Brain and eyes of Kerygmachela reveal protocerebral ancestry of the panarthropod head

A new interpretation of Pikaia reveals the origins of the chordate body plan

Our understanding of the evolutionary origin of Chordata, one of the most disparate and ecologically significant animal phyla, is hindered by a lack of unambiguous stem-group relatives. Problematic Cambrian fossils that have been considered as candidate chordates include vetulicolians,1Yunnanozoon,2 and the iconic Pikaia.3 However, their phylogenetic placement has remained poorly constrained, impeding reconstructions of character evolution along the chordate stem lineage. Here we reinterpret the morphology of Pikaia, providing evidence for a gut canal and, crucially, a dorsal nerve cord-a robust chordate synapomorphy. The identification of these structures underpins a new anatomical model of Pikaia that shows that this fossil was previously interpreted upside down. We reveal a myomere configuration intermediate between amphioxus and vertebrates and establish morphological links between Yunnanozoon, Pikaia, and uncontroversial chordates. In this light, we perform a new phylogenetic analysis, using a revised, comprehensive deuterostome dataset, and establish a chordate stem lineage. We resolve vetulicolians as a paraphyletic group comprising the earliest diverging stem chordates, subtending a grade of more derived stem-group chordates comprising Yunnanozoon and Pikaia. Our phylogenetic results reveal the stepwise acquisition of characters diagnostic of the chordate crown group. In addition, they chart a phase in early chordate evolution defined by the gradual integration of the pharyngeal region with a segmented axial musculature, supporting classical evolutionary-developmental hypotheses of chordate origins4 and revealing a "lost chapter" in the history of the phylum.

The early Cambrian Kylinxia zhangi and evolution of the arthropod head

The early Cambrian Kylinxia zhangi occupies a pivotal position in arthropod evolution, branching from the euarthropod stem lineage between radiodonts (Anomalocaris and relatives) and "great-appendage" arthropods.1,2 Its combination of appendage and exoskeletal features is viewed as uniquely bridging the morphologies of so-called "lower" and "upper" stem-group euarthropods.3,4 Microtomographic study of new specimens of Kylinxia refines and corrects previous interpretation of head structures in this species. Phylogenetic analyses incorporating new data reinforce the placement of Kylinxia in the euarthropod stem group but support new hypotheses of head evolution. The head of Kylinxia is composed of six segments, as in extant mandibulates, e.g., insects.5 In Kylinxia, these are an anterior sclerite associated with an unpaired median eye and paired lateral eyes (thus three rather than five eyes as was previously described1), deutocerebral frontal-most appendages, and four pairs of biramous appendages (rather than two pairs of uniramous appendages). Phylogenetic trees suggest that a six-segmented head in the euarthropod crown group was already acquired by a common ancestor with Kylinxia. The segmental alignment and homology of spinose frontal-most appendages between radiodonts and upper stem-group euarthropods6,7,8,9,10 is bolstered by morphological similarities and inferred phylogenetic continuity between Kylinxia and other stem-group euarthropods.

Developmental and genomic insight into the origin of the tardigrade body plan

Tardigrada is an ancient lineage of miniaturized animals. As an outgroup of the well-studied Arthropoda and Onychophora, studies of tardigrades hold the potential to reveal important insights into body plan evolution in Panarthropoda. Previous studies have revealed interesting facets of tardigrade development and genomics that suggest that a highly compact body plan is a derived condition of this lineage, rather than it representing an ancestral state of Panarthropoda. This conclusion was based on studies of several species from Eutardigrada. We review these studies and expand on them by analyzing the publicly available genome and transcriptome assemblies of Echiniscus testudo, a representative of Heterotardigrada. These new analyses allow us to phylogenetically reconstruct important features of genome evolution in Tardigrada. We use available data from tardigrades to interrogate several recent models of body plan evolution in Panarthropoda. Although anterior segments of panarthropods are highly diverse in terms of anatomy and development, both within individuals and between species, we conclude that a simple one-to-one alignment of anterior segments across Panarthropoda is the best available model of segmental homology. In addition to providing important insight into body plan diversification within Panarthropoda, we speculate that studies of tardigrades may reveal generalizable pathways to miniaturization.

Cambrian lobopodians shed light on the origin of the tardigrade body plan

Phylum Tardigrada (water bears), well known for their cryptobiosis, includes small invertebrates with four paired limbs and is divided into two classes: Eutardigrada and Heterotardigrada. The evolutionary origin of Tardigrada is known to lie within the lobopodians, which are extinct soft-bodied worms with lobopodous limbs mostly discovered at sites of exceptionally well-preserved fossils. Contrary to their closest relatives, onychophorans and euarthropods, the origin of morphological characters of tardigrades remains unclear, and detailed comparison with the lobopodians has not been well explored. Here, we present detailed morphological comparison between tardigrades and Cambrian lobopodians, with a phylogenetic analysis encompassing most of the lobopodians and three panarthropod phyla. The results indicate that the ancestral tardigrades likely had a Cambrian lobopodian-like morphology and shared most recent ancestry with the luolishaniids. Internal relationships within Tardigrada indicate that the ancestral tardigrade had a vermiform body shape without segmental plates, but possessed cuticular structures surrounding the mouth opening, and lobopodous legs terminating with claws, but without digits. This finding is in contrast to the long-standing stygarctid-like ancestor hypothesis. The highly compact and miniaturized body plan of tardigrades evolved after the tardigrade lineage diverged from an ancient shared ancestor with the luolishaniids.

Interpreting fossilized nervous tissues

Paleoneuranatomy is an emerging subfield of paleontological research with great potential for the study of evolution. However, the interpretation of fossilized nervous tissues is a difficult task and presently lacks a rigorous methodology. We critically review here cases of neural tissue preservation reported in Cambrian arthropods, following a set of fundamental paleontological criteria for their recognition. These criteria are based on a variety of taphonomic parameters and account for morphoanatomical complexity. Application of these criteria shows that firm evidence for fossilized nervous tissues is less abundant and detailed than previously reported, and we synthesize here evidence that has stronger support. We argue that the vascular system, and in particular its lacunae, may be central to the understanding of many of the fossilized peri-intestinal features known across Cambrian arthropods. In conclusion, our results suggest the need for caution in the interpretation of evidence for fossilized neural tissue, which will increase the accuracy of evolutionary scenarios. Also see the video abstract here: https://youtu.be/2_JlQepRTb0.

Putting heads together

Cambrian fossils reveal ancestry of the segmented brain in arthropods

The lower Cambrian lobopodian Cardiodictyon resolves the origin of euarthropod brains

For more than a century, the origin and evolution of the arthropod head and brain have eluded a unifying rationale reconciling divergent morphologies and phylogenetic relationships. Here, clarification is provided by the fossilized nervous system of the lower Cambrian lobopodian Cardiodictyon catenulum, which reveals an unsegmented head and brain comprising three cephalic domains, distinct from the metameric ventral nervous system serving its appendicular trunk. Each domain aligns with one of three components of the foregut and with a pair of head appendages. Morphological correspondences with stem group arthropods and alignments of homologous gene expression patterns with those of extant panarthropods demonstrate that cephalic domains of C. catenulum predate the evolution of the euarthropod head yet correspond to neuromeres defining brains of living chelicerates and mandibulates.

Internal anatomy of a fossilized embryonic stage of the Cambrian-Ordovician scalidophoran Markuelia

The Wangcun fossil Lagerstätte in Hunan, South China, has yielded hundreds of fossilized embryos of Markuelia hunanensis representing different developmental stages. Internal tissues have only rarely been observed, impeding further understanding of the soft tissue anatomy, phylogenetic affinity and evolutionary significance of Markuelia. In this study, we used synchrotron radiation X-ray tomographic microscopy (SRXTM) to study a new collection of fossil embryos from the Wangcun fossil Lagerstätte. We describe specimens exhibiting a spectrum of preservation states, the best of which preserves palisade structures underneath the cuticle of the head and tail, distinct from patterns of centripetal mineralization of the cuticle and centrifugal mineralization of hypha-like structures, seen elsewhere in this specimen and other fossils within the same assemblage. Our computed tomographic reconstruction of these mineralization phases preserves the gross morphology of (i) longitudinal structures associated with the tail spines, which we interpret as the proximal ends of longitudinal muscles, and (ii) a ring-shaped structure internal to the introvert, which we interpret as a ring-shaped brain, as anticipated of the cycloneuralian affinity of Markuelia. This is the first record of a fossilized nervous system in a scalidophoran, and the first instance in Orsten-style preservation, opening the potential for further such records within this widespread mode of high-fidelity three-dimensional preservation.

Evolution: The arthropod brain - a saga in three parts

The arthropod brain comprises three segments and innervates three appendages. How it evolved has been a matter of intense debate. New Burgess Shale fossils preserving nervous tissue bring us closer to solving this mystery but evaluating the many hypotheses they could fit is not easy.

A three-eyed radiodont with fossilized neuroanatomy informs the origin of the arthropod head and segmentation

In addition to being among the most iconic and bizarre-looking Cambrian animals, radiodonts are a group that offers key insight into the acquisition of the arthropod body plan by virtue of their phylogenetic divergence prior to all living members of the phylum. Nonetheless, radiodont fossils are rare and often fragmentary, and contentions over their interpretation have hindered resolution of important evolutionary conundrums. Here, we describe 268 specimens of Stanleycaris hirpex from the Cambrian Burgess Shale, including many exceptionally preserved whole-body specimens, informing the most complete reconstruction of a radiodont to date. The trunk region of Stanleycaris has up to 17 segments plus two pairs of filiform caudal blades. The recognition of dorsal sclerotic segmentation of the trunk cuticle and putative unganglionated nerve cords provides new insight into the relative timing of acquisition of segmental traits, the epitome of the arthropod body plan. In addition to the pair of stalked lateral eyes, the short head unexpectedly bears a large median eye situated behind a preocular sclerite on an anteriorly projecting head lobe. Upon re-evaluation, similar median eyes can be identified in other Cambrian panarthropods demonstrating a deep evolutionary continuity. The exquisitely preserved brain of Stanleycaris is consistent with the hypothesized deutocerebral innervation of the frontal appendages, reconciling neuroanatomical evidence with external morphology in support of an ancestrally bipartite head and brain for arthropods. We propose that the integration of this bipartite head prior to the acquisition of most segmental characters exclusively in the arthropod trunk may help explain its developmental differentiation.

The origin and early evolution of arthropods

The rise of arthropods is a decisive event in the history of life. Likely the first animals to have established themselves on land and in the air, arthropods have pervaded nearly all ecosystems and have become pillars of the planet's ecological networks. Forerunners of this saga, exceptionally well-preserved Palaeozoic fossils recently discovered or re-discovered using new approaches and techniques have elucidated the precocious appearance of extant lineages at the onset of the Cambrian explosion, and pointed to the critical role of the plankton and hard integuments in early arthropod diversification. The notion put forward at the beginning of the century that the acquisition of extant arthropod characters was stepwise and represented by the majority of Cambrian fossil taxa is being rewritten. Although some key traits leading to Euarthropoda are indeed well documented along a diversified phylogenetic stem, this stem led to several speciose and ecologically diverse radiations leaving descendants late into the Palaeozoic, and a large part, if not all of the Cambrian euarthropods can now be placed on either of the two extant lineages: Mandibulata and Chelicerata. These new observations and discoveries have altered our view on the nature and timing of the Cambrian explosion and clarified diagnostic characters at the origin of extant arthropods, but also raised new questions, especially with respect to cephalic plasticity. There is now strong evidence that early arthropods shared a homologous frontalmost appendage, coined here the cheira, which likely evolved into antennules and chelicerae, but other aspects, such as brain and labrum evolution, are still subject to active debate. The early evolution of panarthropods was generally driven by increased mastication and predation efficiency and sophistication, but a wealth of recent studies have also highlighted the prevalent role of suspension-feeding, for which early panarthropods developed their own adaptive feedback through both specialized appendages and the diversification of small, morphologically differentiated larvae. In a context of general integumental differentiation and hardening across Cambrian metazoans, arthrodization of body and limbs notably prompted two diverging strategies of basipod differentiation, which arguably became founding criteria in the divergence of total-groups Mandibulata and Chelicerata. The kinship of trilobites and their relatives remains a source of disagreement, but a recent topological solution, termed the 'deep split', could embed Artiopoda as sister taxa to chelicerates and constitute definitive support for Arachnomorpha. Although Cambrian fossils have been critical to all these findings, data of exceptional quality have also been accumulating from other Palaeozoic Konservat-Lagerstätten, and a better integration of this information promises a much more complete and elaborate picture of early arthropod evolution in the near future. From the broader perspective of a total-evidence approach to the understanding of life's history, and despite persisting systematic debates and new interpretative challenges, various advances based on palaeontological evidence open the prospect of finally using the full potential of the most diverse animal phylum to investigate macroevolutionary patterns and processes.

Serial Homology and Segment Identity in the Arthropod Head

The anterior-most unit of the crown-group arthropod body plan includes three segments, the pre-gnathal segments, that contain three neuromeres that together comprise the brain. Recent work on the development of this anterior region has shown that its three units exhibit many developmental differences to the more posterior segments, to the extent that they should not be considered serial homologs. Building on this revised understanding of the development of the pre-gnathal segments, we suggest a novel scenario for arthropod head evolution. We posit an expansion of an ancestral single-segmented head at the transition from Radiodonta to Deuteropoda in the arthropod stem group. The expanded head subdivided into three segmental units, each maintaining some of the structures of the ancestral head. This scenario is consistent with what we know of head evolution from the fossil record and helps reconcile some of the debates about early arthropod evolution.

The evolution of biramous appendages revealed by a carapace-bearing Cambrian arthropod

Biramous appendages are a common feature among modern marine arthropods that evolved deep in arthropod phylogeny. The branched appendage of Cambrian arthropods has long been considered as the ancient biramous limb, sparking numerous investigations on its origin and evolution. Here, we report a new arthropod, Erratus sperare gen. et sp. nov., from the Lower Cambrian (Stage 3, 520 Ma) Chengjiang biota of Yunnan, China, with unique trunk appendages formed of lateral anomalocaridid-type flaps and ventral subconical endopods. These appendages represent an intermediate stage of biramous limb evolution, i.e. from 'two pairs of flap appendages' in radiodonts to 'flap + endopod' in Erratus, to 'exopod + endopod' in the rest of carapace-bearing arthropods that populate the basal region of the upper-stem lineage arthropods (deuteropods). The new species occupies a phylogenetic position at the first node closer to deuteropods than to radiodonts, and therefore pinpoints the earliest occurrence of the endopod within Deuteropoda. The primitive endopod is weakly sclerotized, and has unspecialized segments without endites or claw. The findings might support previous claims that the outer branch of the biramous limb of fossil marine arthropods, such as trilobites, is not a true exopod, but is instead a modified exite. This article is part of the theme issue 'The impact of Chinese palaeontology on evolutionary research'.

New opabiniid diversifies the weirdest wonders of the euarthropod stem group

Once considered 'weird wonders' of the Cambrian, the emblematic Burgess Shale animals Anomalocaris and Opabinia are now recognized as lower stem-group euarthropods and have provided crucial data for constraining the polarity of key morphological characters in the group. Anomalocaris and its relatives (radiodonts) had worldwide distribution and survived until at least the Devonian. However, despite intense study, Opabinia remains the only formally described opabiniid to date. Here we reinterpret a fossil from the Wheeler Formation of Utah as a new opabiniid, Utaurora comosa nov. gen. et sp. By visualizing the sample of phylogenetic topologies in treespace, our results fortify support for the position of U. comosa beyond the nodal support traditionally applied. Our phylogenetic evidence expands opabiniids to multiple Cambrian stages. Our results underscore the power of treespace visualization for resolving imperfectly preserved fossils and expanding the known diversity and spatio-temporal ranges within the euarthropod lower stem group.

Extensive loss of Wnt genes in Tardigrada

Background

Wnt genes code for ligands that activate signaling pathways during development in Metazoa. Through the canonical Wnt (cWnt) signaling pathway, these genes regulate important processes in bilaterian development, such as establishing the anteroposterior axis and posterior growth. In Arthropoda, Wnt ligands also regulate segment polarity, and outgrowth and patterning of developing appendages. Arthropods are part of a lineage called Panarthropoda that includes Onychophora and Tardigrada. Previous studies revealed potential roles of Wnt genes in regulating posterior growth, segment polarity, and growth and patterning of legs in Onychophora. Unlike most other panarthropods, tardigrades lack posterior growth, but retain segmentation and appendages. Here, we investigated Wnt genes in tardigrades to gain insight into potential roles that these genes play during development of the highly compact and miniaturized tardigrade body plan.

Results

We analyzed published genomes for two representatives of Tardigrada, Hypsibius exemplaris and Ramazzottius varieornatus. We identified single orthologs of Wnt4, Wnt5, Wnt9, Wnt11, and WntA, as well as two Wnt16 paralogs in both tardigrade genomes. We only found a Wnt2 ortholog in H. exemplaris. We could not identify orthologs of Wnt1, Wnt6, Wnt7, Wnt8, or Wnt10. We identified most other components of cWnt signaling in both tardigrade genomes. However, we were unable to identify an ortholog of arrow/Lrp5/6, a gene that codes for a Frizzled co-receptor of Wnt ligands. Additionally, we found that some other animals that have lost several Wnt genes and are secondarily miniaturized, like tardigrades, are also missing an ortholog of arrow/Lrp5/6. We analyzed the embryonic expression patterns of Wnt genes in H. exemplaris during developmental stages that span the establishment of the AP axis through segmentation and leg development. We detected expression of all Wnt genes in H. exemplaris besides one of the Wnt16 paralogs. During embryo elongation, expression of several Wnt genes was restricted to the posterior pole or a region between the anterior and posterior poles. Wnt genes were expressed in distinct patterns during segmentation and development of legs in H. exemplaris, rather than in broadly overlapping patterns.

Conclusions

Our results indicate that Wnt signaling has been highly modified in Tardigrada. While most components of cWnt signaling are conserved in tardigrades, we conclude that tardigrades have lost Wnt1, Wnt6, Wnt7, Wnt8, and Wnt10, along with arrow/Lrp5/6. Our expression data may indicate a conserved role of Wnt genes in specifying posterior identities during establishment of the AP axis. However, the loss of several Wnt genes and the distinct expression patterns of Wnt genes during segmentation and leg development may indicate that combinatorial interactions among Wnt genes are less important during tardigrade development compared to many other animals. Based on our results, and comparisons to previous studies, we speculate that the loss of several Wnt genes in Tardigrada may be related to a reduced number of cells and simplified development that accompanied miniaturization and anatomical simplification in this lineage.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-021-01954-y.

New multipodomerous appendages of stem-group euarthropods from the Cambrian (Stage 4) Guanshan Konservat-Lagerstätte

Stem-group euarthropods are important for understanding the early evolutionary and ecological history of the most species-rich animal phylum on Earth. Of particular interest are fossil taxa that occupy a phylogenetic position immediately crownwards of radiodonts, for this part of the euarthropod tree is associated with the appearance of several morphological features that characterize extant members of the group. Here, we report two new euarthropods from the Cambrian Stage 4 Guanshan Biota of South China. The fuxianhuiid Alacaris? sp. is represented by isolated appendages composed of a gnathobasic protopodite and an endite-bearing endopod of at least 20 podomeres. This material represents the youngest occurrence of the family Chengjiangocarididae, and its first record outside the Chengjiang and Xiaoshiba biotas. We also describe Lihuacaris ferox gen. et sp. nov. based on well-preserved and robust isolated appendages. Lihuacaris ferox exhibits an atypical combination of characters including an enlarged rectangular base, 11 endite-bearing podomeres and a hypertrophied distal element bearing 8-10 curved spines. Alacaris? sp. appendages display adaptations for macrophagy. Lihuacaris ferox appendages resemble the frontal appendages of radiodonts, as well as the post-oral endopods of chengjiangocaridid fuxianhuids and other deuteropods with well-documented raptorial/predatory habits. Lihuacaris ferox contributes towards the record of endemic biodiversity in the Guanshan Biota.

Leanchoiliidae reveals the ancestral organization of the stem euarthropod brain

Fossils provide insights into how organs may have diversified over geological time.¹ However, diversification already accomplished early in evolution can obscure ancestral events leading to it. For example, already by the mid-Cambrian period, euarthropods had condensed brains typifying modern mandibulate lineages.² However, the demonstration that extant euarthropods and chordates share orthologous developmental control genes defining the segmental fore-, mid-, and hindbrain suggests that those character states were present even before the onset of the Cambrian.³ Fossilized nervous systems of stem Euarthropoda might, therefore, be expected to reveal ancestral segmental organization, from which divergent arrangements emerged. Here, we demonstrate unsurpassed preservation of cerebral tissue in Kaili leanchoiliids revealing near-identical arrangements of bilaterally symmetric ganglia identified as the proto-, deuto-, and tritocerebra disposed behind an asegmental frontal domain, the prosocerebrum, from which paired nerves extend to labral ganglia flanking the stomodeum. This organization corresponds to labral connections hallmarking extant euarthropod clades⁴ and to predicted transformations of presegmental ganglia serving raptorial preocular appendages of Radiodonta.⁵ Trace nervous system in the gilled lobopodian Kerygmachela kierkegaardi⁶ suggests an even deeper prosocerebral ancestry. An asegmental prosocerebrum resolves its location relative to the midline asegmental sclerite of the radiodontan head, which persists in stem Euarthropoda.⁷ Here, data from two Kaili Leanchoilia, with additional reference to Alalcomenaeus,^8,9 demonstrate that Cambrian stem Euarthropoda confirm genomic and developmental studies^10-15 claiming that the most frontal domain of the euarthropod brain is a unique evolutionary module distinct from, and ancestral to, the fore-, mid-, and hindbrain.

Evolution: Oh, my Cambrian nerves

Developmental gene expression suggests a cryptic subdivision of the anterior brain in euarthropods. A new study illustrates delicate details of the nervous system from exceptionally preserved 500-million-year-old Chinese fossils, supporting the bipartite origin of the anterior brain among Cambrian representatives.

Cambrian comb jellies from Utah illuminate the early evolution of nervous and sensory systems in ctenophores

Ctenophores are a group of predatory macroinvertebrates whose controversial phylogenetic position has prompted several competing hypotheses regarding the evolution of animal organ systems. Although ctenophores date back at least to the Cambrian, they have a poor fossil record due to their gelatinous bodies. Here, we describe two ctenophore species from the Cambrian of Utah, which illuminate the early evolution of nervous and sensory features in the phylum. Thalassostaphylos elegans has 16 comb rows, an oral skirt, and an apical organ with polar fields. Ctenorhabdotus campanelliformis has 24 comb rows, an oral skirt, an apical organ enclosed by a capsule and neurological tissues preserved as carbonaceous films. These are concentrated around the apical organ and ciliated furrows, which connect to a circumoral nerve ring via longitudinal axons. C. campanelliformis deviates from the neuroanatomy of living ctenophores and demonstrates a substantial complexity in the nervous system of Cambrian ctenophores.

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Two species of rare fossil ctenophores are described from the Cambrian of Utah

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Fossil ctenophores preserve remains of nervous tissue and sensory structures

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Neurological structures include an oral nerve ring and giant longitudinal axons

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Cambrian ctenophores had a more complex neuroanatomy than living species

The origin and evolution of the euarthropod labrum

A widely (although not universally) accepted model of arthropod head evolution postulates that the labrum, a structure seen in almost all living euarthropods, evolved from an anterior pair of appendages homologous to the frontal appendages of onychophorans. However, the implications of this model for the interpretation of fossil arthropods have not been fully integrated into reconstructions of the euarthropod stem group, which remains in a state of some disorder. Here I review the evidence for the nature and evolution of the labrum from living taxa, and reconsider how fossils should be interpreted in the light of this. Identification of the segmental identity of head appendage in fossil arthropods remains problematic, and often rests ultimately on unproven assertions. New evidence from the Cambrian stem-group euarthropod Parapeytoia is presented to suggest that an originally protocerebral appendage persisted well up into the upper stem-group of the euarthropods, which prompts a re-evaluation of widely-accepted segmental homologies and the interpretation of fossil central nervous systems. Only a protocerebral brain was implicitly present in a large part of the euarthropod stem group, and the deutocerebrum must have been a relatively late addition.

Disparate compound eyes of Cambrian radiodonts reveal their developmental growth mode and diverse visual ecology

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

Arthropod Origins: Integrating Paleontological and Molecular Evidence

Phylogenomics underpins a stable and mostly well-resolved hypothesis for the interrelationships of extant arthropods. Exceptionally preserved fossils are integrated into this framework by coding their morphological characters, as exemplified by total-evidence dating approaches that treat fossils as dated tips in analyses numerically dominated by molecular data. Cambrian fossils inform on the sequence of character acquisition in the arthropod stem group and in the stems of its main extant clades. The arthropod head problem incorporates unique appendage combinations and remains of the nervous system in fossils into a scheme mostly based on neuroanatomy and Hox expression domains for extant forms. Molecular estimates of arthropod origins in the Cryogenian or Ediacaran predate a coherent picture from the arthropod fossil record, which commences as trace fossils in the earliest Cambrian. Probabilistic morphological clock analysis of trilobites, which exemplify the earliest arthropod body fossils, supports a Cambrian origin, without the need to posit an unfossilized Ediacaran history.

A Tube-Dwelling Early Cambrian Lobopodian

Facivermis yunnanicus [1, 2] is an enigmatic worm-like animal from the early Cambrian Chengjiang Biota of Yunnan Province, China. It is a small (<10 cm) bilaterian with five pairs of spiny anterior arms, an elongated body, and a swollen posterior end. The unusual morphology of Facivermis has prompted a history of diverse taxonomic interpretations, including among annelids [1, 3], lophophorates [4], and pentastomids [5]. However, in other studies, Facivermis is considered to be more similar to lobopodians [2, 6-8]-the fossil grade from which modern panarthropods (arthropods, onychophorans, and tardigrades) are derived. In these studies, Facivermis is thought to be intermediate between cycloneuralian worms and lobopodians. Facivermis has therefore been suggested to represent an early endobenthic-epibenthic panarthropod transition [6] and to provide crucial insights into the origin of paired appendages [2]. However, the systematic affinity of Facivermis was poorly supported in a previous phylogeny [6], partially due to incomplete understanding of its morphology. Therefore, the evolutionary significance of Facivermis remains unresolved. In this study, we re-examine Facivermis from new material and the holotype, leading to the discovery of several new morphological features, such as paired eyes on the head and a dwelling tube. Comprehensive phylogenetic analyses using parsimony, Bayesian inference, and maximum likelihood all support Facivermis as a luolishaniid in a derived position within the onychophoran stem group rather than as a basal panarthropod. In contrast to previous studies, we therefore conclude that Facivermis provides a rare early Cambrian example of secondary loss to accommodate a highly specialized tube-dwelling lifestyle.

Fossils from South China redefine the ancestral euarthropod body plan

BACKGROUND

Early Cambrian Lagerstätten from China have greatly enriched our perspective on the early evolution of animals, particularly arthropods. However, recent studies have shown that many of these early fossil arthropods were more derived than previously thought, casting uncertainty on the ancestral euarthropod body plan. In addition, evidence from fossilized neural tissues conflicts with external morphology, in particular regarding the homology of the frontalmost appendage.

RESULTS

Here we redescribe the multisegmented megacheirans Fortiforceps and Jianfengia and describe Sklerolibyon maomima gen. et sp. nov., which we place in Jianfengiidae, fam. nov. (in Megacheira, emended). We find that jianfengiids show high morphological diversity among megacheirans, both in trunk ornamentation and head anatomy, which encompasses from 2 to 4 post-frontal appendage pairs. These taxa are also characterized by elongate podomeres likely forming seven-segmented endopods, which were misinterpreted in their original descriptions. Plesiomorphic traits also clarify their connection with more ancestral taxa. The structure and position of the "great appendages" relative to likely sensory antero-medial protrusions, as well as the presence of optic peduncles and sclerites, point to an overall homology with the anterior head of radiodontans. This is confirmed by our Bayesian phylogeny, which places jianfengiids as the basalmost euarthropods, paraphyletic with other megacheirans, and in contiguity with isoxyids and radiodontans.

CONCLUSIONS

Sklerolibyon and other jianfengiids expand the disparity of megacheirans and suggest that the common euarthropod ancestor possessed a remarkable phenotypic variability associated with the externalized cephalon, as well as endopods that were already heptopodomerous, which differs from previous hypotheses and observations. These animals also demonstrate that the frontalmost pair of arthrodized appendage is homologous between radiodontans and megacheirans, refuting the claim that the radiodontan frontal appendages evolved into the euarthropod labrum, and questioning its protocerebral identity. This evidence based on external anatomy now constitutes a solid benchmark upon which we should address issues of homology, with the help of carefully examined palaeoneurological data.

Proclivity of nervous system preservation in Cambrian Burgess Shale-type deposits

Recent investigations on neurological tissues preserved in Cambrian fossils have clarified the phylogenetic affinities and head segmentation in pivotal members of stem-group Euarthropoda. However, palaeoneuroanatomical features are often incomplete or described from single exceptional specimens, raising concerns about the morphological interpretation of fossilized neurological structures and their significance for early euarthropod evolution. Here, we describe the central nervous system (CNS) of the short great-appendage euarthropod Alalcomenaeus based on material from two Cambrian Burgess Shale-type deposits of the American Great Basin, the Pioche Formation (Stage 4) and the Marjum Formation (Drumian). The specimens reveal complementary ventral and lateral views of the CNS, preserved as a dark carbonaceous compression throughout the body. The head features a dorsal brain connected to four stalked ventral eyes, and four pairs of segmental nerves. The first to seventh trunk tergites overlie a ventral nerve cord with seven ganglia, each associated with paired sets of segmental nerve bundles. Posteriorly, the nerve cord features elongate thread-like connectives. The Great Basin fossils strengthen the original description—and broader evolutionary implications—of the CNS in Alalcomenaeus from the early Cambrian (Stage 3) Chengjiang deposit of South China. The spatio-temporal recurrence of fossilized neural tissues in Cambrian Konservat-Lagerstätten across North America (Pioche, Burgess Shale, Marjum) and South China (Chengjiang, Xiaoshiba) indicates that their preservation is consistent with the mechanism of Burgess Shale-type fossilization, without the need to invoke alternative taphonomic pathways or the presence of microbial biofilms.

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.

Canadia spinosa and the early evolution of the annelid nervous system

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.

Trilobite compound eyes with crystalline cones and rhabdoms show mandibulate affinities

Most knowledge about the structure, function, and evolution of early compound eyes is based on investigations in trilobites. However, these studies dealt mainly with the cuticular lenses and little was known about internal anatomy. Only recently some data on crystalline cones and retinula cells were reported for a Cambrian trilobite species. Here, we describe internal eye structures of two other trilobite genera. The Ordovician Asaphus sp. reveals preserved crystalline cones situated underneath the cuticular lenses. The same is true for the Devonian species Archegonus (Waribole) warsteinensis, which in addition shows the fine structure of the rhabdom in the retinula cells. These results suggest that an apposition eye with a crystalline cone is ancestral for Trilobita. The overall similarity of trilobite eyes to those of myriapods, crustaceans, and hexapods corroborates views of a phylogenetic position of trilobites in the stem lineage of Mandibulata.

Little is known about the internal anatomy of early eyes. Here, Scholtz and colleagues show the internal eye structures from fossils of two genera of trilobites, which support an ancestral apposition eye with crystalline cones in Trilobita and a close affinity with Mandibulata.

Cambrian Sessile, Suspension Feeding Stem-Group Ctenophores and Evolution of the Comb Jelly Body Plan

The origin of ctenophores (comb jellies) is obscured by their controversial phylogenetic position, with recent phylogenomic analyses resolving either sponges or ctenophores as the sister group of all other animals. Fossil taxa can provide morphological evidence that may elucidate the origins of derived characters and shared ancestries among divergent taxa, providing a means to "break" long branches in phylogenetic trees. Here we describe new fossil material from the early Cambrian Chengjiang Biota, Yunnan Province, China, including the putative cnidarian Xianguangia, the new taxon Daihua sanqiong gen et sp. nov., and Dinomischus venustus, informally referred to as "dinomischids" here. "Dinomischids" possess a basal calyx encircled by 18 tentacles that surround the mouth. The tentacles carry pinnules, each with a row of stiff filamentous structures interpreted as very large compound cilia of a size otherwise only known in ctenophores. Together with the Cambrian tulip animal Siphusauctum and the armored Cambrian scleroctenophores, they exhibit anatomies that trace ctenophores to a sessile, polypoid stem lineage. This body plan resembles the polypoid, tentaculate morphology of cnidarians, including a blind gastric cavity partitioned by mesenteries. We propose that comb rows are derived from tentacles with paired sets of pinnules that each bear a row of compound cilia. The scleroctenophores exhibit paired comb rows, also observed in Siphusauctum, in addition to an organic skeleton, shared as well by Dinomischus, Daihua, and Xianguangia. We formulate a hypothesis in which ctenophores evolved from sessile, polypoid suspension feeders, sharing similarities with cnidarians that suggest either a close relationship between these two phyla, a striking pattern of early convergent evolution, or an ancestral condition for either metazoans or eumetazoans.

The last common ancestor of Ecdysozoa had an adult terminal mouth

The Ecdysozoa is a major animal clade whose main uniting feature is a distinctive growth strategy that requires the periodical moulting of the external cuticle. The staggering diversity within Ecdysozoa has prompted substantial efforts to reconstruct their origin and early evolution. Based on palaentological and developmental data, we proposed a scenario for the early evolution of the ecdysozoan clade Panarthropoda (Onychophora, Tardigrada, Euarthropoda), and postulated that a terminal mouth is ancestral for this lineage. In light of the accompanying comment by Claus Nielsen, we take this opportunity to clarify the significance of our argumentation for Panarthropoda in the phylogenetic context of Ecdysozoa, and Bilateria more broadly. We conclude that the ancestral ecdysozoan most likely had an adult terminal mouth, and that the last common ancestors of all the phyla that constitute Ecdysozoa almost certainly also had an adult terminal mouth. The occurrence of a ventral-facing mouth in various adult ecdysozoans - particularly panarthropods - is the result of convergence. Despite the paucity of embryological data on fossil taxa, we contemplate the likelihood that a developmentally early ventral mouth opening could be ancestral for Ecdysozoa, and if so, then this would represent a symplesiomorphy of Bilateria as a whole.

Bilateral Jaw Elements in Amiskwia sagittiformis Bridge the Morphological Gap between Gnathiferans and Chaetognaths

Amiskwia sagittiformis Walcott 1911 is an iconic soft-bodied taxon from the Burgess Shale [1-3]. It was originally interpreted as a chaetognath [1], but it was later interpreted as a pelagic nemertean [2] or considered of uncertain affinity [3]. Part of this ambiguity is due to direct comparisons with members of the crown groups of extant phyla [4] and a lack of clarity regarding the systematic position of chaetognaths, which would allow for assessing character polarity in the phylum with respect to outgroups. Here, we show that Amiskwia preserves a bilaterally arranged set of head structures visible in relief and high reflectivity. These structures are best interpreted as jaws situated within an expanded pharyngeal complex. Morphological studies have highlighted a likely homology between bilateral and chitinous jaw elements in gnathiferans and chaetognaths [5], which is congruent with a shared unique Hox gene that suggests a close relationship between Gnathifera and Chaetognatha [6]. Molecular phylogenetic studies have recently found gnathiferans to be a deep branch of Spiralia and Chaetognaths either a sister group to Spiralia [7] or forming a clade with gnathiferans [6, 8]. Our phylogenetic analyses render Gnathifera paraphyletic with respect to Chaetognatha, and we therefore suggest that Amiskwia is best interpreted as a stem chaetognath, but crown gnathiferan.

Molecular palaeontology illuminates the evolution of ecdysozoan vision

Colour vision is known to have arisen only twice-once in Vertebrata and once within the Ecdysozoa, in Arthropoda. However, the evolutionary history of ecdysozoan vision is unclear. At the molecular level, visual pigments, composed of a chromophore and a protein belonging to the opsin family, have different spectral sensitivities and these mediate colour vision. At the morphological level, ecdysozoan vision is conveyed by eyes of variable levels of complexity; from the simple ocelli observed in the velvet worms (phylum Onychophora) to the marvellously complex eyes of insects, spiders, and crustaceans. Here, we explore the evolution of ecdysozoan vision at both the molecular and morphological level; combining analysis of a large-scale opsin dataset that includes previously unknown ecdysozoan opsins with morphological analyses of key Cambrian fossils with preserved eye structures. We found that while several non-arthropod ecdysozoan lineages have multiple opsins, arthropod multi-opsin vision evolved through a series of gene duplications that were fixed in a period of 35-71 million years (Ma) along the stem arthropod lineage. Our integrative study of the fossil and molecular record of vision indicates that fossils with more complex eyes were likely to have possessed a larger complement of opsin genes.

Analyses of nervous system patterning genes in the tardigrade Hypsibius exemplaris illuminate the evolution of panarthropod brains

BACKGROUND

Both euarthropods and vertebrates have tripartite brains. Several orthologous genes are expressed in similar regionalized patterns during brain development in both vertebrates and euarthropods. These similarities have been used to support direct homology of the tripartite brains of vertebrates and euarthropods. If the tripartite brains of vertebrates and euarthropods are homologous, then one would expect other taxa to share this structure. More generally, examination of other taxa can help in tracing the evolutionary history of brain structures. Tardigrades are an interesting lineage on which to test this hypothesis because they are closely related to euarthropods, and whether they have a tripartite brain or unipartite brain has recently been a focus of debate.

RESULTS

We tested this hypothesis by analyzing the expression patterns of six3, orthodenticle, pax6, unplugged, and pax2/5/8 during brain development in the tardigrade Hypsibius exemplaris-formerly misidentified as Hypsibius dujardini. These genes were expressed in a staggered anteroposterior order in H. exemplaris, similar to what has been reported for mice and flies. However, only six3, orthodenticle, and pax6 were expressed in the developing brain. Unplugged was expressed broadly throughout the trunk and posterior head, before the appearance of the nervous system. Pax2/5/8 was expressed in the developing central and peripheral nervous system in the trunk.

CONCLUSION

Our results buttress the conclusion of our previous study of Hox genes-that the brain of tardigrades is only homologous to the protocerebrum of euarthropods. They support a model based on fossil evidence that the last common ancestor of tardigrades and euarthropods possessed a unipartite brain. Our results are inconsistent with the hypothesis that the tripartite brain of euarthropods is directly homologous to the tripartite brain of vertebrates.