Abstract Xandarellida is a clade of artiopodan euarthropods known exclusively from the early Cambrian (Series 2, Stage 3) Chengjiang biota of South China, apart from the possible inclusion of Phytophilaspis Ivantsov, 1999 from the early Cambrian Sinsk Biota of Siberia. A rare euarthropod taxon represented by four specimens from the Emu Bay Shale (Cambrian Series 2, Stage 4) of Kangaroo Island, South Australia, represents the first record of Xandarellida from East Gondwana. A new monotypic genus, Austroxandarella n. gen., is based on A. poikar n. sp., being most readily distinguished by its elongate pygidium. Close affinities to Xandarella Hou et al., 1991 are suggested by a thorax of 10 tergites, with progressive elongation of the posterior three tergites relative to the seven anterior tergites of subequal length—this elongation representing the dorsoventral decoupling of biramous appendage pairs and tergites shared by all xandarellids. Discovery of an Australian xandarellid adds more support for biogeographic affinities between East Gondwana and South China in the early Cambrian. UUID: https://zoobank.org/8a0f56ec-6d12-49ee-8ffd-a8dca97df99f

The Emu Bay Shale (EBS) of South Australia is anomalous among Cambrian Lagerstätten because it captures anatomical information that is rare in Burgess Shale-type fossils, and because of its inferred nearshore setting, the nature of which has remained controversial. Intensive study, combining outcrop and borehole data with a compilation of >25,000 fossil specimens, reveals that the EBS biota inhabited a fan delta complex within a tectonically active basin. Preservation of soft-bodied organisms in this setting is unexpected and further underscores differences between the EBS and other Cambrian Lagerstätten. Environmental conditions, including oxygen fluctuations, slope instability, high suspended sediment concentrations, and episodic high-energy events, inhibited colonization of the lower prodelta by all but a few specialist species but favored downslope transportation and preservation of other largely endemic, shallow-water benthos. The EBS provides extraordinary insight into early Cambrian animal diversity from Gondwana. These results demonstrate how environmental factors determined community composition and provide a framework for understanding this unique Konservat-Lagerstätte.

Abstract Trilobites were notably flexible in the moulting behaviours they employed, producing a variety of moult configurations preserved in the fossil record. Investigations seeking to explain this moulting variability and its potential impacts are few, despite abundant material being available for study. We present the first quantitative study on moulting in a single trilobite species using a dataset of almost 500 moult specimens of Estaingia bilobata from the Cambrian (Series 2, Stage 4) Emu Bay Shale, South Australia. Specimens were categorized by moulting mode (Salter's or Sutural Gape) and their associated configurations, and their body proportions measured from both a museum collection (including a bycatch sample) and a randomly‐collected field sample. This enabled analysis of the proportion of E. bilobata specimens displaying the Sutural Gape and Salter's modes of moulting and their different configurations, and tests for association between moulting behaviour and body proportions. The results show a wide range of E. bilobata moulting configurations in all samples, suggesting that configurations represent definable instances in a largely continuous spectrum of variation. Analyses comparing body proportions of specimens showing the two modes of moulting were non‐significant, suggesting there is no true association between moulting behaviour and body proportion, except for a single significant result for body length. All results were relatively consistent between the museum and field samples. However, removing accessioned specimens from the museum sample brought results even further in line with the field sample, supporting the need for consideration of museum collection bias in palaeontological analyses.

Abstract Artiopoda was a diverse group of Palaeozoic euarthropods that proliferated in the early Palaeozoic, epitomized by the ubiquitous trilobites. Their possible phylogenetic position outside mandibulates and chelicerates offers the potential for understanding the evolution of euarthropods in more detail. However, this opportunity remains unexploited given that identification of deep‐splitting artiopodans remains to be fully explored. Here, we describe a new non‐trilobite artiopodan from the lower Cambrian Sirius Passet Lagerstätte, North Greenland. Thulaspis tholops gen. et sp. nov. is a large species with a broad, domed head shield, followed by a trunk consisting of 15 thoracic tergites and a small pygidium, giving the body an ovoid appearance when viewed dorsally. Thulaspis is distinctive with its rounded genae and anterior thoracic pleural tips, as well as short pleural spines posteriorly. A heart‐shaped hypostome with an anterior lobe is present. Appendages, partly obscured by the tergal skeleton, have many moderate length gnathobasic spines, and large flap‐like exopods with a fringe of small setae. Cladistic analyses recover Thulaspis as the sister taxon to Squamacula, a genus found in the Chengjiang and Emu Bay Shale biotas, in either a polytomy with a number of artiopodan taxa or as a sister group to all other artiopodans, indicating an important role in understanding the roots of artiopodan anatomy and evolution.

Abstract Malformed trilobite specimens present important insight into understanding how this extinct arthropod group recovered from developmental or moulting malfunctions, pathologies, and injuries. Previously documented examples of malformed trilobite specimens are often considered in isolation, with few studies reporting on multiple malformations in the same species. Here we report malformed specimens of the ellipsocephaloid trilobiteEstaingia bilobatafrom the Emu Bay ShaleKonservat-Lagerstätte(CambrianSeries 2, Stage 4) on Kangaroo Island, South Australia. Ten malformed specimens exhibiting injuries, pathologies, and a range of teratologies are documented. Furthermore, five examples of mangled exoskeletons are presented, indicative of predation onE. bilobata. Considering the position of malformed and normal specimens ofE. bilobatain bivariate space, we demonstrate that the majority of malformed specimens cluster among the larger individuals. Such specimens may exemplify larger forms successfully escaping predation attempts, but could equally represent individuals exhibiting old injuries that were made during earlier (smaller) growth stages that have healed through subsequent moulting events. The available evidence from the Emu Bay Shale suggests that this small, extremely abundant trilobite likely played an important role in the structure of the local ecosystem, occupying a low trophic level and being preyed upon by multiple durophagous arthropods. Furthermore, the scarcity of malformedE. bilobataspecimens demonstrates how rarely injuries, developmental malfunctions, and pathological infestations occurred within the species.

Two species of Radiodonta (stem-group Euarthropoda) from the Emu Bay Shale (Cambrian Series 2, Stage 4), Kangaroo Island, South Australia, are revised based on new field collections and insights from recent phylogenetic analyses and advances in knowledge of radiodonts globally. Anomalocaris briggsi Nedin, 1995, the most common Emu Bay Shale radiodont, is designated the type species of a new monotypic genus of Tamisiocarididae, Echidnacaris gen. nov. The less common species, previously identified as Anomalocaris aff. canadensis Whiteaves, 1892, is formally named Anomalocaris daleyae sp. nov. Oral cones are assigned to both Echidnacaris briggsi comb. nov. and A. daleyae based on that of the latter species being found in association with pairs of frontal appendages. The Echidnacaris briggsi oral cone is the best preserved for the family Tamisiocarididae; it is triradial, with three large plates and a more pervasive ornament of nodes than in any other known radiodont. Shared characters of the Echidnacaris and Anomalocaris oral cones add support for a sister group relationship between Tamisiocarididae and Anomalocarididae. Unique eye characters documented in E. briggsi, such as being sessile and encircled by an eye sclerite, are unknown in the other tamisiocaridids, Tamisiocaris and Houcaris, and are tentatively regarded as diagnostic for Echidnacaris. An ovate head element resembling that of Tamisiocaris borealis is assigned to E. briggsi, informed by the sister group relationship between these taxa. Isolated radiodont body flaps and sets of setal blades in the Emu Bay Shale cannot be confidently assigned to a species, although relative abundance suggests that many or most are likely E. briggsi. The inner attachment margin of the body flaps is sharply defined and may represent a suture at which flaps are shed in moulting. http://zoobank.org/urn:lsid:zoobank.org:pub:AEFDB294-AE8F-426D-9805-FC701798A986

The paleo-ecological settings of Burgess Shale-type (BST) fossils have been the subject of extensive investigations, particularly with regard to the role of anoxia in their exceptional preservation. Our understanding of the complexity of both paleontological and geochemical environmental proxies has evolved with increased knowledge of niche communities and marine geochemical and biogeochemical processes. Accordingly, dated approaches should be reevaluated. The commonly applied paleoredox proxies V/Cr, Ni/Co, and V/(V + Ni), as well as the redox proxy V/Al2O3 and productivity proxy Ba/Al2O3, were all tested on the Wheeler Formation (Utah, USA) for which a fluctuating oxycline environment has been well established based on paleontological features. Subsequently, these proxies were applied to three additional BST fossil-bearing deposits: the Emigsville Member of the Kinzers Formation (Pennsylvania, USA), the Burgess Shale (British Columbia, CA) and the Emu Bay Shale (South Australia, AU). Their host shales lack perceptible V of marine origin, indicative of oxic conditions, in contrast to the Wheeler and brine-related strata from the Burgess Shale which include additional marine input of V in some beds, consistent with varying redox conditions. The black shales of the Longs Park Member (Kinzers) have elevated V/Al2O3 indicative of anoxic conditions. Molybdenum concentrations are relatively low in most samples, but minor variations tend to correlate with V/Al2O3 patterns. Ba/Al2O3 indicates high productivity with deposition of BST fossils in the Wheeler Formation, and diagenetic remobilisation of Ba owing to sediment anoxia in the Burgess Shale. Our findings show that redox proxy V/Al2O3 can be effectively applied to BST samples, whereas Ba/Al is a reliable proxy for low to moderate paleoproductivity levels and may be useful as a secondary or tangential proxy for redox conditions. Taphonomic processes leading to BST preservation occur in a variety of environments characterised by oxyclines, matgrounds, variable sedimentation rates, hypersalinity and/or anoxia. KEY POINTS V/Al2O3 and Ba/Al2O3 are effective proxies for anoxia and productivity in BST deposits. BST preservation can occur in a variety of depositional environments. Marine anoxia is not required for BST preservation.

Cambrian carnage: Trilobite predator-prey interactions in the Emu Bay Shale of South Australia

The exceptional fossil record of trilobites provides our best window on developmental processes in early euarthropods, but data on growth dynamics are limited. Here, we analyse post-embryonic axial growth in the Cambrian trilobite Estaingia bilobata from the Emu Bay Shale, South Australia. Using threshold models, we show that abrupt changes in growth trajectories of different body sections occurred in two phases, closely associated with the anamorphic/epimorphic and meraspid/holaspid transitions. These changes are similar to the progression to sexual maturity seen in certain extant euarthropods and suggest that the onset of maturity coincided with the commencement of the holaspid period. We also conduct hypothesis testing to reveal the likely controls of observed axial growth gradients and suggest that size may better explain growth patterns than moult stage. The two phases of allometric change in E. bilobata, as well as probable differing growth regulation in the earliest post-embryonic stages, suggest that observed body segmentation patterns in this trilobite were the result of a complex series of changing growth controls that characterized different ontogenetic intervals. This indicates that trilobite development is more complex than previously thought, even in early members of the clade.

The Ediacaran–Cambrian transition was a time of profound reorganisation of the biosphere, coinciding with the assembly of Gondwana and increasing atmospheric and oceanic oxygenation. The lower Cambrian marine sediments of the Stansbury Basin, South Australia, were deposited at low northern paleolatitudes on the western margin of the emerging Pacific Ocean. Here we report results of a multi-pronged investigation employing trace and rare earth element (REE) abundances, total organic carbon (TOC) contents and pyritic sulfur isotopic compositions (δ34Spy) in carbonaceous shales from three formations within the Normanville and Kanmantoo groups (sequences Є1.2 to Є2.2). TOC ranges from ≤0.5 wt% in the Emu Bay Shale, through 0.4–1.9 wt% in the Talisker Formation, to 0.2–2.6 wt% in the Heatherdale Shale. Covariance is demonstrated between trace elements and organic matter, with comparative uranium enrichment in the Heatherdale Shale likely linked to increased primary productivity. Heavy REEs and yttrium are typically more depleted than light REEs. Provenance appears to evolve through the upper Normanville sediments into the Kanmantoo Group, corroborating published detrital zircon interpretations that suggest an increasing dominance of southerly derived sources. The prevailing paleoredox regime for the Heatherdale Shale and the Talisker Formation was dysoxic, evolving into progressively more reducing conditions up section, the latter exhibiting a secular decline in δ34Spy (from +10 to −11‰ VCDT). Conversely, redox proxies concur in demonstrating that the basal fossiliferous Emu Bay Shale accumulated beneath an oxic water column. Comparison of trace element and REE distributions with those of slightly older sequences on the Yangtze Platform, South China, reveals striking similarities, implying that the trace element chemistry of the Paleo-Pacific and Asian oceans was homogeneous. KEY POINTS Trace and rare earth element, total organic carbon and sulfur isotopic data provide a multi-faceted profile of shales from three formations in the Stansbury Basin. The prevailing paleoredox regime for the Heatherdale Shale and Talisker Formation was dysoxic, whereas the basal Emu Bay Shale accumulated beneath an oxic water column. The strong covariance between TOC and normalised Mo, V and U in these shales highlights the evolving bioproductivity and oxygenation of the early Cambrian ocean. Similar element signatures in lower Cambrian black shales, South China, reflect the homogeneous biogeochemistry of the Paleo-Pacific and Asian oceans.

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.

In the recent publication by Caron & Aria (2020), the proposed new family-group name ‘Teratopodidae’ is not available because it is not formed from the stem of an available generic name (ICZN 1999; Art. 11.7.1.1). A replacement name is proposed here, along with corrected Remarks. urn:lsid:zoobank.org:act:41E7E111-B628-42DC-AE3A-0290D5F25DB3 After the type genus Collinsovermis. Acinocricus Conway Morris & Robison, 1988, unnamed form from Emu Bay Shale (García-Bellido et al. 2013), Collinsium Yang et al., 2015, Collinsovermis Caron & Aria, 2020. Luolishaniid lobopodians with stout bodies and paired spinules on anterior lobopods up to c. 2.5 times as long as lobopod diameter, and spaced apart c. 1/3 of lobopod diameter. Collinsovermidae differ from other luolishaniid lobopodians by the length and spacing of their appendicular spinules, as diagnosed above. The need to create a familial-level rank arises with the demonstrated cladistic grouping of Collinsovermis,Acinocricus and Collinsium, which is helpful in discussing diversity across an ever larger luolishaniid clade. The extent of the presence of long and elongate dorsal spines is another potential character that could inform the monophyly of this group or a sub-group within Collinsovermidae, since spines at least twice as long as body diameter in Luolishania are present only on a few post-frontal somites, possibly from somite 2 to somite 4, whereas they cover most of the body in Acinocricus,Collinsium and Collinsovermis. The seemingly shorter spine length in the EBS taxon and its incompleteness cast doubt on the validity of this character, although it may mean that the EBS form occupies a basal position within Collinsovermidae. Likewise, we identified that an antero-lateral change in spine number per set characterised Acinocricus,Collinsovermis and, less markedly, Collinsium, but we cannot be sure at present whether this trait is diagnostic of Collinsovermidae as a whole. This published work and the nomenclatural act it contains, have been registered in ZooBank: http://zoobank.org/References/D5551921-7DCA-4B7E-B8B1-85FF5EBD711D

As a classic Burgess-Shale-Type Konservat-Lagerstätte, the Guanshan biota is well known for its abundant soft-bodied fossils with high taxonomic diversity. Here, we report a new locality of the Guanshan biota in the Kunming area, Yunnan, South China with several new forms being first documented. In particular: a naraoiid euarthropod, the dorsal exoskeleton of which has characters of both naraoiids and Emucaris; and a discoidal animal, identified as a small eldonioid-like metazoan, which has a counterpart from the Emu Bay Shale Lagerstätte of South Australia, and which further strengths the biogeographic connection between the early Cambrian biotas of South China and Australia. Our new discoveries not only expand the palaeogeographic distribution of the Guanshan biota, but also reveal that its biodiversity is much higher than previously thought, and can be justifiably regarded as a community successor to the Chengjiang biota.Jun Zhao [zhaojunpaleo@126.com], Yunnan Key Laboratory for Palaeobiology, Institute of Palaeontology, Yunnan University, Kunming, Yunnan, 650091, China; Yunnan Key Laboratory of International Rivers and Transboundary Eco-Security, Yunnan University, Kunming 650091, China; MEC International Joint Laboratory for Palaeobiology and Palaeoenvironment, Institute of Palaeontology, Yunnan University, Kunming, Yunnan, 650091, China; Yujing Li [yujingli@ynu.edu.cn], Yunnan Key Laboratory for Palaeobiology, Institute of Palaeontology, Yunnan University, Kunming, Yunnan, 650091, China; MEC International Joint Laboratory for Palaeobiology and Palaeoenvironment, Institute of Palaeontology, Yunnan University, Kunming, Yunnan, 650091, China; Paul A. Selden [selden@ku.edu], Department of Geology, University of Kansas, 1457 Jayhawk Boulevard, Lawrence, KS 66045, USA; Natural History Museum, Cromwell Road, London SW7 5BD, UK; Peiyun Cong [cong@ynu.edu.cn], Yunnan Key Laboratory for Palaeobiology, Institute of Palaeontology, Yunnan University, Kunming, Yunnan, 650091, China; MEC International Joint Laboratory for Palaeobiology and Palaeoenvironment, Institute of Palaeontology, Yunnan University, Kunming, Yunnan, 650091, China.

Abstract Trilobites are one of the most diverse and abundant fossil groups from the early Palaeozoic, and as such are useful for answering important questions about early animal evolution, including developmental processes. Ontogenetic information for a large number of trilobite species has been published, but cases where multiple articulated specimens are known across the full range of developmental stages are rare. The early Cambrian (Series 2, Stage 4) Emu Bay Shale biota from Kangaroo Island (South Australia) is numerically dominated by trilobites, particularly articulated specimens of the ellipsocephaloid Estaingia bilobata, which are present in densities of >600 individuals per square metre on certain bedding planes. Here we describe the essentially complete post‐embryonic ontogenetic series of E. bilobata from the Emu Bay Shale, and investigate patterns of growth relating to articulation and segmentation in this early Cambrian arthropod. Estaingia bilobata exhibits the hypoprotomeric mode of growth, with the epimorphic phase (the cessation of trunk segment generation) reached prior to the onset of the holaspid period. The meraspid pygidium had an extended equilibrium period in which anterior segment release into the thorax was matched by subterminal segment generation. Previously undocumented morphological features of E. bilobata, including the hypostome and bispinose pleural tips in holaspides, are also described. The growth characteristics and morphological features of E. bilobata documented herein strengthen close phylogenetic relationships between the Estaingiidae, Ellipsocephalidae and Xystriduridae.

The fossiliferous lower Cambrian (Series 2) successions along the north coast of Kangaroo Island, South Australia—known collectively as the Kangaroo Island Group—can be divided into two main areas: a western succession located between Snelling Beach and Smith Bay, which comprises the Mt McDonnell Formation (base), Stokes Bay Sandstone and Smith Bay Shale; and an eastern succession that extends from Emu Bay to Point Marsden, represented by exposures of the White Point Conglomerate (base), Marsden Sandstone, Emu Bay Shale and Boxing Bay Formation (top), with an overlap in the Cape d’Estaing/Emu Bay area. Some previous interpretations of the Kangaroo Island Group stratigraphy suggest that the western succession stratigraphically underlies the eastern succession. Most of the previously reported and described Cambrian fossils come from the eastern succession, especially the internationally significant Emu Bay Shale Konservat-Lagerstätte. Here we report the trilobites Redlichia takooensis and Balcoracania dailyi from the Smith Bay Shale near Freestone Creek, indicating that the eastern and western successions are at least partly contemporaneous. The present investigation indicates that the Smith Bay Shale of the western succession can be correlated with the stratigraphic interval represented by the Marsden Sandstone and the overlying Emu Bay Shale of the eastern succession. The Kangaroo Island Group was deposited as part of a fan delta system with the eastern succession representing the proximal part, and the western succession representing the distal part of the fan delta. The lack of substantial conglomerate units within the Kangaroo Island Group to the west of Cape d’Estaing suggests that the tectonic uplift that led to the deposition of the White Point Conglomerate was concentrated in the area immediately to the north of Emu Bay.

The trilobite Redlichia Cossmann, 1902 is an abundant element of the lower Cambrian (Series 2, Stage 4) Emu Bay Shale (EBS) Konservat-Lagerstätte on Kangaroo Island, South Australia. Well-preserved, fully articulated specimens from this deposit are known to reach lengths of up to 25 cm, representing one of the largest known Cambrian trilobites. Until now, all Redlichia specimens from the EBS have been referred to Redlichia takooensis Lu, 1950, a species originally described from South China. Previous work recognized considerable differences in exoskeletal morphology among specimens of varying sizes, which was attributed to ontogeny. However, close examination of a large collection of recently acquired specimens shows that this variation actually represents two distinct morphs, interpreted here as separate species: R. takooensis, and a large, new species, Redlichia rex sp. nov. An analysis of morphological variation in holaspides (‘adults’) of the more common R. takooensis reveals considerable ontogenetic change occurred even during this later phase of growth. Some specimens of both Redlichia species from the EBS also exhibit exceptionally preserved soft-part anatomy, particularly the antennae and biramous appendages. Here, appendages (antenniform and biramous) and digestive structures are described, and biramous appendage reconstructions of R. rex sp. nov. are presented, which show a striking resemblance to some early Cambrian trilobites from South China. In particular, R. rex has a tripartite exopodite, as well as a dorsoventrally deep protopodite with gnathobasic spines used to shred or crush food items. Based on recent phylogenetic analyses, it is possible that an exopodite with tripartite subdivisions represents the plesiomorphic condition for Artiopoda (trilobites and kin). The digestive system of R. takooensis exhibits a series of paired digestive glands in the cephalon and anterior thorax, similar to those described for a number of other Cambrian and Ordovician trilobites.http://zoobank.org/urn:lsid:zoobank.org:pub:507BEAFC-4AFA-43F4-A5C4-49E4B58C658E

Chancelloria australilonga sp. nov. is described from the Emu Bay Shale (Cambrian Series 2, Stage 4) of Kangaroo Island, South Australia. The new species has a typical sac-like body, equipped with a meshwork of dozens of distinctive rosette-like sclerites. Each sclerite comprises one central ray and six or seven lateral rays, and is characterized by a pair of remarkably long and subparallel adapical rays. Chancelloria australilonga is one of the few chancelloriid species preserved with an apical orifice with an obvious peripheral tuft. The tuft consists of a series of single-rayed sclerites with inflated bases. The preservation of the specimens, coupled with sedimentological information and their general rarity in the Emu Bay Shale, suggests short-distance transport and subsequent deposition in a nearshore, low-oxygen preservational trap. Additionally, the known distribution of chancelloriids in the well-documented Konservat-Lagerstätten representing a variety of sedimentary environments, indicates that chancelloriids, especially the genus Chancelloria, were likely adapted to an extensive range of habitats.http://zoobank.org/urn:lsid:zoobank.org:pub:E46167B4-186A-4521-B3D7-DC3F48F72A30

We describe the exceptionally well-preserved non-trilobite artiopodan Zhiwenia coronata gen. et sp. nov. from the Cambrian Stage 3 Xiaoshiba Lagerstätte in Yunnan, China. The exoskeleton consists of a cephalic shield with dorsal sutures expressed aslateral notches that accommodate stalked lateral eyes, an elongate trunk composed of 20 tergites-the first of which is reduced-and a short tailspine with marginal spines. Appendicular data include a pair of multi-segmented antennae, and homonomous biramous trunk limbs consisting of an endopod with at least seven podomeres and a flattened exopod with lamellae. Although the presence of cephalic notches and a reduced first trunk tergite invites comparisons with the petalopleurans Xandarella, Luohiniella and Cindarella, the proportions and exoskeletal tagmosis of Zhiwenia do not closely resemble those of any major group within Trilobitomorpha. Parsimony and Bayesian phylogenetic analyses consistently support Zhiwenia as sister-taxon to the Emu Bay Shale artiopodan Australimicola spriggi, and both of them as closely related to Acanthomeridion from the Chengjiang. This new monophyletic clade, Protosutura nov., occupies a basal phylogenetic position within Artiopoda as sister-group to Trilobitomorpha and Vicissicaudata, illuminates the ancestral organization of these successful euarthropods, and leads to a re-evaluation of the evolution of ecdysial dorsal sutures within the group.

Trilobites dominate the Emu Bay Shale (EBS) assemblage (Cambrian Series 2, Stage 4, South Australia) in terms of numbers, with Estaingia bilobata Pocock 1964 being extremely abundant, and the larger Redlichia takooensis Lu 1950, being common. Many specimens within the EBS represent complete moulted exoskeletons, which is unusual for Cambrian fossil deposits. The abundance of complete moults provides an excellent record that has allowed the recognition of various recurrent moult configurations for both species, enabling the inference of movement sequences required to produce such arrangements. Moult configurations of E. bilobata are characterized by slight displacement of the joined rostral plate and librigenae, often accompanied by detachment of the cranidium, suggesting ecdysis was achieved by anterior withdrawal via opening of the cephalic sutures. Moulting in R. takooensis often followed the same method, but configurations show greater displacement of cephalic sclerites, suggesting more vigorous movement by the animal during moulting. Both species also show rare examples of Salter's configuration, with the entire cephalon anteriorly inverted, and several other unusual configurations. These results indicate that moulting in trilobites was a more variable process than originally thought. In contrast, other Cambrian Konservat-Lagerstätten with an abundance of trilobites (e.g. Wheeler Shale, USA, and Mount Stephen Trilobite Beds, Canada) show larger numbers of ‘axial shields’ and isolated sclerites, often interpreted as disarticulated exuviae. This points to a higher level of disturbance from factors, such as animal activity, depositional processes or water movement, compared to that of the EBS, where quiescent conditions and intermittent seafloor anoxia contributed to an unparalleled trilobite moulting record.

Abstract Rare specimens of eldonioids recovered from the lower Cambrian (Series 2, Stage 4) Emu Bay Shale (EBS) Konservat-Lagerstätte represent the first record of the group for the Cambrian of East Gondwana. The disc-shaped body of the EBS taxon bears fine concentric corrugations on the dorsal surface and, ventrally, a series of internal lobes that have primary and secondary bifurcations, as well as a coiled sac. It appears to be most similar toRotadiscusandPararotadiscusof the Cambrian Chengjiang and Kaili biotas of South China, respectively. While the structure of the internal lobes would indicate that this occurrence in the EBS represents a new taxon within the Rotadiscidae, lack of detail regarding the precise number of internal lobes and the condition of the circumoral tentacles warrants a more conservative approach in leaving the genus and species under open nomenclature. The EBS specimens also host trace fossils, including the remains of a burrow, which are generally lacking in the body-fossil-bearing layers of the Konservat-Lagerstätte interval. These traces appear to have been made by small organisms and are similar to traces associated with the discs ofPararotadiscus guizhouensis(Zhao and Zhu, 1994) from the Kaili Biota. The available taphonomic, paleoenvironmental, and ichnological evidence indicates that the EBS eldonioids are most likely vagrants that were transported or settled into the ‘preservational trap’ and subsequently exposed on the substrate for a brief period before burial, thereby allowing organisms to exploit their carcasses for nutrients or other purposes.