest trilobite species. Reduced major axis correlations between the lengths of cephala and pygidia and the total sagittal length of complete individuals have high support and were used to standardise all incomplete specimens to total axial length. Phylogenetic studies into Cope's rule tend to use supertrees, composite trees or a single tree selected through a fit criterion. Here, for the first time, all trees recovered from a maximum parsimony analysis were analysed equally. Maximum likelihood was used to fit four evolutionary models: random walk, directional, Ornstein–Uhlenbeck (evolution towards an adaptive optimum) and stasis. These were compared equally using Akaike weights. Fitting of evolutionary models by maximum likelihood supports stasis as consistently the most likely model across all trees with low support for directionality.