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Exceptionally preserved fossil eggs and embryos provide critical information regarding paleoembryogenesis, reproductive strategies, and the early ontogeny of early arthropods, but the rarity of preservation of both eggs and egg-bearing organisms in situ limits their use in detailed evolutionary developmental (evo-devo) studies. Burgess Shale-type deposits preserve rare instances of egg-bearing arthropods as carbonaceous compressions; however, the eggs are usually poorly preserved with no compelling evidence of embryos. We describe the first record of a brooding specimen of Waptia cf. W. fieldensis from the Spence Shale, a Cambrian (Wuliuan Stage) Burgess Shale-type deposit in northeastern Utah and southeastern Idaho. This is the first record of an egg-bearing arthropod from the Spence Shale and it exhibits two distinct modes of preservation among eggs within the single clutch: carbonization and phosphatization. Unlike the egg-bearing Burgess Shale specimens, many eggs of this Utah specimen are also preserved three-dimensionally. In addition, synchrotron radiation X-ray tomographic microscopy reveals internal distributions of mineral phases, along with potential remnants of the egg membrane and attachment structures, but, as in the Burgess Shale, no explicit traces of developing embryos. The distinct modes of preservation highlight the existence of diagenetic microenvironments within some eggs, but not in others during fossilization.

Radiodonts (stem Euarthropoda) were ecologically diverse, but species generally displayed limited functional specialization of appendages along the body axis compared with crown group euarthropods. This is puzzling, because such functional specialization is considered to have been an important driver of euarthropod ecological diversification. One way to circumvent this constraint could have been the functional specialization of different parts of the frontal appendages, known to have been ecologically important in radiodonts. This hypothesis has yet to be tested explicitly. Here we redescribe the poorly known mid-Cambrian hurdiid radiodont Stanleycaris hirpex from the Burgess Shale (Stephen Formation) and quantitatively assess functional specialization of the frontal appendages of stem euarthropods. The appendages of Stanleycaris are composed of 14 podomeres, variously differentiated by their possession of pectinate endites, mono- to trifurcate medial gnathites, and outer spines. The oral cone is tetraradially organized and can be uniquely distinguished from those of other hurdiids by the presence of 28 rather than 32 smooth tridentate plates. Our phylogenetic analysis finds Stanleycaris in a grade of hurdiids retaining plesiomorphic raptorial appendicular functionality alongside derived adaptations for sweep feeding and large, bilaterally opposed gnathites. We conclude that the latter performed a masticatory function, convergent with gnathal structures like mandibles in various panarthropods. Taken together, Stanleycaris and similar hurdiids provide an extreme example of the evolution of division of labor within the appendage of a stem euarthropod and suggest that this innovation may have facilitated the functional transition, from raptorial to sweep feeding, at the origin of the hurdiid clade.

The chelicerates are a ubiquitous and speciose group of animals that has a considerable ecological effect on modern terrestrial ecosystems—notably as predators of insects and also, for instance, as decomposers1. The fossil record shows that chelicerates diversified early in the marine ecosystems of the Palaeozoic era, by at least the Ordovician period2. However, the timing of chelicerate origins and the type of body plan that characterized the earliest members of this group have remained controversial. Although megacheirans3–5 have previously been interpreted as chelicerate-like, and habeliidans6 (including Sanctacaris7,8) have been suggested to belong to their immediate stem lineage, evidence for the specialized feeding appendages (chelicerae) that are diagnostic of the chelicerates has been lacking. Here we use exceptionally well-preserved and abundant fossil material from the middle Cambrian Burgess Shale (Marble Canyon, British Columbia, Canada) to show that Mollisonia plenovenatrix sp. nov. possessed robust but short chelicerae that were placed very anteriorly, between the eyes. This suggests that chelicerae evolved a specialized feeding function early on, possibly as a modification of short antennules. The head also encompasses a pair of large compound eyes, followed by three pairs of long, uniramous walking legs and three pairs of stout, gnathobasic masticatory appendages; this configuration links habeliidans with euchelicerates (‘true’ chelicerates, excluding the sea spiders). The trunk ends in a four-segmented pygidium and bears eleven pairs of identical limbs, each of which is composed of three broad lamellate exopod flaps, and endopods are either reduced or absent. These overlapping exopod flaps resemble euchelicerate book gills, although they lack the diagnostic operculum9. In addition, the eyes of M. plenovenatrix were innervated by three optic neuropils, which strengthens the view that a complex malacostracan-like visual system10,11 might have been plesiomorphic for all crown euarthropods. These fossils thus show that chelicerates arose alongside mandibulates12 as benthic micropredators, at the heart of the Cambrian explosion. Mollisonia plenovenatrix, a small predatory arthropod from the Burgess Shale dated to about 508 million years ago, is morphologically close to horseshoe crabs, which suggests chelicerates arose as micropredators early during the Cambrian explosion.

Cambrian deposits bearing exceptional preservation of soft-bodied fossils offer unique insights into the diversity and paleobiology of the Cambrian biota. In recent years, a growing number of studies have provided evidence that highly labile tissues, such as nervous tissues, may be more common in fossils from Burgess Shale-type deposits than previously appreciated. These discoveries have provided novel insights into the anatomy and phylogeny of Cambrian animals and have also challenged the basic understanding of the limits on the resolution of anatomical information that may be captured by Burgess Shale-type preservation. Evidence used to infer primary blood chemistry was recently reported from the iconic Burgess Shale fossil Marrella splendens, the first such report from a Cambrian Lagerstatte. Diminutive crystals of a copper sulfide mineral found in association with Marrella were interpreted as evidence that the animal's blood contained the Cu-bearing protein hemocyanin. Copper minerals in shales in general, and in the Burgess Shale in particular, however, may have a complex geologic history. Using microscopy and SEM-EDS elemental mapping of material from the Walcott Quarry as well as from the Burgess Shale fossil quarry at Marble Canyon, we assess the timing of emplacement of Cu-sulfides in fossil-bearing mudrocks of the Burgess Shale. These data demonstrate that copper minerals are most conspicuously concentrated within veins that are oriented sub-perpendicular to bedding. These late-stage fluid flow paths allowed Cu-mineralizing fluids to infiltrate bedding plane-parallel cracks, which commonly run through Burgess Shale fossils and extend into the matrix around them. Based upon the relationships of mineral phases associated with fossils, it appears that Cu-mineralization occurred after metamorphic volatilization of a large proportion of the carbonaceous material that originally comprised the fossils. We find that copper minerals in the Burgess Shale are of demonstrably post-Cambrian origin, a secondary overprint that cannot be used to infer original blood chemistry or tissue composition of its fossils.

Synopsis We use a series of hydrodynamic experiments on abstracted models to explore whether primitive vertebrates may have swum under various conditions without a clearly-differentiated tail fin. Cambrian vertebrates had post-anal stubby tails, some had single dorsal and ventral fins, but none had yet evolved a clearly differentiated caudal fin typical of post-Cambrian fishes, and must have relied on their long and flexible laterally-compressed bodies for locomotion, i.e., by bending their bodies side-to-side in order to propagate waves from head to tail. We approach this problem experimentally based on an abstracted model of Metaspriggina walcotti from the 506-million-year old Burgess Shale by using oscillating thin flexible plates while varying the tail fin geometry from rectangular to uniform, and finally to a no tail–fin condition. Despite a missing tail fin, this study supports the observation that the abstracted Metaspriggina model can generate a strong propulsive force in cruise conditions, both away from, and near the sea bed (in ground effect). However, when the abstracted Metaspriggina model moves in ground effect, a weaker performance is observed, indicating that Metaspriggina may not necessarily have been optimized for swimming near the sea bed. When considering acceleration from rest, we find that the Metaspriggina model's performance is not significantly different from other morphological models (abstracted truncate tail and abstracted heterocercal tail). Statistical analysis shows that morphological parameters, swimming modes, and ground effect all play significant roles in thrust performance. While the exact relationships of Cambrian vertebrates are still debated, as agnathans, they share some general characteristics with modern cyclostomes, in particular an elongate body akin to lampreys. Lampreys, as anguilliform swimmers, are considered to be some of the most efficient swimmers using a particular type of suction thrust induced by the traveling body wave as it travels from head to tail. Our current experiments suggest that Metaspriggina’s ability in acceleration from rest, through possibly a similar type of suction thrust, which is defined as the ability to generate low pressure on upstream facing sections of the body, might have evolved early in response to increasing predator pressure during the Cambrian Explosion.

Anomalocaris canadensis, a soft-bodied stem-group arthropod from the Burgess Shale, is considered the largest predator of the Cambrian period. Thanks to a series of lateral flexible lobes along its dorso-ventrally compressed body, it is generally regarded as an efficient swimmer, well-adapted to its predatory lifestyle. Previous theoretical hydrodynamic simulations have suggested a possible optimum in swimming performance when the lateral lobes performed as a single undulatory lateral fin, comparable to the pectoral fins in skates and rays. However, the role of the unusual fan-like tail of Anomalocaris has not been previously explored. Swimming efficiency and maneuverability deduced from direct hydrodynamic analysis are here studied in a towing tank facility using a three-vane physical model designed as an abstraction of the tail fin. Through direct force measurements, it was found that the model exhibited a region of steady-state lift and drag enhancement at angles of attack greater than 25° when compared with a triangular-shaped reference model. This would suggest that the resultant normal force on the tail fin of Anomalocaris made it well-suited for turning maneuvers, giving it the ability to turn quickly and through small radii of curvature. These results are consistent with an active predatory lifestyle, although detailed kinematic studies integrating the full organism, including the lateral lobes, would be required to test the effect of the tail fin on overall swimming performance. This study also highlights a possible example of evolutionary convergence between the tails of Anomalocaris and birds, which, in both cases, are well-adapted to efficient turning maneuvers.

Tokummia katalepsis from the Burgess Shale had a pair of mandibles and maxilliped claws, showing that large bivalved arthropods from the Cambrian period are forerunners of myriapods and pancrustaceans, thereby providing a basis for the origin of the hyperdiverse mandibulate body plan. Fossils from the famous 508-million-year-old Burgess Shale in Canada have been vital for shaping our understanding of the origin and early evolution of arthropods, the group of invertebrate animals recognized by their segmented bodies with jointed limbs and an exoskeleton. In recent years, research has found support for a single group of arthropods known as mandibulates that comprises insects, crustaceans and myriapods (centipedes and millipedes) but excludes chelicerates (spiders, scorpions and their allies). Few fossils have been found to illuminate the earliest mandibulates. Cedric Aria and Jean-Bernard Caron now show that this gap is partially filled by the arrival of the Burgess Shale fossil Tokummia katalepsis, whose anatomy allows the reconstruction of the anatomical and evolutionary history of this important animal group. Retracing the evolutionary history of arthropods has been one of the greatest challenges in biology1. During the past decade, phylogenetic analyses of morphological and molecular data2,3,4 have coalesced towards the conclusion that Mandibulata, the most diverse and abundant group of animals, is a distinct clade from Chelicerata, in that its members possess post-oral head appendages specialized for food processing, notably the mandible2,5. The origin of the mandibulate body plan, however, which encompasses myriapods, crustaceans and hexapods, has remained poorly documented1,6. Here we show that Tokummia katalepsis gen. et sp. nov., a large bivalved arthropod from the 508 million-year-old Marble Canyon fossil deposit (Burgess Shale, British Columbia), has unequivocal mandibulate synapomorphies, including mandibles and maxillipeds, as well as characters typically found in crustaceans, such as enditic, subdivided basipods and ring-shaped trunk segments. Tokummia and its closest relative, Branchiocaris (in Protocarididae, emended), also have an anteriormost structure housing a probable bilobed organ, which could support the appendicular origin of the labrum7. Protocaridids are retrieved with Canadaspis and Odaraia (in Hymenocarina, emended) as part of an expanded mandibulate clade, refuting the idea that these problematic bivalved taxa, as well as other related forms, are representatives of the basalmost euarthropods8,9. Hymenocarines now illustrate that the subdivision of the basipod and the presence of proximal endites are likely to have been ancestral conditions critical for the evolution of coxal and pre-coxal features in mandibulates10,11. The presence of crustaceomorph traits in the Cambrian larvae of various clades basal to Mandibulata is reinterpreted as evidence for the existence of distinct ontogenetic niches among stem arthropods. Larvae would therefore have constituted an important source of morphological novelty during the Cambrian period, and, through heterochronic processes, may have contributed to the rapid acquisition of crown-group characters and thus to greater evolutionary rates during the early radiation of euarthropods12.

The origin of the arthropod carapace, an enlargement of cephalic tergites, can be traced back to the Cambrian period. However, its disparity and evolution are still not fully understood. Here, we describe a new ‘bivalved’ arthropod,Fibulacaris nereidisgen. et sp. nov., based on 102 specimens from the middle Cambrian (Wuliuan Stage) Burgess Shale, Marble Canyon area in British Columbia's Kootenay National Park, Canada. The laterally compressed carapace covers most of the body. It is fused dorsally and merges anteriorly into a conspicuous postero-ventrally recurved rostrum as long as the carapace and positioned between a pair of backwards-facing pedunculate eyes. The body is homonomous, with approximately 40 weakly sclerotized segments bearing biramous legs with elongate endopods, and ends in a pair of small flap-like caudal rami.Fibulacaris nereidisis interpreted as a suspension feeder possibly swimming inverted, in a potential case of convergence with some branchiopods. A Bayesian phylogenetic analysis places it within a group closely related to the extinct Hymenocarina.Fibulacaris nereidisis unique in its carapace morphology and overall widens the ecological disparity of Cambrian arthropods and suggests that the evolution of a ‘bivalved' carapace and an upside-down lifestyle may have occurred early in stem-group crustaceans.

Exceptional fossils reveal annelid nervous system evolution., Annelid worms are a disparate, primitively segmented clade of bilaterians that first appear during the early Cambrian Period. Reconstructing their early evolution is complicated by the extreme morphological diversity in early diverging lineages, rapid diversification, and sparse fossil record. Canadia spinosa, a Burgess Shale fossil polychaete, is redescribed as having palps with feeding grooves, a dorsal median antenna and biramous parapodia associated with the head and flanking a ventral mouth. Carbonaceously preserved features are identified as a terminal brain, circumoral connectives, a midventral ganglionated nerve cord and prominent parapodial nerves. Phylogenetic analysis recovers neuroanatomically simple extant taxa as the sister group of other annelids, but the phylogenetic position of Canadia suggests that the annelid ancestor was reasonably complex neuroanatomically and that reduction of the nervous system occurred several times independently in the subsequent 500 million years of annelid evolution.

Brood care, including the carrying of eggs or juveniles, is a form of parental care, which, like other parental traits [1], enhances offspring fitness with variable costs and benefits to the parents [2]. Attempts to understand why and how parental care evolved independently in numerous animal groups often emphasize the role of environmental pressures such as predation, ephemeral resources, and, more generally, the harshness of environment. The fossil record can, in principle, provide minimum age constraints on the evolution of life-history traits, including brood care and key information on the reproductive strategies of extinct organisms. New, exceptionally preserved specimens of the weakly sclerotized arthropod Waptia fieldensis from the middle Cambrian (ca. 508 million years ago) Burgess Shale, Canada, provide the oldest example of in situ eggs with preserved embryos in the fossil record. The relatively small clutch size, up to 24 eggs, and the relatively large diameter of individual eggs, some over 2 mm, contrast with the high number of small eggs-found without preserved embryos-in the bivalved bradoriid arthropod Kunmingella douvillei from the Chengjiang biota (ca. 515 million years ago). The presence of these two different parental strategies suggests a rapid evolution of a variety of modern-type life-history traits, including extended investment in offspring survivorship, soon after the Cambrian emergence of animals. Together with previously described brooded eggs in ostracods from the Upper Ordovician (ca. 450 million years ago), these new findings suggest that the presence of a bivalved carapace played a key role in the early evolution of parental care in arthropods.

Yuknessia Walcott, 1919 recently was transferred from the green algae to the Phylum Hemichordata on the basis of new details observed for the type species, Y. simplex, from the Burgess Shale Formation (Cambrian Stage 5) of British Columbia. This has prompted reexamination of material attributed to Yuknessia from various Cambrian localities in South China. Findings preclude both a Yuknessia and a hemichordate affinity for all of the Chinese study material, and most of this material is formally transferred to Fuxianospira Chen and Zhou, 1997, a taxon common in the Chengjiang biota. Comparable material from the Cambrian Marjum, Wheeler, and Burgess Shale formations of North America is also assigned to Fuxianospira, and this reassignment expands both the paleogeographic and stratigraphic range of this taxon. All aspects of the study specimens, including details obtained from scanning electron microscopy, are consistent with an algal affinity, as proposed in the original descriptions of the Chinese material.

The molecularly defined clade Ecdysozoa1 comprises the panarthropods (Euarthropoda, Onychophora and Tardigrada) and the cycloneuralian worms (Nematoda, Nematomorpha, Priapulida, Loricifera and Kinorhyncha). These disparate phyla are united by their means of moulting, but otherwise share few morphological characters—none of which has a meaningful fossilization potential. As such, the early evolutionary history of the group as a whole is largely uncharted. Here we redescribe the 508-million-year-old stem-group onychophoran Hallucigenia sparsa2, 3, 4, 5, 6 from the mid-Cambrian Burgess Shale. We document an elongate head with a pair of simple eyes, a terminal buccal chamber containing a radial array of sclerotized elements, and a differentiated foregut that is lined with acicular teeth. The radial elements and pharyngeal teeth resemble the sclerotized circumoral elements and pharyngeal teeth expressed in tardigrades7, 8, 9, stem-group euarthropods10, 11, 12 and cycloneuralian worms13. Phylogenetic results indicate that equivalent structures characterized the ancestral panarthropod and, seemingly, the ancestral ecdysozoan, demonstrating the deep homology of panarthropod and cycloneuralian mouthparts, and providing an anatomical synapomorphy for the ecdysozoan supergroup.

Decay experiments are becoming a more widespread tool in evaluating the fidelity of the fossil record. Character interpretations of fossil specimens stand to benefit from an understanding of how decay can result in changes in morphology and, potentially, total character loss. We performed a decay experiment for the Class Enteropneusta to test the validity of anatomical interpretations of the Burgess Shale enteropneust Spartobranchus tenuis and to determine how the preservation of morphological features compares with the sequence of character decay in extant analogues. We used three species of enteropneust (Saccoglossus pusillus, Harrimania planktophilus, and Balanoglossus occidentalis) representing the two major families of Enteropneusta. Comparisons between decay sequences suggest that morphological characters decay in a consistent and predictable manner within Enteropneusta, and do not support the hypothesis of stemward slippage. The gill bars and nuchal skeleton were the most decay resistant, whereas the gill pores and pre-oral ciliary organ were unequivocally the most decay prone. Decay patterns support the identification of the nuchal skeleton, gill bars, esophageal organ, trunk, and proboscis in Spartobranchus tenuis and corroborate a harrimaniid affinity. Bias due to the taphonomic loss of taxonomically informative characters is unlikely. The morphologically simple harrimaniid body plan can be seen, therefore, to be plesiomorphic within the enteropneusts. Discrepancies between the sequence of decay in a laboratory setting and fossil preservation also exist. These discrepancies are highlighted not to discredit the use of modern decay studies but rather to underline their non-actualistic nature. Paleoenvironmental variables besides decay, such as the timeframe between death and early diagenesis as well as postmortem transport, are discussed relative to decay data. These experiments reinforce the strength of a comprehensive understanding of decay sequences as a benchmark against which to describe fossil taxa and understand the conditions leading to fossilization.

Characterized by atypical frontalmost appendages, leanchoiliids are early arthropods whose phylogenetic placement has been much debated. Morphological interpretations have differed, some of which concern critical characters such as the number of eyes and head appendages, but methodological approaches also have diverged. Here, we describe a new leanchoiliid, Yawunik kootenayi gen. et sp. nov., based on 42 specimens from the newly discovered Marble Canyon locality of the Burgess Shale (Kootenay National Park, British Columbia; middle Cambrian). This new morphotype demonstrates the presence of a four-segmented head in leanchoiliids, along with two small antero-median eyes in addition to lateral eyes. Yawunik is characterized by a 12-segmented trunk and a carinate, lanceolate telson adorned with minute spines. The ‘great appendages’ of the animal bear teeth on their two distal rami, which would have enhanced their ability to grasp prey. Attitudes of specimens, resulting from burial at multiple aspects of bedding, suggest the ‘great appendages’ were flexible and capable of antero-posterior rotation. We also discuss the nature of intersegmental tissues and filaments present within the ‘great appendages’. Our phylogenetic analyses extend the monophyly of leanchoiliids to include Haikoucaris and Yohoia in a new clade, the Cheiromorpha nom. nov. (within Heptopodomera nom. nov.). Other nodes are poorly resolved unless implied weights are used, and in this case, the topology is critically sensitive to the coding prerogative of inapplicable states (NAs). Both the traditional ‘Arachnomorpha’ hypothesis (NAs as additional states) and the more recently favoured ‘Artiopoda + Crustacea’ (NAs as uncertainties) were obtained using the same data set and outgroup. This result stresses, first, the historical importance of polarization over data content in scenarios of early arthropod evolution, and second, a pressing need to investigate the impacts of coding inapplicables, especially given the inflating effect of implied weights.

Waptia fieldensisWalcott, 1912 is one of the iconic animals from the middle Cambrian Burgess Shale biota that had lacked a formal description since its discovery at the beginning of the twentieth century. This study, based on over 1800 specimens, finds thatW. fieldensisshares general characteristics with pancrustaceans, as previous authors had suggested based mostly on its overall aspect. The cephalothorax is covered by a flexible, bivalved carapace and houses a pair of long multisegmented antennules, palp-bearing mandibles, maxillules, and four pairs of appendages with five-segmented endopods—the anterior three pairs with long and robust enditic basipods, the fourth pair with proximal annulations and lamellae. The post-cephalothorax has six pairs of lamellate and fully annulated appendages which appear to be extensively modified basipods rather than exopods. The front part of the body bears a pair of stalked eyes with the first ommatidia preserved in a Burgess Shale arthropod, and a median ‘labral’ complex flanked by lobate projections with possible affinities to hemi-ellipsoid bodies.Waptiaconfirms the mandibulate affinity of hymenocarines, retrieved here as part of an expanded Pancrustacea, thereby providing a novel perspective on the evolutionary history of this hyperdiverse group. We construe thatWaptiawas an active swimming predator of soft prey items, using its anterior appendages for food capture and manipulation, and also potentially for clinging to epibenthic substrates.

To investigate the phylogenetic affinity of Yuknessia simplex Walcott, 1919, scanning electron microscopy was applied to the Burgess Shale (Cambrian Series 3, Stage 5) type material and to new material from the Trilobite Beds (Yoho National Park) and specimens from the Cambrian of Utah. On the basis of fine-scale details observed using this approach, including banding structure interpreted as fusellae, Yuknessia Walcott, 1919 is transferred from the algae, where it resided for nearly a century, to the extant taxon Pterobranchia (Phylum Hemichordata). Considered as such, Yuknessia specimens from the Trilobite Beds and Spence Formation (Utah) are amongst the oldest known colonial pterobranchs. Two morphs regarded herein as two different species are recognized from the Trilobite Beds based on tubarium morphology. Yuknessia simplex has slender erect tubes whereas Yuknessia stephenensis n. sp., which is also known in Utah, has more robust erect tubes. The two paratypes of Y. simplex designated by Walcott (1919) are formally removed from Yuknessia and are reinterpreted respectively as an indeterminate alga and Dalyia racemata Walcott, 1919, a putative red alga.

Background Chelicerata represents a vast clade of mostly predatory arthropods united by a distinctive body plan throughout the Phanerozoic. Their origins, however, with respect to both their ancestral morphological features and their related ecologies, are still poorly understood. In particular, it remains unclear whether their major diagnostic characters were acquired early on, and their anatomical organization rapidly constrained, or if they emerged from a stem lineage encompassing an array of structural variations, based on a more labile “panchelicerate” body plan. Results In this study, we reinvestigated the problematic middle Cambrian arthropod Habelia optata Walcott from the Burgess Shale, and found that it was a close relative of Sanctacaris uncata Briggs and Collins (in Habeliida, ord. nov.), both retrieved in our Bayesian phylogeny as stem chelicerates. Habelia possesses an exoskeleton covered in numerous spines and a bipartite telson as long as the rest of the body. Segments are arranged into three tagmata. The prosoma includes a reduced appendage possibly precursor to the chelicera, raptorial endopods connected to five pairs of outstandingly large and overlapping gnathobasic basipods, antennule-like exopods seemingly dissociated from the main limb axis, and, posteriorly, a pair of appendages morphologically similar to thoracic ones. While the head configuration of habeliidans anchors a seven-segmented prosoma as the chelicerate ground pattern, the peculiar size and arrangement of gnathobases and the presence of sensory/tactile appendages also point to an early convergence with the masticatory head of mandibulates. Conclusions Although habeliidans illustrate the early appearance of some diagnostic chelicerate features in the evolution of euarthropods, the unique convergence of their cephalons with mandibulate anatomies suggests that these traits retained an unusual variability in these taxa. The common involvement of strong gnathal appendages across non-megacheirans Cambrian taxa also illustrates that the specialization of the head as the dedicated food-processing tagma was critical to the emergence of both lineages of extant euarthropods—Chelicerata and Mandibulata—and implies that this diversification was facilitated by the expansion of durophagous niches. Electronic supplementary material The online version of this article (10.1186/s12862-017-1088-7) contains supplementary material, which is available to authorized users.

Background Chelicerata represents a vast clade of mostly predatory arthropods united by a distinctive body plan throughout the Phanerozoic. Their origins, however, with respect to both their ancestral morphological features and their related ecologies, are still poorly understood. In particular, it remains unclear whether their major diagnostic characters were acquired early on, and their anatomical organization rapidly constrained, or if they emerged from a stem lineage encompassing an array of structural variations, based on a more labile “panchelicerate” body plan. Results In this study, we reinvestigated the problematic middle Cambrian arthropod Habelia optata Walcott from the Burgess Shale, and found that it was a close relative of Sanctacaris uncata Briggs and Collins (in Habeliida, ord. nov.), both retrieved in our Bayesian phylogeny as stem chelicerates. Habelia possesses an exoskeleton covered in numerous spines and a bipartite telson as long as the rest of the body. Segments are arranged into three tagmata. The prosoma includes a reduced appendage possibly precursor to the chelicera, raptorial endopods connected to five pairs of outstandingly large and overlapping gnathobasic basipods, antennule-like exopods seemingly dissociated from the main limb axis, and, posteriorly, a pair of appendages morphologically similar to thoracic ones. While the head configuration of habeliidans anchors a seven-segmented prosoma as the chelicerate ground pattern, the peculiar size and arrangement of gnathobases and the presence of sensory/tactile appendages also point to an early convergence with the masticatory head of mandibulates. Conclusions Although habeliidans illustrate the early appearance of some diagnostic chelicerate features in the evolution of euarthropods, the unique convergence of their cephalons with mandibulate anatomies suggests that these traits retained an unusual variability in these taxa. The common involvement of strong gnathal appendages across non-megacheiran Cambrian taxa also illustrates that the specialization of the head as the dedicated food-processing tagma was critical to the emergence of both lineages of extant euarthropods—Chelicerata and Mandibulata—and implies that this diversification was facilitated by the expansion of durophagous niches.

Summary Annelida is one of the most speciose (∼17,000 species) and ecologically successful phyla. Key to this success is their flexible body plan with metameric trunk segments and bipartite heads consisting of a prostomium bearing sensory structures and a peristomium containing the mouth. The flexibility of this body plan has traditionally proven problematic for reconstructing the evolutionary relationships within the Annelida. Although recent phylogenies have focused on resolving the interrelationships of the crown group [1–3], many questions remain regarding the early evolution of the annelid body plan itself, including the origin of the head [4]. Here we describe an abundant and exceptionally well-preserved polychaete with traces of putative neural and vascular tissues for the first time in a fossilized annelid. Up to three centimeters in length, Kootenayscolex barbarensis gen. et sp. nov. is described based on more than 500 specimens from Marble Canyon [5] and several specimens from the original Burgess Shale site (both in British Columbia, Canada). K. barbarensis possesses biramous parapodia along the trunk, bearing similar elongate and thin notochaetae and neurochaetae. A pair of large palps and one median antenna project from the anteriormost dorsal margin of the prostomium. The mouth-bearing peristomium bears neuropodial chaetae, a condition that is also inferred in Canadia and Burgessochaeta from the Burgess Shale, suggesting a chaetigorous origin for the peristomial portion of the head and a secondary loss of peristomial parapodia and chaetae in modern polychaetes.

Summary Chaetognaths (arrow worms) are a separate phylum (Chaetognatha) of small carnivorous animals, dominantly pelagic, and a major component of today's plankton [1, 2]. The position of Chaetognatha among metazoan phyla remains equivocal—neither morphological nor molecular data provide definitive evidence [3]. Originating early in the Cambrian period [4], if not earlier [5], chaetognaths quickly became important members of marine metazoan communities [6]. Chaetognath grasping spines, originally reported as conodonts, occur worldwide in many Cambrian marine sediments [6, 7]. Fossilized chaetognath bodies, in contrast, are very rare: only two unequivocal specimens have been reported, both from the early Cambrian of China [8, 9]. Here we describe Capinatator praetermissus , a new genus and species, based on ∼50 specimens from several middle Cambrian Burgess Shale localities in British Columbia, many of which preserve evidence of soft tissues. Capinatator praetermissus reached body lengths of nearly 10 cm exclusive of fins, a much larger size than that of most living forms. Clusters of specimens preserving the body indicate that they were rapidly buried, providing indirect evidence that they swam near the seabed. The feeding apparatus comprises up to ∼25 spines in each half, almost double the maximum number in living chaetognaths. Early chaetognaths apparently occupied ecological niches associated with predatory euarthropods. The large body size and high number of grasping spines in C. praetermissus may indicate that miniaturization and migration to a planktonic lifestyle were secondary.

Radiodonts, a clade of Cambro-Devonian stem group euarthropods, have classically been regarded as nektonic apex predators. However, many aspects of radiodont morphology and ecology have remained unclear because of the typically fragmentary nature of fossil material. Here, we describe a new hurdiid radiodont based on abundant and exceptionally well-preserved fossils from the Burgess Shale (Marble Canyon area, British Columbia, Canada). Cambroraster falcatus gen. et sp. nov . is characterized by an extra-large horseshoe-shaped head carapace, bearing conspicuous posterolateral spinous processes, and partially covering a short trunk with eight pairs of lateral flaps. Each of the pair of frontal appendages possess five mesially curving rake-like endites equipped with a series of anteriorly directed hooked spines, altogether surrounding the oral cone. This feeding apparatus suggests a micro to macrophagous sediment-sifting feeding ecology. Cambroraster illuminates the evolution of Hurdiidae and evinces the exploitation of the diversifying infauna by these large and specialized nektobenthic carnivores in the aftermath of the Cambrian explosion.

Lagerstättenfrom the Precambrian–Cambrian transition have traditionally been a relatively untapped resource for understanding the paleoecology of the “Cambrian explosion.” This quantitative paleoecological study is based on 10,238 fossil specimens belonging to 100 animal species, 11 phyla, and 15 ecological categories from the lower Cambrian (Series 2, Stage 3) Chengjiang biota (Mafang locality near Haikou, Yunnan Province, China). Fossils were systematically collected within a 2.5-meter-thick sequence divided into ten stratigraphic intervals. Each interval represents an induced time-averaged assemblage of various event (obrution) beds of unknown duration. Overall, the different fossil assemblages are taxonomically and ecologically similar, suggesting the presence of a single community type recurring throughout the Mafang section. The Mafang community is dominated by epibenthic vagile hunters or scavengers, sessile suspension feeders, and infaunal vagile hunters or scavengers represented primarily by arthropods, brachiopods, and priapulids, respectively. Most species have low abundance and low occurrence frequencies, whereas a few species are numerically abundant and occur frequently. Overall, in structure and ecology the Mafang community is comparable to the Middle Cambrian (Series 3, Stage 5) Burgess Shale biota (Walcott Quarry, Yoho National Park, British Columbia, Canada). This suggests that, despite variations in species identity within taxonomic and ecological groups, the structure and ecology of Cambrian Burgess Shale-type communities remained relatively stable until at least the Middle Cambrian (Series 3, Stage 5) in subtidal to relatively deep-water offshore settings in siliciclastic soft-substrate environments.

The morphology of two new bivalved arthropods, Loricicaris spinocaudatus gen. et sp. nov. and Nereocaris briggsi sp. nov. from the middle Cambrian (Series 3, Stage 5) Burgess Shale Formation (Collins Quarry locality on Mount Stephen, Yoho National Park, British Columbia, Canada), is described. The material was originally assigned to the genus Branchiocaris, but exhibits distinctive character combinations meriting its assignment to other taxa. Loricicaris spinocaudatus possesses an elongate and spinose abdomen comparable to the contemporaneous Perspicaris and Canadaspis, as well as chelate second head appendages and subtriangular exopods, comparable to Branchiocaris. Nereocaris briggsi possesses a laterally compressed carapace, elongate and delicate appendages and a medial eye located between a pair of lateral eyes on a rhomboidal eye stalk. Although undoubtedly congeneric with Nereocaris exilis from a slightly younger horizon of the Burgess Shale Formation, N. briggsi differs in overall proportions and segment number, warranting assignment to a new species. The newly described taxa were coded into an extensive cladistic analysis of 755 characters, and 312 extinct and extant panarthropods, including a variety of Cambrian bivalved arthropods from both the Burgess Shale and the Chengjiang Lagerstatten. Cambrian bivalved arthropods consistently resolved as a paraphyletic assemblage at the base of Arthropoda. Important innovations in arthropod history such as the specialization of the deutocerebral head appendages and a shift from a nekton-benthic deposit feeding habit to a benthic scavenging/predatory habit, the symplesiomorphic feeding condition of Euarthropoda (crown-group arthropods), were found to have occurred among basal bivalved arthropods.

In Cambrian fossil Lagerstatten like the Burgess Shale, exceptionally preserved arthropods constitute a large part of the taxonomic diversity, providing opportunities to study the early evolution o ...

Analysis of exceptionally preserved fossils of the Cambrian hyolith Haplophrentis leads to a proposed evolutionary relationship with Lophophorata, the group containing brachiopods and phoronids, on the basis of a newly described tentacular feeding apparatus. Hyoliths were shelly fossils that were common throughout the Palaeozoic. They look like drinking horns with a lid (the operculum), and even had two curved protruberances (called helens) that the drinking horn could stand on, making a tripod with the horn-shaped shell. Because hyoliths are so distinctive, their affinities are difficult to determine, although they are thought to belong to a group of invertebrates known as Lophotrochozoa—annelids, molluscs and lophophore-bearing animals such as brachiopods. Joseph Moysiuk et al. report that a cache of hyoliths from the famous Cambrian Burgess Shales of Canada has remarkable soft-tissue preservation, revealing that they are indeed lophophorates, probably akin to brachiopods, and perhaps taking in the tommotiids, another extinct and enigmatic group of what palaeontologists refer to as 'small shelly fossils'. Hyoliths are abundant and globally distributed ‘shelly’ fossils that appear early in the Cambrian period and can be found throughout the 280 million year span of Palaeozoic strata1,2. The ecological and evolutionary importance of this group has remained unresolved, largely because of their poorly constrained soft anatomy and idiosyncratic scleritome, which comprises an operculum, a conical shell and, in some taxa, a pair of lateral spines (helens)3,4,5. Since their first description over 175 years ago, hyoliths have most often been regarded as incertae sedis4,6, related to molluscs7,8 or assigned to their own phylum1,2. Here we examine over 1,500 specimens of the mid-Cambrian hyolith Haplophrentis from the Burgess Shale and Spence Shale Lagerstatten. We reconstruct Haplophrentis as a semi-sessile, epibenthic suspension feeder that could use its helens to elevate its tubular body above the sea floor3,9,10,11,12. Exceptionally preserved soft tissues include an extendable, gullwing-shaped, tentacle-bearing organ surrounding a central mouth, which we interpret as a lophophore, and a U-shaped digestive tract ending in a dorsolateral anus. Together with opposing bilateral sclerites and a deep ventral visceral cavity, these features indicate an affinity with the lophophorates (brachiopods, phoronids and tommotiids), substantially increasing the morphological disparity of this prominent group.

Background The combination of a meager fossil record of vermiform enteropneusts and their disparity with the tubicolous pterobranchs renders early hemichordate evolution conjectural. The middle Cambrian Oesia disjuncta from the Burgess Shale has been compared to annelids, tunicates and chaetognaths, but on the basis of abundant new material is now identified as a primitive hemichordate. Results Notable features include a facultative tubicolous habit, a posterior grasping structure and an extensive pharynx. These characters, along with the spirally arranged openings in the associated organic tube (previously assigned to the green alga Margaretia), confirm Oesia as a tiered suspension feeder. Conclusions Increasing predation pressure was probably one of the main causes of a transition to the infauna. In crown group enteropneusts this was accompanied by a loss of the tube and reduction in gill bars, with a corresponding shift to deposit feeding. The posterior grasping structure may represent an ancestral precursor to the pterobranch stolon, so facilitating their colonial lifestyle. The focus on suspension feeding as a primary mode of life amongst the basal hemichordates adds further evidence to the hypothesis that suspension feeding is the ancestral state for the major clade Deuterostomia. Electronic supplementary material The online version of this article (doi:10.1186/s12915-016-0271-4) contains supplementary material, which is available to authorized users.

Extant arthropods are diverse and ubiquitous, forming a major constituent of most modern ecosystems. Evidence from early Palaeozoic Konservat Lagerstätten indicates that this has been the case since the Cambrian. Despite this, the details of arthropod origins remain obscure, although most hypotheses regard the first arthropods as benthic predators or scavengers such as the fuxianhuiids or megacheirans (‘great-appendage’ arthropods). Here, we describe a new arthropod from the Tulip Beds locality of the Burgess Shale Formation (Cambrian, series 3, stage 5) that possesses a weakly sclerotized thorax with filamentous appendages, encased in a bivalved carapace, and a strongly sclerotized, elongate abdomen and telson. A cladistic analysis resolved this taxon as the basal-most member of a paraphyletic grade of nekto-benthic forms with bivalved carapaces. This grade occurs at the base of Arthropoda (panarthropods with arthropodized trunk limbs) and suggests that arthrodization (sclerotization and jointing of the exoskeleton) evolved to facilitate swimming . Predatory and fully benthic habits evolved later in the euarthropod stem-lineage and are plesiomorphically retained in pycnogonids (sea spiders) and euchelicerates (horseshoe crabs and arachnids).

The first arthropod trackways are described from the Middle Cambrian Burgess Shale Formation of Canada. Trace fossils, including trackways, provide a rich source of biological and ecological information, including direct evidence of behaviour not commonly available from body fossils alone. The discovery of large arthropod trackways is unique for Burgess Shale-type deposits. Trackway dimensions and the requisite number of limbs are matched with the body plan of a tegopeltid arthropod. Tegopelte , one of the rarest Burgess Shale animals, is over twice the size of all other benthic arthropods known from this locality, and only its sister taxon, Saperion , from the Lower Cambrian Chengjiang biota of China, approaches a similar size. Biomechanical trackway analysis demonstrates that tegopeltids were capable of rapidly skimming across the seafloor and, in conjunction with the identification of gut diverticulae in Tegopelte , supports previous hypotheses on the locomotory capabilities and carnivorous mode of life of such arthropods. The trackways occur in the oldest part (Kicking Horse Shale Member) of the Burgess Shale Formation, which is also known for its scarce assemblage of soft-bodied organisms, and indicate at least intermittent oxygenated bottom waters and low sedimentation rates.

Agnostids (agnostinids and eodiscinids) are a widespread and biostratigraphically important group of Cambro-Ordovician euarthropods whose evolutionary affinities have been highly controversial. Their dumbbell-shaped calcified tergum was traditionally suggested to unite them with trilobites, but agnostinids have alternatively been interpreted as stem-crustaceans, based on Orsten larval material from the Cambrian of Sweden. We describe exceptionally preserved soft tissues from mature individuals of the agnostinids Peronopsis and Ptychagnostus from the middle Cambrian (Wuliuan Stage) Burgess Shale (Walcott Quarry and Marble Canyon, British Columbia, Canada), facilitating the testing of alternative hypotheses. The digestive tract includes conspicuous ramifying cephalic diverticulae. The cephalon carries one pair of elongate spinous antennules projecting to the front, two pairs of appendages with distally setose, oar-like exopods, and three pairs of presumably biramous appendages with endopods sporting club-shaped exites. The trunk bears five appendage pairs, at least the first two of which are similar to the posteriormost cephalic pairs. The combined evidence supports a nektobenthic and detritivorous lifestyle for agnostinids. A head with six appendiferous segments contrasts strikingly with the four known in trilobites and five typical of mandibulates. Agnostinids are retrieved as the sister group to polymeroid trilobites in our phylogeny, implying that crustacean-like morphologies evolved homoplastically. This result highlights the variability in segmental composition of the artiopodan head. Finally, our study emphasizes the continued role of Burgess Shale-type fossils in resolving the affinities of problematic biomineralizing taxa.

The association of trace fossils and non-biomineralized carapaces has been reported from Cambrian Lagerstätten worldwide, but the abundance, ichnodiversity, taphonomy and ecological significance of such associations have yet to be fully investigated. Two main end-member hypotheses are explored based on the study of a relatively wide variety of trace fossils preserved associated to Tuzoia carapaces from the middle Cambrian Burgess Shale in British Columbia. In the ecological Tuzoia garden hypothesis, the bacterially enriched surface of carapaces provides opportunities for intricate ecologic interactions among trophic levels. In the taphonomic shielding hypothesis, the trace fossil–carapace association results from preferential preservation of traces as controlled by compaction independent of any association in life. In an attempt to better understand the role of the carapace as a medium for preservation of trace fossils and to evaluate the effects of mechanical stress related to burial, a numerical model was developed. Results indicate that the carapace can shield underlying sediment from mechanical stress for a finite time, differentially protecting trace fossils during the initial phase of burial and compaction. However, this taphonomic model alone fails to fully explain relatively high-density assemblages displaying a diversity of structures spatially confined within the perimeter of carapaces or branching patterns recording re-visitation.

To better understand temporal variations in species diversity and composition, ecological attributes, and environmental influences for the Middle Cambrian Burgess Shale community, we studied 50,900 fossil specimens belonging to 158 genera (mostly monospecific and non-biomineralized) representing 17 major taxonomic groups and 17 ecological categories. Fossils were collected in situ from within 26 massive siliciclastic mudstone beds of the Greater Phyllopod Bed (Walcott Quarry — Fossil Ridge). Previous taphonomic studies have demonstrated that each bed represents a single obrution event capturing a predominantly benthic community represented by census- and time-averaged assemblages, preserved within habitat. The Greater Phyllopod Bed (GPB) corresponds to an estimated depositional interval of 10 to 100 KA and thus potentially preserves community patterns in ecological and short-term evolutionary time. The community is dominated by epibenthic vagile deposit feeders and sessile suspension feeders, represented primarily by arthropods and sponges. Most species are characterized by low abundance and short stratigraphic range and usually do not recur through the section. It is likely that these are stenotopic forms (i.e., tolerant of a narrow range of habitats, or having a narrow geographical distribution). The few recurrent species tend to be numerically abundant and may represent eurytopic organisms (i.e., tolerant of a wide range of habitats, or having a wide geographical distribution). Rarefaction curves demonstrate variation in species richness through the GPB and suggest that more stenotopic species could still be discovered with additional sampling. Comparisons between richness and evenness trends suggest that the community is relatively stable overall, despite gradual species turnover through time, especially in assemblages from younger beds. Less diverse assemblages with low species evenness possibly represent the onset of less favourable environmental conditions. Fossil occurrences in individual beds were analysed using a range of statistical techniques (Correspondence Analysis, Canonical Correspondence Analysis, Minimum Spanning Tree, Indicator Species Analysis, Mantel Test) to extract community patterns. Results suggest the presence of four fossil assemblages based on distinct species associations. The different assemblages presumably reflect variations in environmental and ecological conditions, some acting through time, leading to species turnover. “Disturbances” (e.g., changes in paleo-redox conditions), differences in substrate firmness, and limited taphonomic biases are probably the main factors contributing to community structure. The influence of ecological factors, however, is also predicted from non-random patterns of species recurrences in successive events. Preliminary comparisons with Lower Cambrian Chengjiang-type assemblages of southern China suggest that the overall structure and ecology of Cambrian communities remained relatively stable until at least the Middle Cambrian in subtidal siliciclastic soft substrate environments. Comparisons with modern marine benthic ecosystems further suggest the Burgess Shale community was probably highly dependent on immigration from a regional pool of species after each burial event. This could support the view that species availability, habitat characteristics, and recolonisation processes were more important in structuring the community in the long-term than species interactions or environmental variations at a local scale.

Background One of the first phyla to acquire biomineralized skeletal elements in the Cambrian, brachiopods represent a vital component in unraveling the early evolution and relationships of the Lophotrochozoa. Critical to improving our understanding of lophotrochozoans is the origin, evolution and function of unbiomineralized morphological features, in particular features such as chaetae that are shared between brachiopods and other lophotrochozoans but are poorly understood and rarely preserved. Micromitra burgessensis and Paterina zenobia from the middle Cambrian Burgess Shale are among the most remarkable examples of fossilized chaetae-bearing brachiopods. The form, functional morphology, evolutionary and ecological significance of their chaetae are studied herein. Results Like in Recent forms, the moveable but semi-rigid chaetae fringe both the dorsal and ventral mantle margins, but in terms of length, the chaetae of Burgess Shale taxa can exceed twice the maximum length of the shell from which it projects. This is unique amongst Recent and fossil brachiopod taxa and given their size, prominence and energy investment to the organism certainly had an important functional significance. Micromitra burgessensis individuals are preserved on hard skeletal elements, including conspecific shells, Tubulella and frequently on the spicules of the sponge Pirania muricata, providing direct evidence of an ecological association between two species. Morphological analysis and comparisons with fossil and extant brachiopod chaetae point to a number of potential functions, including sensory, defence, feeding, defouling, mimicry and spatial competition. Conclusions Our study indicates that it is feasible to link chaetae length to the lack of suitable substrate in the Burgess Shale environment and the increased intraspecific competition associated with this. Our results however, also lend support to the elongated chaetae as an example of Batesian mimicry, of the unpalatable sponge Pirania muricata. We also cannot discount brachiopod chaetae acting as a sensory grille, extending the tactile sensitivity of the mantle into the environment, as an early warning system to approaching predators. Electronic supplementary material The online version of this article (doi:10.1186/s12862-015-0314-4) contains supplementary material, which is available to authorized users.

The degree to which the original community composition of the Middle Cambrian Burgess Shale was altered through transport and decay and how taphonomic conditions varied through time and across taxa is poorly understood. To address these issues, variation in fossil preservation was analyzed through a vertical succession of 26 bed assemblages, each representing a single obrution event, within the 7-m-thick Greater Phyllopod Bed of the Walcott Quarry. More than 50,000 specimens belonging to 158 genera—mostly benthic, monospecific and nonbiomineralized—were included in this analysis. The decay gradient of the polychaete Burgessochaeta setigera was used as a taphonomic threshold to estimate how far decay had proceeded in each bed assemblage. Qualitative comparisons of the degree of preservation of 15 species, representing an array of different body plans, demonstrate that all bed assemblages contain a mix of articulated and in situ dissociated or completely dissociated organisms interpreted respective...

Holmer, L.E. and Caron, J-B. 2006. A soft-shelled spinose stem group brachiopod with pedicle from the Middle Cambrian Burgess Shale. — Acta Zoologica (Stockholm) 87 : 273-290 A new genus and species of a Middle Cambrian stem group brachiopod, Acanthotretella spinosa n. gen. and n. sp., is described from the Burgess Shale Formation. Most of the 42 specimens studied came from the Greater Phyllopod bed (Walcott Quarry) and were collected from five bed assemblages, each representing a single obrution event. Specimens are probably preserved within their original habitat. In contrast to all brachiopods known from the Burgess Shale, the shells of the new stem group brachiopod are often deformed and do not show signs of brittle breakage, which suggests that the valves were originally either entirely organic in composition or, more likely, had just a minor mineral component. Acanthotretella spinosa differs from all the other described Cambrian brachiopods in that it is covered by long, slender and possibly partly mineralized spines that are posteriorly inclined at an oblique angle away from the anterior margin. The spines penetrate the shell and are mainly comparable with the thorn-like organic objects that have been inferred from early siphonotretoid brachiopods. The pedicle was slender and was composed of a central coelomic region and emerged from an apical foramen at the end of an internal pedicle tube. The finding of a pedicle attached to the macrobenthic algae Dictyophycus and other epibenthos implies that A. spinosa did not have an infaunal mode of life. The visceral region and interior characters are poorly preserved.

Odontogriphus omalus was originally described as a problematic non-biomineralized lophophorate organism. Here we re-interpret Odontogriphus based on 189 new specimens including numerous exceptionally well preserved individuals from the Burgess Shale collections of the Royal Ontario Museum. This additional material provides compelling evidence that the feeding apparatus in Odontogriphus is a radula of molluscan architecture comprising two primary bipartite tooth rows attached to a radular membrane and showing replacement by posterior addition. Further characters supporting molluscan affinity include a broad foot bordered by numerous ctenidia located in a mantle groove and a stiffened cuticular dorsum. Odontogriphus has a radula similar to Wiwaxia corrugata but lacks a scleritome. We interpret these animals to be members of an early stem-group mollusc lineage that probably originated in the Neoproterozoic Ediacaran Period, providing support for the retention of a biomat-based grazing community from the late Precambrian Period until at least the Middle Cambrian.

Although Banffia constricta was first described in 1911, the affinities of this soft-bodied fossil animal have remained unclear. Banffia is redescribed here, on the basis of over 300 specimens. Banffia has a bipartite body measuring up to 10 cm in length. The entire body is axially twisted along a spiral, clockwise as seen from the front. The anterior section consists of two fused carapace-like units with a prominent crown-like structure at the anterior end surrounding the mouth. This crown-like structure is composed of three concentric circlets. A single antenniform outgrowth lies posterior to the mouth. The posterior section of Banffia is composed of 40 to 50 lightly sclerotised segments. The anus is terminal, and situated in a caudal notch. The gut is straight. Simple serially repeated structures along the gut may represent metameric mid-gut diverticulae. Banffia is considered to have been an epibenthic gregarious animal and possibly a deposit-feeder.Banffia is putatively classified as an end-member of the phylum Vetulicolia, in a new class, the Banffozoa. If the vetulicolids represent a stem-group deuterostome, the absence of gills and a possible endostyle in Banffia is problematic. The presence of mid-gut diverticulae in Banffia may represent a link with the protostomes.

The discovery of a new naraoiid nektaspid in the Upper Silurian (Pridolian) of southeastern Ontario significantly extends the range of this unusual group. Nektaspids are nonmineralized arthropods typical of Early and Middle Cambrian soft-bottom communities, but were thought to have become extinct in the Late Ordovician. The unique holotype specimen ofNaraoia bertiensisn. sp. comes from aKonservat–Lagerstättedeposit renowned for its eurypterid fauna (the Williamsville Member of the Bertie Formation).Naraoia bertiensislacks thoracic segments and is morphologically similar toNaraoia compactafrom the Middle Cambrian Burgess Shale, save for the presence of a long ventral cephalic doublure and a subtly pointed posterior shield. To examine the phylogenetic relationships of the new naraoiid, we coded characters of the holotype specimen and of nine previously described nektaspids. The results confirm a sister taxon relationship betweenNaraoia compactaandNaraoia bertiensisand the monophyly of nektaspid forms lacking thoracic segments (family Naraoiidae). This latter group may have arisen from an ancestral segment-bearing form through heterochronic loss of thoracic segments early in the Cambrian. The disjunct occurrence of a naraoiid nektaspid in the Late Silurian resembles the reappearance of other “Lazarus taxa” that were thought to have been eliminated during mass extinction events. The naraoiid lineage survived the Late Ordovician biotic crisis, but in this case the “Lazarus effect” seems likely to be taphonomic in origin.

Understanding the structure of benthic communities in the Cambrian remains a major challenge. Direct evidence for species interrelationships is rare and therefore past ecological interactions typically cannot be reconstructed with great accuracy. Here we reveal the community patterns and modes of life of brachiopods – one of the most important filter-feeding groups of Cambrian ecosystems – from the Cambrian Burgess Shale LagerstÀtte. Burgess Shale brachiopods attached to a range of hard substrates, including skeletal debris, conspecific brachiopods and enigmatic tubes, with an overwhelming preference for attachment on the demosponge Pirania muricata. The dominance of P. muricata as a substrate choice – even in bedding assemblages where P. muricata individuals are rare – and similarities to the gregarious attachment strategies of extant brachiopod species suggests that brachiopods larvae in the Burgess Shale community selected their attachment substrates. The distribution of brachiopod taxa is also intricately linked with the presence of suitable hard substrates, with species declining in bedding assemblages where their preferred hard substrates are absent. In addition, brachiopods in the Burgess Shale community are predominantly low epifaunal tierers and do not exploit niches high in the water column, despite the presence of suitable attachment sites. Our analysis of tiering height versus host height indicates that there is no selection by brachiopod larvae in regard to the height of attachment and individuals attached at the first point of contact with the selected substrate. Through comparisons with the ‘early’ Cambrian Chengjiang Biota, we confirm that by the ‘middle’ Cambrian (Series 3, Stage 5) brachiopods had already developed a range of attachment strategies similar to some modern brachiopod populations. Our results provide significant insight into the ecological constraints and adaptability of brachiopods in the earliest animal communities of the Cambrian.

We herein describe Surusicaris elegans gen. et sp. nov. (in Isoxyidae, amended), a middle (Series 3, Stage 5) Cambrian bivalved arthropod from the new Burgess Shale deposit of Marble Canyon (Kootenay National Park, British Columbia). Surusicaris exhibits 12 simple, partly undivided biramous trunk limbs with long tripartite caeca, which may illustrate a plesiomorphic “fused” condition of exopod and endopod. We construe also that the head is made of five somites (= four segments), including two eyes, one pair of anomalocaridid-like frontalmost appendages, and three pairs of poorly sclerotized uniramous limbs. This fossil may therefore be a candidate for illustrating the origin of the plesiomorphic head condition in euarthropods, and questions the significance of the “two-segmented head” in, e.g., fuxianhuiids. The frontalmost appendage in isoxyids is intriguingly disparate, bearing similarities with both dinocaridids and euarthropods. In order to evaluate the relative importance of bivalved arthropods, such as Surusicaris, in the hypothetical structuro-functional transition between the dinocaridid frontal appendage and the pre-oral—arguably deutocerebral—appendage of euarthropods, we chose a phenetic approach and computed morphospace occupancy for the frontalmost appendages of 36 stem and crown taxa. Results show different levels of evolutionary decoupling between frontalmost appendage disparity and body plans. Variance is greatest in dinocaridids and “stem bivalved” arthropods, but these groups do not occupy the morphospace homogeneously. Rather, the diversity of frontalmost appendages in “stem bivalved” arthropods, distinct in its absence of clear clustering, is found to link the morphologies of “short great appendages,” chelicerae and antennules. This find fits the hypothesis of an increase in disparity of the deutocerebral appendage prior to its diversification in euarthropods, and possibly corresponds to its original time of development. The analysis of this pattern, however, is sensitive to the—still unclear—extent of polyphyly of the “stem bivalved” taxa.

Exceptionally preserved fossils are the product of complex interplays of biological and geological processes including burial, autolysis and microbial decay, authigenic mineralization, diagenesis, metamorphism, and finally weathering and exhumation. Determining which tissues are preserved and how biases affect their preservation pathways is important for interpreting fossils in phylogenetic, ecological, and evolutionary frameworks. Although laboratory decay experiments reveal important aspects of fossilization, applying the results directly to the interpretation of exceptionally preserved fossils may overlook the impact of other key processes that remove or preserve morphological information. Investigations of fossils preserving non-biomineralized tissues suggest that certain structures that are decay resistant (e.g., the notochord) are rarely preserved (even where carbonaceous components survive), and decay-prone structures (e.g., nervous systems) can fossilize, albeit rarely. As we review here, decay resistance is an imperfect indicator of fossilization potential, and a suite of biological and geological processes account for the features preserved in exceptional fossils.

Ecological interactions, including symbiotic associations such as mutualism, parasitism and commensalism are crucial factors in generating evolutionary novelties and strategies. Direct examples of species interactions in the fossil record generally involve organisms attached to sessile organisms in an epibiont or macroboring relationship. Here we provide support for an intimate ecological association between a calcareous brachiopod (Nisusia) and the stem group mollusc Wiwaxia from the Burgess Shale. Brachiopod specimens are fixed to Wiwaxia scleritomes, the latter showing no signs of decay and disarticulation, suggesting a live association. We interpret this association as the oldest unambiguous example of a facultative ectosymbiosis between a sessile organism and a mobile benthic animal in the fossil record. The potential evolutionary advantage of this association is discussed, brachiopods benefiting from ease of attachment, increased food supply, avoidance of turbid benthic conditions, biofoul and possible protection from predators, suggesting commensalism (benefiting the symbiont with no impact for the host). While Cambrian brachiopods are relatively common epibionts, in particular on sponges, the association of Nisusia with the motile Wiwaxia is rare for a brachiopod species, fossil or living, and suggests that symbiotic associations were already well established and diversified by the "middle'' (Series 3, Stage 5) Cambrian.

Recently discovered mammal-bearing layers near La Roche-Blanche-Gergovie, Clermont-Ferrand basin (France) make it possible to attribute an Early Miocene age (Aquitanian, MN1) to the upper part of these series, hitherto considered as Late Oligocene. Palaeoenvironmental inferences are drawn from the micro-mammal fauna, from a complete and connected posterior leg of Mesaceratherium pauliacense as well as from ostracod community-bearing beds. Up-dating of the mammal faunas from old mammal localities in the Clermont basin and recently discovered ones point out, for the first time, a narrow subsident channel, near the Western Limagne main fault, testifying that the rift activity continued during Aquitanian times.

Burgess Shale-type fossil assemblages provide the best evidence of the 'Cambrian explosion'. Here we report the discovery of an extraordinary new soft-bodied fauna from the Burgess Shale. Despite its proximity (ca. 40 km) to Walcott's original locality, the Marble Canyon fossil assemblage is distinct, and offers new insights into the initial diversification of metazoans, their early morphological disparity, and the geographic ranges and longevity of many Cambrian taxa. The arthropod-dominated assemblage is remarkable for its high density and diversity of soft-bodied fossils, as well as for its large proportion of new species (22% of total diversity) and for the preservation of hitherto unreported anatomical features, including in the chordate Metaspriggina and the arthropod Mollisonia. The presence of the stem arthropods Misszhouia and Primicaris, previously known only from the early Cambrian of China, suggests that the palaeogeographic ranges and longevity of Burgess Shale taxa may be underestimated.

Burgess Shale-type deposits are renowned for their exquisite preservation of soft-bodied organisms, representing a range of animal body plans that evolved during the Cambrian ‘explosion’. However, the rarity of these fossil deposits makes it difficult to reconstruct the broader-scale distributions of their constituent organisms. By contrast, microscopic skeletal elements represent an extensive chronicle of early animal evolution—but are difficult to interpret in the absence of corresponding whole-body fossils. Here, we provide new observations on the dorsal spines of the Cambrian lobopodian (panarthropod) wormHallucigenia sparsafrom the Burgess Shale (Cambrian Series 3, Stage 5). These exhibit a distinctive scaly microstructure and layered (cone-in-cone) construction that together identify a hitherto enigmatic suite of carbonaceous and phosphatic Cambrian microfossils—including material attributed toMongolitubulus,RushtonitesandRhombocorniculum—as spines ofHallucigenia-type lobopodians. Hallucigeniids are thus revealed as an important and widespread component of disparate Cambrian communities from late in the Terreneuvian (Cambrian Stage 2) through the ‘middle’ Cambrian (Series 3); their apparent decline in the latest Cambrian may be partly taphonomic. The cone-in-cone construction of hallucigeniid sclerites is shared with the sclerotized cuticular structures (jaws and claws) in modern onychophorans. More generally, our results emphasize the reciprocal importance and complementary roles of Burgess Shale-type fossils and isolated microfossils in documenting early animal evolution.

Examination of a fossil enteropneust, Spartobranchus tenuis (Walcott, 1911), from the Cambrian-period Burgess Shale shows that they looked similar to modern enteropneusts but lived in tubes, like modern pterobranchs; the findings shed light on the common ancestor of enteropneusts and pterobranchs, and hence the origin of chordates. Enteropneusts, or acorn worms, are mud-dwelling creatures that live from the foreshore down to the deep ocean. They are related to pterobranchs, small, colonial, tube-dwelling animals that superficially look completely different. Both are related to echinoderms (starfish and allies) and chordates — the group of animals that includes ourselves. Here Jean-Bernard Caron and colleagues describe fossil enteropneusts from the famous Cambrian Burgess Shale of British Columbia. They look much like some modern enteropneusts, but lived in tubes, more like modern pterobranchs. This adds to the diversity of the fossil fauna and gives pointers to what the common ancestor of enteropneusts and pterobranchs looked like. It might also fuel the debate about whether the ancestor of chordates was a free-living worm, or a sessile, colonial creature. Hemichordates are a marine group that, apart from one monospecific pelagic larval form, are represented by the vermiform enteropneusts and minute colonial tube-dwelling pterobranchs. Together with echinoderms, they comprise the clade Ambulacraria1. Despite their restricted diversity, hemichordates provide important insights into early deuterostome evolution, notably because of their pharyngeal gill slits2. Hemichordate phylogeny has long remained problematic3,4, not least because the nature of any transitional form that might serve to link the anatomically disparate enteropneusts and pterobranchs is conjectural. Hence, inter-relationships have also remained controversial. For example, pterobranchs have sometimes been compared to ancestral echinoderms1. Molecular data identify enteropneusts as paraphyletic, and harrimaniids5,6,7,8 as the sister group of pterobranchs. Recent molecular phylogenies suggest that enteropneusts are probably basal within hemichordates, contrary to previous views9, but otherwise provide little guidance as to the nature of the primitive hemichordate7,8. In addition, the hemichordate fossil record is almost entirely restricted to peridermal skeletons of pterobranchs, notably graptolites10,11. Owing to their low preservational potentials, fossil enteropneusts are exceedingly rare12,13,14,15, and throw no light on either hemichordate phylogeny or the proposed harrimaniid–pterobranch transition. Here we describe an enteropneust, Spartobranchus tenuis (Walcott, 1911), from the Middle Cambrian-period (Series 3, Stage 5) Burgess Shale. It is remarkably similar to the extant harrimaniids, but differs from all known enteropneusts in that it is associated with a fibrous tube that is sometimes branched. We suggest that this is the precursor of the pterobranch periderm, and supports the hypothesis that pterobranchs are miniaturized and derived from an enteropneust-like worm5,6. It also shows that the periderm was acquired before size reduction and acquisition of feeding tentacles, and that coloniality emerged through aggregation of individuals, perhaps similar to the Cambrian rhabdopleurid Fasciculitubus11. The presence of both enteropneusts and pterobranchs in Middle Cambrian strata, suggests that hemichordates originated at the onset of the Cambrian explosion.

The Middle Cambrian Pikaia gracilens (Walcott) has an iconic position as a Cambrian chordate, but until now no detailed description has been available. Here on the basis of the 114 available specimens we review its anatomy, confirm its place in the chordates and explore with varying degrees of confidence its relationships to both extant and extinct chordates and other deuterostomes. The body of Pikaia is fusiform, laterally compressed and possesses about 100 myomeres. The head is small, bilobed and bears two narrow tentacles. There is no evidence for eyes. Apart from a thin dorsal fin (without finrays) and a series of at least nine bilaterally arranged appendages with possible pharyngeal pores at the anterior end, there are no other external features. In addition to the musculature the internal anatomy includes an alimentary canal, the anterior of which forms a prominent lenticular unit that is almost invariably preserved in positive relief. The cavity is interpreted as pharyngeal, implying that the mouth itself was almost terminal. The posterior extension of the gut is unclear although the anus appears to have been terminal. The most prominent internal structure is a reflectively preserved unit, possibly hollow, termed here the dorsal organ. Although formerly interpreted as a notochord its position and size make this less likely. Its original function remains uncertain, but it could have formed a storage organ. Ventral to the dorsal organ a narrower strand of tissue is interpreted as representing the nerve chord and notochord. In addition to these structures, there is also evidence for a vascular system, including a ventral blood vessel. The position of Pikaia in the chordates is largely based on the presence of sigmoidal myomeres, and the more tentative identification of a notochord. In many other respects, Pikaia differs from the expected nature of primitive chordates, especially as revealed in amphioxus and the Cambrian record (including Cathaymyrus, Haikouichthys, Metaspriggina, Myllokunmingia, and Zhongxiniscus). Whilst the possibility that Pikaia is simply convergent on the chordates cannot be dismissed, we prefer to build a scenario that regards Pikaia as the most stem-ward of the chordates with links to the phylogenetically controversial yunnanozoans. This hypothesis has implications for the evolution of the myomeres, notochord and gills. Finally, the wealth of material of Pikaia indicates that, although by definition there must be some sort of taphonomic imprint, the consistency of preservational details allows a reliable reconstruction of the anatomy and does not significantly erode phylogenetically relevant characters.