A new probable stem lineage crustacean with three-dimensionally preserved soft parts from the Herefordshire (Silurian) Lagerstätte, UK

A vicissicaudatan arthropod from the Silurian Herefordshire Lagerstätte, UK

A new arthropod, Carimersa neptuni gen. et sp. nov., is described from the Silurian (Wenlock Series) Herefordshire Konservat-Lagerstätte, UK. The head bears pedunculate eyes and five pairs of appendages. Triflagellate antennae are followed by two pairs of uniramous limbs each with an endopod bearing a pronounced gnathobasic basipod. The posterior two pairs of head limbs and all trunk limbs bear an endopod, exopod and filamentous exite. The trunk consists of 10 appendage-bearing segments followed by an apodous abdomen of four segments. The arthropod resolves as sister taxon to Kodymirus and Eozetetes + Aglaspidida. It is the first representative of Vicissicaudata reported from the Herefordshire Lagerstätte and the first Silurian example with well-preserved appendages. The preservation of a cluster of radiolarians apparently captured by the trunk appendages is the first direct association of predator and prey discovered in the Herefordshire fauna, and suggests that Carimersa was a nektobenthic form that used its gnathobasic basipods in microdurophagy.

Waptia fieldensis Walcott, a mandibulate arthropod from the middle Cambrian Burgess Shale

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

A new crustacean from the Herefordshire (Silurian) Lagerstätte, UK, and its significance in malacostracan evolution

Cascolus ravitis gen. et sp. nov. is a three-dimensionally preserved fossil crustacean with soft parts from the Herefordshire (Silurian) Lagerstätte, UK. It is characterized by a head with a head shield and five limb pairs, and a thorax (pereon) with nine appendage-bearing segments followed by an apodous abdomen (pleon). All the appendages except the first are biramous and have a gnathobase. The post-mandibular appendages are similar one to another, and bear petal-shaped epipods that probably functioned as a part of the respiratory-circulatory system. Cladistic analysis resolves the new taxon as a stem-group leptostracan (Malacostraca). This well-preserved arthropod provides novel insights into the evolution of appendage morphology, tagmosis and the possible respiratory-circulatory physiology of a basal malacostracan.

The nature of non-appendicular anterior paired projections in Palaeozoic total-group Euarthropoda

Recent studies have clarified the segmental organization of appendicular and exoskeletal structures in the anterior region of Cambrian stem-group Euarthropoda, and thus led to better understanding of the deep evolutionary origins of the head region in this successful animal group. However, there are aspects of the anterior organization of Palaeozoic euarthropods that remain problematic, such as the morphological identity and significance of minute limb-like projections on the anterior region in stem and crown-group representatives. Here, we draw attention to topological and morphological similarities between the frontal filaments of extant Crustacea and the embryonic frontal processes of Onychophora, and distinctive anterior paired projections observed in several extinct total-group Euarthropoda. Anterior paired projections are redescribed in temporally and phylogenetically distant fossil taxa, including the gilled lobopodians Kerygmachela kierkegaardi and Pambdelurion whittingtoni, the bivalved stem-euarthropod Canadaspis perfecta, the larval pycnogonid Cambropycnogon klausmuelleri, and the mandibulate Tanazios dokeron. Developmental data supporting the homology of the 'primary antennae' of Onychophora, the 'frontal appendages' of lower-stem Euarthropoda, and the hypostome/labrum complex of Deuteropoda, argue against the morphological identity of the anterior paired projections of extant and extinct panarthropods as a pair of pre-ocular appendages. Instead, we regard the paired projections of fossil total-group euarthropods as non-appendicular evaginations with a likely protocerebral segmental association, and a possible sensorial function. The widespread occurrence of pre-ocular paired projections among extant and extinct taxa suggests their potential homology as fundamentally ancestral features of the anterior body organization in Panarthropoda. Non-appendicular paired projections with a sensorial function may reflect a critical--yet previously overlooked--component of the panarthropod ground pattern.

Waptia revisited: Intimations of behaviors

The middle Cambrian taxon Waptia fieldensis offers insights into early evolution of sensory arrangements that may have supported a range of actions such as exploratory behavior, burrowing, scavenging, swimming, and escape, amongst others. Less elaborate than many modern pancrustaceans, specific features of Waptia that suggest a possible association with the pancrustacean evolutionary trajectory, include mandibulate mouthparts, a single pair of antennae, reflective triplets on the head comparable to ocelli, and traces of brain and optic lobes that conform to the pancrustacean ground pattern. This account revisits an earlier description of Waptia to further interpret the distribution of its overall morphology and receptor arrangements in the context of plausible behavioral repertoires.

Preservation of three-dimensional anatomy in phosphatized fossil arthropods enriches evolutionary inference

External and internal morphological characters of extant and fossil organisms are crucial to establishing their systematic position, ecological role and evolutionary trends. The lack of internal characters and soft-tissue preservation in many arthropod fossils, however, impedes comprehensive phylogenetic analyses and species descriptions according to taxonomic standards for Recent organisms. We found well-preserved three-dimensional anatomy in mineralized arthropods from Paleogene fissure fillings and demonstrate the value of these fossils by utilizing digitally reconstructed anatomical structure of a hister beetle. The new anatomical data facilitate a refinement of the species diagnosis and allowed us to reject a previous hypothesis of close phylogenetic relationship to an extant congeneric species. Our findings suggest that mineralized fossils, even those of macroscopically poor preservation, constitute a rich but yet largely unexploited source of anatomical data for fossil arthropods.

Arthropod fossil data increase congruence of morphological and molecular phylogenies

The relationships of major arthropod clades have long been contentious, but refinements in molecular phylogenetics underpin an emerging consensus. Nevertheless, molecular phylogenies have recovered topologies that morphological phylogenies have not, including the placement of hexapods within a paraphyletic Crustacea, and an alliance between myriapods and chelicerates. Here we show enhanced congruence between molecular and morphological phylogenies based on 753 morphological characters for 309 fossil and Recent panarthropods. We resolve hexapods within Crustacea, with remipedes as their closest extant relatives, and show that the traditionally close relationship between myriapods and hexapods is an artefact of convergent character acquisition during terrestrialisation. The inclusion of fossil morphology mitigates long-branch artefacts as exemplified by pycnogonids: when fossils are included, they resolve with euchelicerates rather than as a sister taxon to all other euarthropods.

A Silurian short-great-appendage arthropod

A new arthropod, Enalikter aphson gen. et sp. nov., is described from the Silurian (Wenlock Series) Herefordshire Lagerstätte of the UK. It belongs to the Megacheira (=short-great-appendage group), which is recognized here, for the first time, in strata younger than mid-Cambrian age. Discovery of this new Silurian taxon allows us to identify a Devonian megacheiran representative, Bundenbachiellus giganteus from the Hunsrück Slate of Germany. The phylogenetic position of megacheirans is controversial: they have been interpreted as stem chelicerates, or stem euarthropods, but when Enalikter and Bundenbachiellus are added to the most comprehensive morphological database available, a stem euarthropod position is supported. Enalikter represents the only fully three-dimensionally preserved stem-group euarthropod, it falls in the sister clade to the crown-group euarthropods, and it provides new insights surrounding the origin and early evolution of the euarthropods. Recognition of Enalikter and Bundenbachiellus as megacheirans indicates that this major arthropod group survived for nearly 100 Myr beyond the mid-Cambrian.

Captopodus poschmanni gen. et sp. nov. a new stem-group arthropod from the Lower Devonian Hunsrück Slate (Germany)

A new arthropod from the Lower Devonian Hunsrück Slate is described on the basis of four specimens. The body of Captopodus poschmanni comprises a head, a trunk with an anal portion. The high number of trunk appendages (≥66 segments) is unusual. The function of one pair of cupola-like structures of the head shield is unclear. The presence of large grasping appendages in the head superficially resembles the 'short great appendages' of other euarthropods and grasping appendages of thylacocephalans. The phylogenetic position of the arthropod cannot be determined in detail, though several morphological aspects indicate a phylogenetic position as a stem lineage representative of the Euarthropoda, the morphology of the trunk appendages seem to indicate a more advanced phylogenetic position. This new taxon underlines the exceptional diversity of arthropods within the Hunsrück Slate in comparison to other Devonian fossil sites and highlights the significance of the Hunsrück Slate for the evolution of early arthropods.

Arthropod phylogeny: an overview from the perspectives of morphology, molecular data and the fossil record

Monophyly of Arthropoda is emphatically supported from both morphological and molecular perspectives. Recent work finds Onychophora rather than Tardigrada to be the closest relatives of arthropods. The status of tardigrades as panarthropods (rather than cycloneuralians) is contentious from the perspective of phylogenomic data. A grade of Cambrian taxa in the arthropod stem group includes gilled lobopodians, dinocaridids (e.g., anomalocaridids), fuxianhuiids and canadaspidids that inform on character acquisition between Onychophora and the arthropod crown group. A sister group relationship between Crustacea (itself likely paraphyletic) and Hexapoda is retrieved by diverse kinds of molecular data and is well supported by neuroanatomy. This clade, Tetraconata, can be dated to the early Cambrian by crown group-type mandibles. The rival Atelocerata hypothesis (Myriapoda+Hexapoda) has no molecular support. The basal node in the arthropod crown group is embroiled in a controversy over whether myriapods unite with chelicerates (Paradoxopoda or Myriochelata) or with crustaceans and hexapods (Mandibulata). Both groups find some molecular and morphological support, though Mandibulata is presently the stronger morphological hypothesis. Either hypothesis forces an unsampled ghost lineage for Myriapoda from the Cambrian to the mid Silurian.

An exceptionally preserved myodocopid ostracod from the Silurian of Herefordshire, UK

An exceptionally preserved new ostracod crustacean from the Silurian of Herefordshire, UK, represents only the third fully documented Palaeozoic ostracod with soft-part preservation. Appendages, gills, gut system, lateral compound eyes and even a medial eye with a Bellonci organ are preserved, allowing assignment of the fossil to a new genus and species of cylindroleberidid myodocope (Myodocopida, Cylindroleberididae). The Bellonci organ is recorded for the first time in fossil ostracods. The find also represents a rare occurrence of gills in fossil ostracods and confirms the earliest direct evidence of a respiratory-cum-circulatory system in the group. The species demonstrates remarkably conserved morphology within myodocopes over a period of 425 Myr. Its shell morphology more closely resembles several families of myodocopes other than the Cylindroleberididae, especially the Cypridinidae and Sarsiellidae, thus questioning the utility of the carapace alone in establishing the affinity of fossil ostracods.

Simple 3D images from fossil and recent micromaterial using light microscopy

Abstract We present a technique for extracting 3D information from small-scale fossil and Recent material and give a summary of other contemporary techniques for 3D methods of investigation. The only hardware needed for the here-presented technique is a microscope that can perform dark field and/or differential interference contrast with a mounted digital camera and a computer. Serial images are taken while the focus is successively shifted from the uppermost end of the specimen to the lowermost end, resulting in about 200 photographs. The data are then processed almost completely automatically by successive use of three freely available programs. Firstly, the stack of images is aligned by the use of CombineZM, which is used to produce a combined image with a high depth of field. Secondly, the aligned images are cropped and sharp edges extracted with the aid of ImageJ. Thirdly, although ImageJ is also capable of producing 3D representations, we preferred to process the image stack further using osirix as it has the facility to export various formats. One of the interesting export formats is a virtual Quicktime movie file (QTVR), which can be used for documentation, and stereo images can also be produced from this Quicktime VR. This method is easy to apply and can be used for documenting specimens in 3D (at least some aspects) without having to prepare them. Therefore, it is particularly useful as a safe method for documenting limited material, before using methods that may destroy the specimen of interest, or to investigate type material that cannot be treated with any preparatory technique. As light microscopes are available in most labs and free computer programs are easily accessible, this method can be readily applied.

The origin and evolution of arthropods

The past two decades have witnessed profound changes in our understanding of the evolution of arthropods. Many of these insights derive from the adoption of molecular methods by systematists and developmental biologists, prompting a radical reordering of the relationships among extant arthropod classes and their closest non-arthropod relatives, and shedding light on the developmental basis for the origins of key characteristics. A complementary source of data is the discovery of fossils from several spectacular Cambrian faunas. These fossils form well-characterized groupings, making the broad pattern of Cambrian arthropod systematics increasingly consensual.

A Silurian 'marrellomorph' arthropod

Xylokorys chledophilia, a new arthropod with three-dimensionally preserved soft tissues, is described from the Herefordshire (Silurian) Lagerstätte of England. The head and trunk are covered by a relatively featureless ovoid carapace, which comprises a domed central part and a flange-like border. The head bears five pairs of appendages. The first is uniramous, with dorsal and ventral projections distally. Appendages two to four are biramous and each endopod terminates in two projections. Appendage five is possibly biramous. The hypostome is very long and subrectangular in outline. There are approximately 35 pairs of biramous trunk appendages. Each exopod comprises a long slender shaft bearing numerous fine filaments; each endopod comprises a ribbon-like shaft bearing paddle-like endites. Morphological comparisons and cladistic analyses of X. chledophilia indicate affinity with Vachonisia rogeri from the Lower Devonian Hunsrück Slate, within the marrellomorphs, but assignment to Marrellomorpha is provisional pending revision of other members of this clade. Xylokorys is the first 'marrellomorph' to be reported from the Silurian. It is interpreted as a benthic particle filter feeder, which may also have consumed prey items.