Mouthparts of the Burgess Shale fossils Odontogriphus and Wiwaxia: implications for the ancestral molluscan radula

The Middle Cambrian lophotrochozoans Odontogriphus omalus and Wiwaxia corrugata have been interpreted as stem-group members of either the Mollusca, the Annelida, or a group containing Mollusca + Annelida. The case for each classification rests on the organisms' unusual mouthparts, whose two to three tooth-rows resemble both the molluscan radula and the jaws of certain annelid worms. Despite their potential significance, these mouthparts have not previously been described in detail. This study examined the feeding apparatuses of over 300 specimens from the 505-million-year-old Burgess Shale, many of which were studied for the first time. Rather than denticulate plates, each tooth row comprises a single axial tooth that is flanked on each side by eight to 16 separate shoehorn-shaped teeth. Tooth rows sat on a grooved basal tongue, and two large lobes flanked the apparatus. New observations-the shape, distribution and articulation of the individual teeth, and the mouthparts' mode of growth-are incompatible with an annelid interpretation, instead supporting a classification in Mollusca. The ancestral molluscan radula is best reconstructed as unipartite with a symmetrical medial tooth, and Odontogriphus and Wiwaxia as grazing deposit-feeders.     

Evolutionary crossroads in developmental biology: annelids

Annelids (the segmented worms) have a long history in studies of animal developmental biology, particularly with regards to their cleavage patterns during early development and their neurobiology. With the relatively recent reorganisation of the phylogeny of the animal kingdom, and the distinction of the super-phyla Ecdysozoa and Lophotrochozoa, an extra stimulus for studying this phylum has arisen. As one of the major phyla within Lophotrochozoa, Annelida are playing an important role in deducing the developmental biology of the last common ancestor of the protostomes and deuterostomes, an animal from which >98% of all described animal species evolved.     

An Odontogriphid from the Upper Permian of Australia

A problematic organism, Bowengriphus perphlegis gen. et sp. nov., is described based on two specimens from the Late Permian Rangal Coal Measures of eastern central Queensland. It displays a double-looped feeding apparatus bearing small conical elements, considered homologous with that of the supposed lophophorate Odontogriphus omalus Conway Morris, 1976, from the Middle Cambrian Burgess Shale of British Columbia, Canada. Odontogriphids are thus interpreted as a group that survived through the Palaeozoic and made the transition from marine to freshwater environments. Recent proposals that odontogriphids are stem-group chordates are not well established.     

多毛纲环节动物软背鳞虫的电镜观察与描述及其与Wiwaxia骨片微结构的比较

通过对现生环节动物门（Annelida）多毛纲（Polychaeta）叶须虫目（Phyllodocidae）鳞沙蚕亚目动物（Aphroditiformia）软背鳞虫（Lepidonotus helotypus Grube,1877）的鳞片、刚毛等疣足附属物的超微结构观察及其与Wiwaxia骨片的比较研究,探讨Wiwaxia的系统分类位置。研究结果表明,软背鳞虫的刚毛与Wiwaxia骨片具有以下相似性：1）两者表面均具有浅纵纹;2）均向身体背中线方向倾斜或弯曲;3）着生方式均以刚毛囊或类似根状基的结构插入软组织。此外,文中首次对鳞片上的锥形突起与Wiwaxia骨片的相似性进行比较和探讨。结果显示,Wiwaxia骨片与鳞沙蚕刚毛具有一定相似性,但在形态上仍存在显著差异（如二者的排列方式不同,Wiwaxia骨片上不存在鳞沙蚕刚毛的覆瓦状结构等）。     

Ecological innovations in the Cambrian and the origins of the crown group phyla

Simulation studies of the early origins of the modern phyla in the fossil record, and the rapid diversification that led to them, show that these are inevitable outcomes of rapid and long-lasting radiations. Recent advances in Cambrian stratigraphy have revealed a more precise picture of the early bilaterian radiation taking place during the earliest Terreneuvian Series, although several ambiguities remain. The early period is dominated by various tubes and a moderately diverse trace fossil record, with the classical ‘Tommotian’ small shelly biota beginning to appear some millions of years after the base of the Cambrian at ca 541 Ma. The body fossil record of the earliest period contains a few representatives of known groups, but most of the record is of uncertain affinity. Early trace fossils can be assigned to ecdysozoans, but deuterostome and even spiralian trace and body fossils are less clearly represented. One way of explaining the relative lack of clear spiralian fossils until about 536 Ma is to assign the various lowest Cambrian tubes to various stem-group lophotrochozoans, with the implication that the groundplan of the lophotrochozoans included a U-shaped gut and a sessile habit. The implication of this view would be that the vagrant lifestyle of annelids, nemerteans and molluscs would be independently derived from such a sessile ancestor, with potentially important implications for the homology of their sensory and nervous systems.     

Poriferan mtDNA and animal phylogeny based on mitochondrial gene arrangements

Phylogenetic relationships among the metazoan phyla are the subject of an ongoing controversy. Analysis of mitochondrial gene arrangements is a powerful tool to investigate these relationships; however, its previous application outside of individual animal phyla has been hampered by the lack of informative out-group data. To address this shortcoming, we determined complete mitochondrial DNA sequences for the demosponges Geodia neptuni and Tethya actinia, two representatives of the most basal animal phylum, the Porifera. With sponges as an outgroup, we investigated phylogenetic relationships of nine bilaterian phyla using both breakpoint analysis of global mitochondrial gene arrangements and maximum parsimony analysis of mitochondrial gene adjacencies. Our results provide strong support for a group that includes protostome (but not deuterostome) coelomate, pseudocoelomate, and acoelomate animals, thus clearly rejecting the Coelomata hypothesis. Two other groups of bilaterian animals, Lophotrochozoa and Ambulacraria, are also supported by our analyses. However, due to the remarkable stability of mitochondrial gene arrangements in Deuterostomia and the Ecdysozoa, conclusions on their evolutionary history cannot be drawn.     

Cambrian cinctan echinoderms shed light on feeding in the ancestral deuterostome

Reconstructing the feeding mode of the latest common ancestor of deuterostomes is key to elucidating the early evolution of feeding in chordates and allied phyla; however, it is debated whether the ancestral deuterostome was a tentaculate feeder or a pharyngeal filter feeder. To address this, we evaluated the hydrodynamics of feeding in a group of fossil stem-group echinoderms (cinctans) using computational fluid dynamics. We simulated water flow past three-dimensional digital models of a Cambrian fossil cinctan in a range of possible life positions, adopting both passive tentacular feeding and active pharyngeal filter feeding. The results demonstrate that an orientation with the mouth facing downstream of the current was optimal for drag and lift reduction. Moreover, they show that there was almost no flow to the mouth and associated marginal groove under simulations of passive feeding, whereas considerable flow towards the animal was observed for active feeding, which would have enhanced the transport of suspended particles to the mouth. This strongly suggests that cinctans were active pharyngeal filter feeders, like modern enteropneust hemichordates and urochordates, indicating that the ancestral deuterostome employed a similar feeding strategy.     

Assembling the lophotrochozoan (=spiralian) tree of life

The advent of numerical methods for analysing phylogenetic relationships, along with the study of morphology and molecular data, has driven our understanding of animal relationships for the past three decades. Within the protostome branch of the animal tree of life, these data have sufficed to establish its two main side branches, the moulting Ecdysozoa and the non-moulting Lophotrochozoa. In this review, I explore our current knowledge of protostome relationships and discuss progress and future perspectives and strategies to increase resolution within the main lophotrochozoan clades. Novel approaches to coding morphological characters are needed by scoring real observations on species selected as terminals. Still, methodological issues, for example, how to deal with inapplicable characters or the coding of absences, may require novel algorithmic developments. Taxon sampling is another key issue, as phyla should include enough species so as to represent their span of anatomical disparity. On the molecular side, phylogenomics is playing an increasingly important role in elucidating animal relationships, but genomic sampling is still fairly limited within the lophotrochozoan protostomes, for which only three phyla are represented in currently available phylogenies. Future work should therefore concentrate on generating novel morphological observations and on producing genomic data for the lophotrochozoan side of the animal tree of life.     

A comparative study of Lower Cambrian Halkieria and Middle Cambrian Wiwaxia

Two Cambrian lepidote metazoans known from different respective types of preservation have been compared in order to elucidate their biology and affinities. The widely distributed Lower Cambrian Halkieria is represented by isolated hollow sclerites, probably of originally calcareous composition. The Middle Cambrian Wiwaxia is known from the Burgess Shale as isolated sclerites (scales and spines) and as more or less complete individuals. Although Halkieria sclerites were mineralized and those of Wiwaxia were probably not, there are fundamental structural and morphological similarities between the two. Both bad an imbricating scaly and spiny armour consisting of hollow sclerites with a longitudinally fibrous structure. The sclerites did not grow, but were probably moulted during the course of ontogenetic growth. Halkieria and Wiwaxia are regarded as closely related. Both are referred to the Order Sachitida He 1980. The sclerite armour of Halkieria is reconstructed on the template provided by Wiwaxia. The interpretation of sachitid sclerites as protective armour is an alternative to the interpretation by Jell (1981, Alcheringa 5 )that sachitid sclerites were respiratory organs in an animal of probable annelid affinities. Sachitids are interpreted as sluggish, benthic deposit feeders that do not belong to any recognized phylum.     

Pelagiella exigua,an early Cambrian stem gastropod with chaetae: lophotrochozoan heritage and conchiferan novelty

Exceptionally well-preserved impressions of two bundles of bristles protrude from the apertures of small, spiral shells of Pelagiella exigua, recovered from the Kinzers Formation (Cambrian, Stage 4, ‘Olenellus Zone’, c. 512 Ma) of Pennsylvania. These impressions are inferred to represent clusters of chitinous chaetae, comparable to those borne by annelid parapodia and some larval brachiopods. They provide an affirmative test in the early metazoan fossil record of the inference, from phylogenetic analyses of living taxa, that chitinous chaetae are a shared early attribute of the Lophotrochozoa. Shells of Pelagiella exhibit logarithmic spiral growth, microstructural fabrics, distinctive external sculptures and muscle scars characteristic of molluscs. Hence, Pelagiella has been regarded as a stem mollusc, a helcionelloid expressing partial torsion, an untorted paragastropod, or a fully torted basal member of the gastropod crown group. The inference that its chaeta-bearing appendages were anterior–lateral, based on their probable functions, prompts a new reconstruction of the anatomy of Pelagiella, with a mainly anterior mantle cavity. Under this hypothesis, two lateral–dorsal grooves, uniquely preserved in Pelagiella atlantoides, are interpreted as sites of attachment for a long left ctenidium and a short one, anteriorly on the right. The orientation of Pelagiella and the asymmetry of its gills, comparable to features of several living vetigastropods, nominate it as the earliest fossil mollusc known to exhibit evidence of the developmental torsion characteristic of gastropods. This key adaptation facilitated an evolutionary radiation, slow at first and rapid during the Ordovician, that gave rise to the remarkable diversification of the Gastropoda.     

A tolloid homologue from the Pacific oyster Crassostrea gigas

The genes governing mesoderm specification have been extensively studied in vertebrates, arthropods and nematodes. The latter two phyla belong to the Ecdysozoan clade but little is understood of the role that these genes might play in the development of the other major protostomal clade, the Lophotrochozoa. As part of a wider project to analyze the functions associated with transforming growth factor beta superfamily members in Lophotrochozoa, we have cloned a gene encoding a tolloid homologue from the bivalve mollusc Crassostrea gigas. Tolloid is a key developmental protein that regulates the activity of bone morphogenetic proteins (BMPs). We have determined the intron-exon structure of the gene encoding C. gigas tolloid and have compared it with those of homologous genes from both protostomes and deuterostomes. In order to analyze the functionality of oyster tolloid the zebrafish embryo has been employed as a reporter organism and we show that over-expression of this protein results in the ventralization of zebrafish embryos at 24h post fertilization. The expression of the C. gigas tolloid gene during embryonic and larval development as well as in adult tissues is also explored.     

Multigene analysis of lophophorate and chaetognath phylogenetic relationships

Maximum likelihood and Bayesian inference analyses of seven concatenated fragments of nuclear-encoded housekeeping genes indicate that Lophotrochozoa is monophyletic, i.e., the lophophorate groups Bryozoa, Brachiopoda and Phoronida are more closely related to molluscs and annelids than to Deuterostomia or Ecdysozoa. Lophophorates themselves, however, form a polyphyletic assemblage. The hypotheses that they are monophyletic and more closely allied to Deuterostomia than to Protostomia can be ruled out with both the approximately unbiased test and the expected likelihood weights test. The existence of Phoronozoa, a putative clade including Brachiopoda and Phoronida, has also been rejected. According to our analyses, phoronids instead share a more recent common ancestor with bryozoans than with brachiopods. Platyhelminthes is the sister group of Lophotrochozoa. Together these two constitute Spiralia. Although Chaetognatha appears as the sister group of Priapulida within Ecdysozoa in our analyses, alternative hypothesis concerning chaetognath relationships could not be rejected.     

Phylogenomic analyses of lophophorates (brachiopods, phoronids and bryozoans) confirm the Lophotrochozoa concept

Based on embryological and morphological evidence, Lophophorata was long considered to be the sister or paraphyletic stem group of Deuterostomia. By contrast, molecular data have consistently indicated that the three lophophorate lineages, Ectoprocta, Brachiopoda and Phoronida, are more closely related to trochozoans (annelids, molluscs and related groups) than to deuterostomes. For this reason, the lophophorate groups and Trochozoa were united to Lophotrochozoa. However, the relationships of the lophophorate lineages within Lophotrochozoa are still largely unresolved. Maximum-likelihood and Bayesian analyses were performed based on a dataset comprising 11,445 amino acid positions derived from 79 ribosomal proteins of 39 metazoan taxa including new sequences obtained from a brachiopod and a phoronid. These analyses show that the three lophophorate lineages are affiliated with trochozoan rather than deuterostome phyla. All hypotheses claiming that they are more closely related to Deuterostomia than to Protostomia can be rejected by topology testing. Monophyly of lophophorates was not recovered but that of Bryozoa including Ectoprocta and Entoprocta and monophyly of Brachiozoa including Brachiopoda and Phoronida were strongly supported. Alternative hypotheses that are refuted include (i) Brachiozoa as the sister group of Mollusca, (ii) ectoprocts as sister to all other Lophotrochozoa including Platyzoa, and (iii) ectoprocts as sister or to all other protostomes except chaetognaths.     

Neural stem cell biology in vertebrates and invertebrates: more alike than different?

Many of the regulatory mechanisms controlling neural stem cell behavior are proving to be conserved between organisms as diverse as worms and man. Common principles are emerging with respect to the regulation of neural stem cell division and the specification of distinct stem and progenitor cell types. Great progress has been made in recent years in identifying the cellular mechanisms underpinning these processes, thanks in large part to the cross-fertilization of research on different model systems. We review here recent findings that highlight hitherto unappreciated similarities in the cell and molecular biology of neural stem cell self-renewal and differentiation between invertebrates and vertebrates. As well as underscoring the possible conservation of stem cell mechanisms across phyla, these similarities are proving to be practically useful in studying neural stem cell biology in health and disease.     

Almost a spider: a 305-million-year-old fossil arachnid and spider origins

Spiders are an important animal group, with a long history. Details of their origins remain limited, with little knowledge of their stem group, and no insights into the sequence of character acquisition during spider evolution. We describe a new fossil arachnid, Idmonarachne brasieri gen. et sp. nov. from the Late Carboniferous (Stephanian, ca 305–299 Ma) of Montceau-les-Mines, France. It is three-dimensionally preserved within a siderite concretion, allowing both laboratory- and synchrotron-based phase-contrast computed tomography reconstruction. The latter is a first for siderite-hosted fossils and has allowed us to investigate fine anatomical details. Although distinctly spider-like in habitus, this remarkable fossil lacks a key diagnostic character of Araneae: spinnerets on the underside of the opisthosoma. It also lacks a flagelliform telson found in the recently recognized, spider-related, Devonian–Permian Uraraneida. Cladistic analysis resolves our new fossil as sister group to the spiders: the spider stem-group comprises the uraraneids and I. brasieri. While we are unable to demonstrate the presence of spigots in this fossil, the recovered phylogeny suggests the earliest character to evolve on the spider stem-group is the secretion of silk. This would have been followed by the loss of a flagelliform telson, and then the ability to spin silk using spinnerets. This last innovation defines the true spiders, significantly post-dates the origins of silk, and may be a key to the group''s success. The Montceau-les-Mines locality has previously yielded a mesothele spider (with spinnerets). Evidently, Late Palaeozoic spiders lived alongside Palaeozoic arachnid grades which approached the spider condition, but did not express the full suite of crown-group autapomorphies.     

The evolution of the Ecdysozoa

Ecdysozoa is a clade composed of eight phyla: the arthropods, tardigrades and onychophorans that share segmentation and appendages and the nematodes, nematomorphs, priapulids, kinorhynchs and loriciferans, which are worms with an anterior proboscis or introvert. Ecdysozoa contains the vast majority of animal species and there is a great diversity of body plans among both living and fossil members. The monophyly of the clade has been called into question by some workers based on analyses of whole genome datasets. We review the evidence that now conclusively supports the unique origin of these phyla. Relationships within Ecdysozoa are also controversial and we discuss the molecular and morphological evidence for a number of monophyletic groups within this superphylum.     

A phylogenetic test of the calcichordate scenario

The calcichordate scenario of Jefferies and colleagues purports to explain the origin and early evolution of the phyla Echinodermata and Chordata. Calcichordate proponents have argued that echinoderms are the sister group of the chordates and urochordates are the sister group of the craniates. These phylogenetic hypotheses, which differ from the traditional groupings, are derived primarily from morphological interpretations of carpoids (solutes, cornutes, and mitrates), an enigmatic fossil group usually held to be primitive stem-group echinoderms. Although the scenario has received only limited support, it has yet to be falsified. The difficulty with falsifying the calcichordate scenario is proving that the morphological interpretations, for example, that carpoids possessed notochords, dorsal hollow nerve cords, and other typical chordate or craniate characters, are incorrect. Here, rather than argue over the interpretation of fossils, the phylogenetic hypotheses embedded within the scenario are tested. It is found that the calcichordate scenario fails such a test, even if both the Recent and fossils forms are coded according to the calcichordate scenario. It is argued that: (1) the erection of scenarios must follow the construction of a cladogram; and (2) fossils are unable to dictate the relationships among phyla. □ Calcichordate scenario, Carpoidea, Deuterostomia, Echinodermata, Chordata, phylogeny, cladistics.     

Characterization of chemoautotrophic bacterial symbionts in a gutless marine worm Oligochaeta, Annelida) by phylogenetic 16S rRNA sequence analysis and in situ hybridization.

The phylogenetic relationships of chemoautotrophic endosymbionts in the gutless marine oligochaete Inanidrilus leukodermatus to chemoautotrophic ecto- and endosymbionts from other host phyla and to free-living bacteria were determined by comparative 16S rRNA sequence analysis. Fluorescent in situ hybridization confirmed that the 16S rRNA sequence obtained from these worms originated from the symbionts. The symbiont sequence is unique to I. leukodermatus. In phylogenetic trees inferred by both distance and parsimony methods, the oligochaete symbiont is peripherally associated with one of two clusters of chemoautotrophic symbionts that belong to the gamma subdivision of the Proteobacteria. The endosymbionts of this oligochaete form a monophyletic group with chemoautotrophic ectosymbionts of a marine nematode. The oligochaete and nematode symbionts are very closely related, although their hosts belong to separate, unrelated animal phyla. Thus, cospeciation between the nematode and oligochaete hosts and their symbionts could not have occurred. Instead, the similar geographic locations and habitats of the hosts may have influenced the establishment of these symbioses.     

Chordate evolution and the three-phylum system

Traditional metazoan phylogeny classifies the Vertebrata as a subphylum of the phylum Chordata, together with two other subphyla, the Urochordata (Tunicata) and the Cephalochordata. The Chordata, together with the phyla Echinodermata and Hemichordata, comprise a major group, the Deuterostomia. Chordates invariably possess a notochord and a dorsal neural tube. Although the origin and evolution of chordates has been studied for more than a century, few authors have intimately discussed taxonomic ranking of the three chordate groups themselves. Accumulating evidence shows that echinoderms and hemichordates form a clade (the Ambulacraria), and that within the Chordata, cephalochordates diverged first, with tunicates and vertebrates forming a sister group. Chordates share tadpole-type larvae containing a notochord and hollow nerve cord, whereas ambulacrarians have dipleurula-type larvae containing a hydrocoel. We propose that an evolutionary occurrence of tadpole-type larvae is fundamental to understanding mechanisms of chordate origin. Protostomes have now been reclassified into two major taxa, the Ecdysozoa and Lophotrochozoa, whose developmental pathways are characterized by ecdysis and trochophore larvae, respectively. Consistent with this classification, the profound dipleurula versus tadpole larval differences merit a category higher than the phylum. Thus, it is recommended that the Ecdysozoa, Lophotrochozoa, Ambulacraria and Chordata be classified at the superphylum level, with the Chordata further subdivided into three phyla, on the basis of their distinctive characteristics.     

Testing the new animal phylogeny: a phylum level molecular analysis of the animal kingdom

The new animal phylogeny inferred from ribosomal genes some years ago has prompted a number of radical rearrangements of the traditional, morphology based metazoan tree. The two main bilaterian clades, Deuterostomia and Protostomia, find strong support, but the protostomes consist of two sister groups, Ecdysozoa and Lophotrochozoa, not seen in morphology based trees. Although widely accepted, not all recent molecular phylogenetic analyses have supported the tripartite structure of the new animal phylogeny. Furthermore, even if the small ribosomal subunit (SSU) based phylogeny is correct, there is a frustrating lack of resolution of relationships between the phyla that make up the three clades of this tree. To address this issue, we have assembled a dataset including a large number of aligned sequence positions as well as a broad sampling of metazoan phyla. Our dataset consists of sequence data from ribosomal and mitochondrial genes combined with new data from protein coding genes (5139 amino acid and 3524 nucleotide positions in total) from 37 representative taxa sampled across the Metazoa. Our data show strong support for the basic structure of the new animal phylogeny as well as for the Mandibulata including Myriapoda. We also provide some resolution within the Lophotrochozoa, where we confirm support for a monophyletic clade of Echiura, Sipuncula and Annelida and surprising evidence of a close relationship between Brachiopoda and Nemertea.