A plurality of morphological characters need not equate with phylogenetic accuracy: A rare genomic change refutes the placement of Solifugae and Pseudoscorpiones in Haplocnemata.

Recent advances in higher-level invertebrate phylogeny have leveraged shared features of genomic architecture to resolve contentious nodes across the tree of life. Yet, the interordinal relationships within Chelicerata have remained recalcitrant given competing topologies in recent molecular analyses. As such, relationships between topologically unstable orders remain supported primarily by morphological cladistic analyses. Solifugae, one such unstable chelicerate order, has long been thought to be the sister group of Pseudoscorpiones, forming the clade Haplocnemata, on the basis of eight putative morphological synapomorphies. The discovery, however, of a shared whole genome duplication placing Pseudoscorpiones in Arachnopulmonata provides the opportunity for a simple litmus test evaluating the validity of Haplocnemata. Here, we present the first developmental transcriptome of a solifuge (Titanopuga salinarum) and survey copy numbers of the homeobox genes for evidence of systemic duplication. We find that over 70% of the identified homeobox genes in T. salinarum are retained in a single copy, while representatives of the arachnopulmonates retain orthologs of those genes as two or more copies. Our results refute the placement of Solifugae in Haplocnemata. Subsequent reevaluation of putative interordinal morphological synapomorphies among chelicerates reveals a high incidence of homoplasy, reversals, and inaccurate coding within Haplocnemata and other small clades, as well as Arachnida more broadly, suggesting existing morphological character matrices are insufficient to resolve chelicerate phylogeny.

Raptorial appendages of the Cambrian apex predator Anomalocaris canadensis are built for soft prey and speed.

The stem-group euarthropod Anomalocaris canadensis is one of the largest Cambrian animals and is often considered the quintessential apex predator of its time. This radiodont is commonly interpreted as a demersal hunter, responsible for inflicting injuries seen in benthic trilobites. However, controversy surrounds the ability of A. canadensis to use its spinose frontal appendages to masticate or even manipulate biomineralized prey. Here, we apply a new integrative computational approach, combining three-dimensional digital modelling, kinematics, finite-element analysis (FEA) and computational fluid dynamics (CFD) to rigorously analyse an A. canadensis feeding appendage and test its morphofunctional limits. These models corroborate a raptorial function, but expose inconsistencies with a capacity for durophagy. In particular, FEA results show that certain parts of the appendage would have experienced high degrees of plastic deformation, especially at the endites, the points of impact with prey. The CFD results demonstrate that outstretched appendages produced low drag and hence represented the optimal orientation for speed, permitting acceleration bursts to capture prey. These data, when combined with evidence regarding the functional morphology of its oral cone, eyes, body flaps and tail fan, suggest that A. canadensis was an agile nektonic predator that fed on soft-bodied animals swimming in a well-lit water column above the benthos. The lifestyle of A. canadensis and that of other radiodonts, including plausible durophages, suggests that niche partitioning across this clade influenced the dynamics of Cambrian food webs, impacting on a diverse array of organisms at different sizes, tiers and trophic levels.

Re-evaluating and dating myriapod diversification with phylotranscriptomics under a regime of dense taxon sampling.

Recent transcriptomic studies of myriapod phylogeny have been based on relatively small datasets with <40 myriapod terminals and variably supported or contradicted the traditional morphological groupings of Progoneata and Dignatha. Here we amassed a large dataset of 104 myriapod terminals, including multiple species for each of the four myriapod classes. Across the tree, most nodes are stable and well supported. Most analyses across a range of gene occupancy levels provide moderate to strong support for a deep split of Myriapoda into Symphyla + Pauropoda (=Edafopoda) and an uncontradicted grouping of Chilopoda + Diplopoda (=Pectinopoda nov.), as in other recent transcriptome-based analyses; no analysis recovers Progoneata or Dignatha as clades. As in all recent multi-locus and phylogenomic studies, chilopod interrelationships resolve with Craterostigmus excluded from Amalpighiata rather than uniting with other centipedes with maternal brood care in Phylactometria. Diplopod ordinal interrelationships are largely congruent with morphology-based classifications. Chilognathan clades that are not invariably advocated by morphologists include Glomerida + Glomeridesmida, such that the volvation-related characters of pill millipedes may be convergent, and Stemmiulida + Polydesmida more closely allied to Juliformia than to Callipodida + Chordeumatida. The latter relationship implies homoplasy in spinnerets and contradicts Nematophora. A time-tree with nodes calibrated by 25 myriapod and six outgroup fossil terminals recovers Cambrian-Ordovician divergences for the deepest splits in Myriapoda, Edafopoda and Pectinopoda, predating the terrestrial fossil record of myriapods as in other published chronograms, whereas age estimates within Chilopoda and Diplopoda overlap with or do not appreciably predate the calibration fossils. The grouping of Chilopoda and Diplopoda is recovered in all our analyses and is formalized as Pectinopoda nov., named for the shared presence of mandibular comb lamellae. New taxonomic proposals for Chilopoda based on uncontradicted clades are Tykhepoda nov. for the three blind families of Scolopendromorpha that share a "sieve-type" gizzard, and Taktikospina nov. for Scolopendromorpha to the exclusion of Mimopidae.

Genetic diversity varies with species traits and latitude in predatory soil arthropods (Myriapoda: Chilopoda).

Aim: To investigate the drivers of intra-specific genetic diversity in centipedes, a group of ancient predatory soil arthropods. Location: Asia, Australasia and Europe. Time Period: Present. Major Taxa Studied: Centipedes (Class: Chilopoda). Methods: We assembled a database of 1245 mitochondrial cytochrome c oxidase subunit I sequences representing 128 centipede species from all five orders of Chilopoda. This sequence dataset was used to estimate genetic diversity for centipede species and compare its distribution with estimates from other arthropod groups. We studied the variation in centipede genetic diversity with species traits and biogeography using a beta regression framework, controlling for the effect of shared evolutionary history within a family. Results: A wide variation in genetic diversity across centipede species (0-0.1713) falls towards the higher end of values among arthropods. Overall, 27.57% of the variation in mitochondrial COI genetic diversity in centipedes was explained by a combination of predictors related to life history and biogeography. Genetic diversity decreased with body size and latitudinal position of sampled localities, was greater in species showing maternal care and increased with geographic distance among conspecifics. Main Conclusions: Centipedes fall towards the higher end of genetic diversity among arthropods, which may be related to their long evolutionary history and low dispersal ability. In centipedes, the negative association of body size with genetic diversity may be mediated by its influence on local abundance or the influence of ecological strategy on long-term population history. Species with maternal care had higher genetic diversity, which goes against expectations and needs further scrutiny. Hemispheric differences in genetic diversity can be due to historic climatic stability and lower seasonality in the southern hemisphere. Overall, we find that despite the differences in mean genetic diversity among animals, similar processes related to life-history strategy and biogeography are associated with the variation within them.

Aquatic stem group myriapods close a gap between molecular divergence dates and the terrestrial fossil record.

Identifying marine or freshwater fossils that belong to the stem groups of the major terrestrial arthropod radiations is a longstanding challenge. Molecular dating and fossils of their pancrustacean sister group predict that myriapods originated in the Cambrian, much earlier than their oldest known fossils, but uncertainty about stem group Myriapoda confounds efforts to resolve the timing of the group's terrestrialization. Among a small set of candidates for membership in the stem group of Myriapoda, the Cambrian to Triassic euthycarcinoids have repeatedly been singled out. The only known Devonian euthycarcinoid, Heterocrania rhyniensis from the Rhynie and Windyfield cherts hot spring complex in Scotland, reveals details of head structures that constrain the evolutionary position of euthycarcinoids. The head capsule houses an anterior cuticular tentorium, a feature uniquely shared by myriapods and hexapods. Confocal microscopy recovers myriapod-like characters of the preoral chamber, such as a prominent hypopharynx supported by tentorial bars and superlinguae between the mandibles and hypopharynx, reinforcing an alliance between euthycarcinoids and myriapods recovered in recent phylogenetic analysis. The Cambrian occurrence of the earliest euthycarcinoids supplies the oldest compelling evidence for an aquatic stem group for either Myriapoda or Hexapoda, previously a lacuna in the body fossil record of these otherwise terrestrial lineages until the Silurian and Devonian, respectively. The trace fossil record of euthycarcinoids in the Cambrian and Ordovician reveals amphibious locomotion in tidal environments and fills a gap between molecular estimates for myriapod origins in the Cambrian and a post-Ordovician crown group fossil record.

Trilobite evolutionary rates constrain the duration of the Cambrian explosion.

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

Biomechanical analyses of Cambrian euarthropod limbs reveal their effectiveness in mastication and durophagy.

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

Phylogeny of Lithobiidae Newport, 1844, with emphasis on the megadiverse genus Lithobius Leach, 1814 (Myriapoda, Chilopoda).

Phylogenetic analyses based on molecular and morphological data were conducted to shed light on relationships within the mostly Palaearctic/Oriental centipede family Lithobiidae, with a particular focus on the Palaearctic genus Lithobius Leach, 1814 (Lithobiidae, Lithobiomorpha), which contains >500 species and subspecies. Previous studies based on morphological data resolved Lithobius as nonmonophyletic, but molecular-based phylogenetic analyses have until now sampled few species. To elucidate species inter-relationships of the genus, test the validity of its classification into subgenera, and infer its relationships with other Lithobiidae, we obtained molecular data (nuclear markers: 18S rRNA, 28S rRNA; mitochondrial markers: 16S rRNA, COI) and 61 morphological characters for 44 species of Lithobius representing four of its eight subgenera and nine other representatives of Lithobiidae. The data were analyzed phylogenetically using maximum-likelihood, parsimony and Bayesian inference. This study suggests that (i) a close relationship between L. giganteus and the pterygotergine Disphaerobius loricatus highlighted in recent morphological analyses is also strongly supported by molecular data, and Pterygoterginae is formally synonymized with Lithobiinae; (ii) the Oriental/Australian genus Australobius is consistently resolved as sister group to all other sampled Lithobiidae by the molecular and combined data; (iii) the subfamily Ethopolyinae may be paraphyletic; (iv) the genus Lithobius is nonmonophyletic; (v) the subgenera Lithobius, Sigibius and Monotarsobius are nonmonophyletic and should not be used in future taxonomic studies; and (vi) there are instances of cryptic species and cases in which subspecies should be elevated to full species status, as identified for some European taxa within Lithobius.

Ancestral morphology of Ecdysozoa constrained by an early Cambrian stem group ecdysozoan.

BACKGROUND: Ecdysozoa are the moulting protostomes, including arthropods, tardigrades, and nematodes. Both the molecular and fossil records indicate that Ecdysozoa is an ancient group originating in the terminal Proterozoic, and exceptional fossil biotas show their dominance and diversity at the beginning of the Phanerozoic. However, the nature of the ecdysozoan common ancestor has been difficult to ascertain due to the extreme morphological diversity of extant Ecdysozoa, and the lack of early diverging taxa in ancient fossil biotas. RESULTS: Here we re-describe Acosmia maotiania from the early Cambrian Chengjiang Biota of Yunnan Province, China and assign it to stem group Ecdysozoa. Acosmia features a two-part body, with an anterior proboscis bearing a terminal mouth and muscular pharynx, and a posterior annulated trunk with a through gut. Morphological phylogenetic analyses of the protostomes using parsimony, maximum likelihood and Bayesian inference, with coding informed by published experimental decay studies, each placed Acosmia as sister taxon to Cycloneuralia + Panarthropoda-i.e. stem group Ecdysozoa. Ancestral state probabilities were calculated for key ecdysozoan nodes, in order to test characters inferred from fossils to be ancestral for Ecdysozoa. Results support an ancestor of crown group ecdysozoans sharing an annulated vermiform body with a terminal mouth like Acosmia, but also possessing the pharyngeal armature and circumoral structures characteristic of Cambrian cycloneuralians and lobopodians. CONCLUSIONS: Acosmia is the first taxon placed in the ecdysozoan stem group and provides a constraint to test hypotheses on the early evolution of Ecdysozoa. Our study suggests acquisition of pharyngeal armature, and therefore a change in feeding strategy (e.g. predation), may have characterised the origin and radiation of crown group ecdysozoans from Acosmia-like ancestors.

Brain structure resolves the segmental affinity of anomalocaridid appendages.

Despite being among the most celebrated taxa from Cambrian biotas, anomalocaridids (order Radiodonta) have provoked intense debate about their affinities within the moulting-animal clade that includes Arthropoda. Current alternatives identify anomalocaridids as either stem-group euarthropods, crown-group euarthropods near the ancestry of chelicerates, or a segmented ecdysozoan lineage with convergent similarity to arthropods in appendage construction. Determining unambiguous affinities has been impeded by uncertainties about the segmental affiliation of anomalocaridid frontal appendages. These structures are variably homologized with jointed appendages of the second (deutocerebral) head segment, including antennae and 'great appendages' of Cambrian arthropods, or with the paired antenniform frontal appendages of living Onychophora and some Cambrian lobopodians. Here we describe Lyrarapax unguispinus, a new anomalocaridid from the early Cambrian Chengjiang biota, southwest China, nearly complete specimens of which preserve traces of muscles, digestive tract and brain. The traces of brain provide the first direct evidence for the segmental composition of the anomalocaridid head and its appendicular organization. Carbon-rich areas in the head resolve paired pre-protocerebral ganglia at the origin of paired frontal appendages. The ganglia connect to areas indicative of a bilateral pre-oral brain that receives projections from the eyestalk neuropils and compound retina. The dorsal, segmented brain of L. unguispinus reinforces an alliance between anomalocaridids and arthropods rather than cycloneuralians. Correspondences in brain organization between anomalocaridids and Onychophora resolve pre-protocerebral ganglia, associated with pre-ocular frontal appendages, as characters of the last common ancestor of euarthropods and onychophorans. A position of Radiodonta on the euarthropod stem-lineage implies the transformation of frontal appendages to another structure in crown-group euarthropods, with gene expression and neuroanatomy providing strong evidence that the paired, pre-oral labrum is the remnant of paired frontal appendages.

Soft-Bodied Fossils Are Not Simply Rotten Carcasses - Toward a Holistic Understanding of Exceptional Fossil Preservation: Exceptional Fossil Preservation Is Complex and Involves the Interplay of Numerous Biological and Geological Processes.

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.

Evolutionary biogeography of the centipede genus Ethmostigmus from Peninsular India: testing an ancient vicariance hypothesis for Old World tropical diversity.

BACKGROUND: Understanding the relative influence of vicariance and dispersal in shaping Old World tropical biodiversity remains a challenge. We aimed to infer the roles of these alternative biogeographic processes using a species time-tree for the centipede genus Ethmostigmus from the Old World tropics. Additionally, we explored fine-scale biogeographic patterns for an endemic radiation of Ethmostigmus from the peninsular Indian Plate (PIP), an area with complex geological and climatic history. RESULTS: Divergence time estimates suggest that Ethmostigmus began diversifying in the Late Cretaceous, 99 (± 25) million years ago (Ma), its early biogeographic history shaped by vicariance. Members of Ethmostigmus in PIP form a monophyletic group that underwent endemic radiation in the Late Cretaceous, 72 (± 25) Ma. In contrast, a new species of Ethmostigmus from north-east India formed a clade with African/Australian species. Fine-scale biogeographic analyses in PIP predict that Indian Ethmostigmus had an ancestor in southern-central parts of the Western Ghats. This was followed by four independent dispersal events from the southern-central Western Ghats to the Eastern Ghats, and between different parts of the Western Ghats in the Cenozoic. CONCLUSIONS: Our results are consistent with Gondwanan break-up driving the early evolutionary history of the genus Ethmostigmus. Multiple dispersal events coinciding with geo-climatic events throughout the Cenozoic shaped diversification in PIP. Ethmostigmus species in PIP are restricted to wet forests and have retained that niche throughout their diversification.

Correction to: Evolutionary biogeography of the centipede genus Ethmostigmus from Peninsular India: testing an ancient vicariance hypothesis for Old World tropical diversity.

Following publication of the original article [1], the authors notified us of an error in the Results section of the Abstract. The original article has been corrected.

Developing an integrated understanding of the evolution of arthropod segmentation using fossils and evo-devo.

Segmentation is fundamental to the arthropod body plan. Understanding the evolutionary steps by which arthropods became segmented is being transformed by the integration of data from evolutionary developmental biology (evo-devo), Cambrian fossils that allow the stepwise acquisition of segmental characters to be traced in the arthropod stem-group, and the incorporation of fossils into an increasingly well-supported phylogenetic framework for extant arthropods based on genomic-scale datasets. Both evo-devo and palaeontology make novel predictions about the evolution of segmentation that serve as testable hypotheses for the other, complementary data source. Fossils underpin such hypotheses as arthropodization originating in a frontal appendage and then being co-opted into other segments, and segmentation of the endodermal midgut in the arthropod stem-group. Insights from development, such as tagmatization being associated with different modes of segment generation in different body regions, and a distinct patterning of the anterior head segments, are complemented by palaeontological evidence for the pattern of tagmatization during ontogeny of exceptionally preserved fossils. Fossil and developmental data together provide evidence for a short head in stem-group arthropods and the mechanism of its formation and retention. Future breakthroughs are expected from identification of molecular signatures of developmental innovations within a phylogenetic framework, and from a focus on later developmental stages to identify the differentiation of repeated units of different systems within segmental precursors.

Jaw elements in Plumulites bengtsoni confirm that machaeridians are extinct armoured scaleworms.

Machaeridians are Palaeozoic animals that are dorsally armoured with serialized, imbricating shell plates that cover or enclose the body. Prior to the discovery of an articulated plumulitid machaeridian from the Early Ordovician of Morocco that preserved unambiguous annelid characters (segmental parapodia with chaetae), machaeridians were a palaeontological mystery, having been previously linked to echinoderms, barnacles, tommotiids (putative stem-group brachiopods) or molluscs. Although the annelid affinities of machaeridians are now firmly established, their position within the phylum and relevance for understanding the early evolution of Annelida is less secure, with competing hypotheses placing Machaeridia in the stem or deeply nested within the crown group of annelids. We describe a scleritome of Plumulites bengtsoni from the Fezouata Formation of Morocco that preserves an anterior jaw apparatus consisting of at least two discrete elements that exhibit growth lines. Although jaws have multiple independent origins within the annelid crown group, comparable jaws are present only within Phyllodocida, the clade that contains modern aphroditiforms (scaleworms and relatives). Phylogenetic analysis places a monophyletic Machaeridia within the crown group of Phyllodocida in total-group Aphroditiformia, consistent with a common origin of machaeridian shell plates and scaleworm elytrae. The inclusion of machaeridians in Aphroditiformia truncates the ghost lineage of Phyllodocida by almost a hundred million years.

Fine-scale appendage structure of the Cambrian trilobitomorph Naraoia spinosa and its ontogenetic and ecological implications.

Trilobitomorphs are a species-rich Palaeozoic arthropod assemblage that unites trilobites with several other lineages that share similar appendage structure. Post-embryonic development of the exoskeleton is well documented for some trilobitomorphs, especially trilobites, but little is known of the ontogeny of their soft parts, limiting understanding of their autecology. Here, we document appendage structure of the Cambrian naraoiid trilobitomorph Naraoia spinosa by computed microtomography, resulting in three-dimensional reconstructions of appendages at both juvenile and adult stages. The adult has dense, strong spines on the protopods of post-antennal appendages, implying a predatory/scavenging behaviour. The absence of such gnathobasic structures, but instead tiny protopodal bristles and a number of endopodal setae, suggests a detritus-feeding strategy for the juvenile. Our data add strong morphological evidence for ecological niche shifting by Cambrian arthropods during their life cycles. A conserved number of appendages across the sampled developmental stages demonstrates that Naraoia ceased budding off new appendages by the mid-juvenile stage.

Chelicerate neural ground pattern in a Cambrian great appendage arthropod.

Preservation of neural tissue in early Cambrian arthropods has recently been demonstrated, to a degree that segmental structures of the head can be associated with individual brain neuromeres. This association provides novel data for addressing long-standing controversies about the segmental identities of specialized head appendages in fossil taxa. Here we document neuroanatomy in the head and trunk of a 'great appendage' arthropod, Alalcomenaeus sp., from the Chengjiang biota, southwest China, providing the most complete neuroanatomical profile known from a Cambrian animal. Micro-computed tomography reveals a configuration of one optic neuropil separate from a protocerebrum contiguous with four head ganglia, succeeded by eight contiguous ganglia in an eleven-segment trunk. Arrangements of optic neuropils, the brain and ganglia correspond most closely to the nervous system of Chelicerata of all extant arthropods, supporting the assignment of 'great appendage' arthropods to the chelicerate total group. The position of the deutocerebral neuromere aligns with the insertion of the great appendage, indicating its deutocerebral innervation and corroborating a homology between the 'great appendage' and chelicera indicated by morphological similarities. Alalcomenaeus and Fuxianhuia protensa demonstrate that the two main configurations of the brain observed in modern arthropods, those of Chelicerata and Mandibulata, respectively, had evolved by the early Cambrian.

Homeosis in a scorpion supports a telopodal origin of pectines and components of the book lungs.

BACKGROUND: The morphological and functional evolution of appendages has played a key role in the diversification of arthropods. While the ancestral arthropod appendage is held to be polyramous, terrestriality is associated with the reduction or loss of appendage rami, which may obscure the homology of different appendage derivatives. Proxies for appendage homology have included surveys of cross-reactive antibodies for wing markers like Nubbin/PDM, which have suggested that the abdominal appendages of arachnids (e.g., book lungs, tracheal tubules) are derived from ancestral gills (epipods). RESULTS: Here, we discovered a rare case of inferred homeosis in a scorpion in which the bilobed genital opercula and the pectines are transformed to walking legs, and an abnormal sternite shows a book lung close to an everted structure comparable to the morphology of some Palaeozoic scorpion fossils. CONCLUSIONS: The observed morphology is consistent with abnormal expression of homeotic genes during embryonic development. The phenotype of this abnormal specimen suggests that the genital opercula, the pectines, and parts of the book lung may be derived from the telopodite of abdominal appendages rather than from epipods. This interpretation contradicts the "ancestral gill" hypothesis but reconciles features of the Palaeozoic scorpion fossil record with the embryology of modern scorpions.

Complex brain and optic lobes in an early Cambrian arthropod.

The nervous system provides a fundamental source of data for understanding the evolutionary relationships between major arthropod groups. Fossil arthropods rarely preserve neural tissue. As a result, inferring sensory and motor attributes of Cambrian taxa has been limited to interpreting external features, such as compound eyes or sensilla decorating appendages, and early-diverging arthropods have scarcely been analysed in the context of nervous system evolution. Here we report exceptional preservation of the brain and optic lobes of a stem-group arthropod from 520 million years ago (Myr ago), Fuxianhuia protensa, exhibiting the most compelling neuroanatomy known from the Cambrian. The protocerebrum of Fuxianhuia is supplied by optic lobes evidencing traces of three nested optic centres serving forward-viewing eyes. Nerves from uniramous antennae define the deutocerebrum, and a stout pair of more caudal nerves indicates a contiguous tritocerebral component. Fuxianhuia shares a tripartite pre-stomodeal brain and nested optic neuropils with extant Malacostraca and Insecta, demonstrating that these characters were present in some of the earliest derived arthropods. The brain of Fuxianhuia impacts molecular analyses that advocate either a branchiopod-like ancestor of Hexapoda or remipedes and possibly cephalocarids as sister groups of Hexapoda. Resolving arguments about whether the simple brain of a branchiopod approximates an ancestral insect brain or whether it is the result of secondary simplification has until now been hindered by lack of fossil evidence. The complex brain of Fuxianhuia accords with cladistic analyses on the basis of neural characters, suggesting that Branchiopoda derive from a malacostracan-like ancestor but underwent evolutionary reduction and character reversal of brain centres that are common to hexapods and malacostracans. The early origin of sophisticated brains provides a probable driver for versatile visual behaviours, a view that accords with compound eyes from the early Cambrian that were, in size and resolution, equal to those of modern insects and malacostracans.

The functional head of the Cambrian radiodontan (stem-group Euarthropoda) Amplectobelua symbrachiata.

BACKGROUND: Segmental composition and homologies of the head of stem-group Euarthropoda have been the foci of recent studies on arthropod origins. An emerging hypothesis suggests that upper-stem group euarthropods possessed a three-segmented head/brain, including an ocular segment (protocerebrum) followed by the deutocerebrum with associated antennae/raptorial limbs and the tritocerebrum, while in the lower stem, head structures of Radiodonta are wholly associated with the protocerebrum and its preceding part. However, this hypothesis is incompletely tested because detailed knowledge on the head components of radiodontans is patchy, and informative articulated specimens are lacking for many taxa. Amplectobelua symbrachiata is the most common radiodontan species in the Chengjiang biota (ca. 520 Ma), normally known as isolated frontal appendages. Here we present detailed descriptions of new articulated specimens that elucidate the morphology and function of its head structures, and discuss their implications for hypotheses about euarthropod cephalic organisation. RESULTS: In addition to a central oval head shield, A. symbrachiata also bears a pair of P-elements connected by an elongated rod. The mouth consists of sets of smooth and tuberculate plates, in contrast to the typical radial oral cones of other radiodontans. Previously identified 'palm-like teeth' are located external to the mouth in the posterior head region, and are interpreted as segmental gnathobase-like structures (GLSs) associated with at least three reduced transitional flaps in a one (pair)-to-one (pair) pattern, consistent with an appendicular nature. Comparisons with other panarthropods show that GLSs are morphologically similar to the mandibles and other gnathobasic mouthparts of euarthropods, as well as to the jaws of onychophorans, indicating their functional integration into the feeding activities of A. symbrachiata. CONCLUSIONS: The functional head of A. symbrachiata must include the reduced transitional segments (and their associated structures), which have been identified in several other radiodontans. This functional view supports the idea that the integration of segments (and associated appendages) into the head region, probably driven by feeding, occurred along the euarthropod stem-lineage. However, the number of reduced transitional segments varies between different groups and it remains uncertain whether GLSs represent proximal or distal parts of appendages. Our study is the first description of appendicular structures other than the frontal appendages in the functional head region of radiodontans, revealing novel feeding structures in the morphological transition from the lower- to the upper- stem-group of Euarthropoda.