The family Emuellidae Pocock, 1970 was established for Emuella Pocock, 1970 and Balcoracania Pocock, 1970 from the Lower Cambrian of South Australia. Based on their peculiar trunk tagmosis, emuellids have been interpreted as the sister group of all other trilobites with dorsal facial sutures, and classified as high as the ordinal level. Cladistic analysis with a range of exemplar taxa of the Olenellina and Redlichiina instead resolves the emuellids within the Redlichiina, with tagmosis into a prothorax and opisthothorax ("telosoma") nonhomologous in olenellines and emuellids. A taxonomic revision of Australian species identifies Balcoracania flindersi as a junior subjective synonym of B. dailyi, whereas the two named species of Emuella are considered to be distinct. Balcoracania dailyi possesses up to 103 thoracic segments, the maximum number recorded in any trilobite

Recent fossil discoveries from the lower Cambrian Emu Bay Shale (EBS) on Kangaroo Island, South Australia, have provided critical insights into the tempo of the Cambrian explosion of animals, such as the origin and seemingly rapid evolution of arthropod compound eyes, as well as extending the geographical ranges of several groups to the East Gondwanan margin, supporting close faunal affinities with South China. The EBS also holds great potential for broadening knowledge on taphonomic pathways involved in the exceptional preservation of fossils in Cambrian Konservat-Lagerstätten. EBS fossils display a range of taphonomic modes for a variety of soft tissues, especially phosphatization and pyritization, in some cases recording a level of anatomical detail that is absent from most Cambrian Konservat-Lagerstätten.

Radiodonts are nektonic stem-group euarthropods that played various trophic roles in Paleozoic marine ecosystems, but information on their vision is limited. Optical details exist only in one species from the Cambrian Emu Bay Shale of Australia, here assigned to Anomalocaris aff. canadensis We identify another type of radiodont compound eye from this deposit, belonging to 'Anomalocaris' briggsi. This ≤4-cm sessile eye has >13,000 lenses and a dorsally oriented acute zone. In both taxa, lenses were added marginally and increased in size and number throughout development, as in many crown-group euarthropods. Both species' eyes conform to their inferred lifestyles: The macrophagous predator A. aff. canadensis has acute stalked eyes (>24,000 lenses each) adapted for hunting in well-lit waters, whereas the suspension-feeding 'A.' briggsi could detect plankton in dim down-welling light. Radiodont eyes further demonstrate the group's anatomical and ecological diversity and reinforce the crucial role of vision in early animal ecosystems.

Trilobites are often considered exemplary for understanding the Cambrian explosion of animal life, due to their unsurpassed diversity and abundance. These biomineralized arthropods appear abruptly in the fossil record with an established diversity, phylogenetic disparity, and provincialism at the beginning of Cambrian Series 2 (∼521 Ma), suggesting a protracted but cryptic earlier history that possibly extends into the Precambrian. However, recent analyses indicate elevated rates of phenotypic and genomic evolution for arthropods during the early Cambrian, thereby shortening the phylogenetic fuse. Furthermore, comparatively little research has been devoted to understanding the duration of the Cambrian explosion, after which normal Phanerozoic evolutionary rates were established. We test these hypotheses by applying Bayesian tip-dating methods to a comprehensive dataset of Cambrian trilobites. We show that trilobites have a Cambrian origin, as supported by the trace fossil record and molecular clocks. Surprisingly, they exhibit constant evolutionary rates across the entire Cambrian, for all aspects of the preserved phenotype: discrete, meristic, and continuous morphological traits. Our data therefore provide robust, quantitative evidence that by the time the typical Cambrian fossil record begins (∼521 Ma), the Cambrian explosion had already largely concluded. This suggests that a modern-style marine biosphere had rapidly emerged during the latest Ediacaran and earliest Cambrian (∼20 million years), followed by broad-scale evolutionary stasis throughout the remainder of the Cambrian.

Two species of 'Anomalocaris' co-occur in the Emu Bay Shale (Cambrian Series 2, Stage 4) at Big Gully, Kangaroo Island. Frontal appendages of 'Anomalocaris briggsi' Nedin, 1995, are more common than those of 'Anomalocaris cf. canadensis' Whiteaves, 1892, at a quarry inland of the wave-cut platform site from which these species were originally described. An oral cone has the three large, node-bearing plates recently documented for 'Anomalocaris canadensis', confirming that 'Anomalocaris' lacks a tetraradial 'Peytoia' oral cone and strengthening the case for the identity of the Australian specimens as 'Anomalocaris'. Disarticulated anomalocaridid body flaps are more numerous in the Emu Bay Shale than in other localities, and they preserve anatomical details not recognized elsewhere. Transverse lines on the anterior part of the flaps, interpreted as strengthening rays or veins in previous descriptions of anomalocaridids, are associated with internal structures consisting of a series of well-bounded, striated blocks or bars. Their structure is consistent with a structural function imparting strength to the body flaps. Setal structures consisting of a series of lanceolate blades are similar to those of other anomalocaridids and are found in isolation or associated with body flaps. A single specimen also preserves putative gut diverticula. The morphology of the appendages, oral cone, gut diverticula and compound eyes of 'Anomalocaris', along with its large size, suggests that it was an active predator, and specimens of coprolites containing trilobite fragments and trilobites with prominent injuries have been cited as evidence of anomalocaridid predation on trilobites. Based on frontal appendage morphology, 'Anomalocaris briggsi' is inferred to have been a predator of soft-bodied animals exclusively and only 'Anomalocaris cf. canadensis' may have been capable of durophagous predation on trilobites, although predation (including possible cannibalism) by 'Redlichia' could also explain the coprolites and damage to trilobite exoskeletons found in the Emu Bay Shale.

An exceptionally preserved specimen of the horseshoe crab Euproops danae (Xiphosurida, Belinurina) in a siderite concretion from the Carboniferous (Upper Pennsylvanian, Virgilian) Lawrence Formation, Kansas, shows anatomical details of the prosomal musculature. The extrinsic appendicular muscles are comparable to those of Limulus polyphemus (the modern American horseshoe crab), demonstrating anatomical conservatism within Xiphosurida that spans two morphologically disparate subgroups, Belinurina and Limulina. The three-dimensional preservation of muscles highlights how siderite concretion fossils (including those of the Mazon Creek Konservat-Lagerstätte) have better preservational fidelity than previously realized and the potential to reveal new anatomical information, especially with regard to the labile tissues of euarthropods.

Durophagy arose in the Cambrian and greatly influenced the diversification of biomineralized defensive structures throughout the Phanerozoic. Spinose gnathobases on protopodites of Cambrian euarthropod limbs are considered key innovations for shell-crushing, yet few studies have demonstrated their effectiveness with biomechanical models. Here we present finite-element analysis models of two Cambrian trilobites with prominent gnathobases—Redlichia rex and Olenoides serratus—and compare these to the protopodites of the Cambrian euarthropod Sidneyia inexpectans and the modern American horseshoe crab, Limulus polyphemus. Results show that L. polyphemus, S. inexpectans and R. rex have broadly similar microstrain patterns, reflecting effective durophagous abilities. Conversely, low microstrain values across the O. serratus protopodite suggest that the elongate gnathobasic spines transferred minimal strain, implying that this species was less well-adapted to masticate hard prey. These results confirm that Cambrian euarthropods with transversely elongate protopodites bearing short, robust gnathobasic spines were likely durophages. Comparatively, taxa with shorter protopodites armed with long spines, such as O. serratus, were more likely restricted to a soft food diet. The prevalence of Cambrian gnathobase-bearing euarthropods and their various feeding specializations may have accelerated the development of complex trophic relationships within early animal ecosystems, especially the ‘arms race' between predators and biomineralized prey.

Abundant material from a new quarry excavated in the lower Cambrian Emu Bay Shale (Kangaroo Island, South Australia) and, particularly, the preservation of softbodied features previously unknown from this Burgess Shaletype locality, permit the revision of two bivalved arthropod taxa described in the late 1970s, 'Isoxys communis' and 'Tuzoia australis'. The collections have also produced fossils belonging to two new species: 'Isoxys glaessneri' and 'Tuzoia sp.' Among the soft parts preserved in these taxa are stalked eyes, digestive structures and cephalic and trunk appendages, rivalling in quality and quantity those described from better-known Lagerstätten, notably the lower Cambrian Chengjiang fauna of China and the middle Cambrian Burgess Shale of Canada.

The central nervous system (CNS) presents unique insight into the behaviors and ecology of extant and extinct animal groups. However, neurological tissues are delicate and prone to rapid decay, and thus their occurrence as fossils is mostly confined to Cambrian Burgess Shale–type deposits and Cenozoic amber inclusions. We describe an exceptionally preserved CNS in the horseshoe crab Euproops danae from the late Carboniferous (Moscovian) Mazon Creek Konservat-Lagerstätte in Illinois, USA. The E. danae CNS demonstrates that the general prosomal synganglion organization has remained essentially unchanged in horseshoe crabs for >300 m.y., despite substantial morphological and ecological diversification in that time. Furthermore, it reveals that the euarthropod CNS can be preserved by molding in siderite and suggests that further examples may be present in the Mazon Creek fauna. This discovery fills a significant temporal gap in the fossil record of euarthropod CNSs and expands the taphonomic scope for preservation of detailed paleoneuroanatomical data in the Paleozoic to siderite concretion Lagerstätten of marginal marine deposits.