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An Ordovician Pycnogonid (Sea Spider) with Serially Subdivided ‘Head' Region
Abstract
The bizarre morphology of living Pycnogonida, known colloquially as sea spiders, has long fueled dissent over their status within the arthropods. Pycnogonids figure prominently in recent analyses of anterior limb homologies and ancestral crown-group euarthropod relationships, with support for the concept of Pycnogonida as sister taxon to Euchelicerata now contested by proponents of a more basal position between Radiodonta and all other arthropods. A challenge to further elucidation of their phylogenetic position is the exceptional rarity and disjunct distribution of pycnogonids in the fossil record, due largely to their fragile unmineralized exoskeletons. New fossil discoveries therefore have the potential to add significantly to knowledge of their evolution, paleoecology, and paleobiogeography. Here we report the first known occurrence of fossil pycnogonids from rocks of Ordovician age, bridging a 65 Myr gap between controversial late Cambrian larval forms and a single documented Silurian specimen. The new taxon, Palaeomarachne granulata n. gen. n. sp., from the Upper Ordovician (ca. 450 Ma) William Lake Konservat-Lagerstätte deposit in Manitoba, Canada, is also the first reported from Laurentia. It is the only record thus far of a fossil sea spider in rocks of demonstrably shallow marine origin. Four incomplete, partially disarticulated molts represent a relatively large, robust animal with a series of five segment-like elements in a ‘head' region that does not incorporate the first of four preserved limb-bearing trunk segments. This unique pattern may reflect the plesiomorphic condition prior to complete fusion of anterior ‘head' elements and first trunk segment to form a cephalosoma, as seen in all eupycnogonids. Palaeomarachne granulata is interpreted as occupying a basal stem-group position in the Pycnogonida.
... 425 Ma) (Siveter et al. 2004) but could date back to the Late Ordovician (c. 450 Ma) (Rudkin et al. 2013) or Late Cambrian (c. 500 Ma) (Waloszek & Dunlop 2002). ...
... 425 Ma; UK) (Siveter et al. 2004). Palaeomarachne granulata Rudkin et al., 2013, which is probably a pycnogonid moult from the Upper Ordovician William Lake Lagerst€ atte (c. 450 Ma; Manitoba, Canada), is distinct due to the serial segmentation of its head (Rudkin et al. 2013). ...
... Palaeomarachne granulata Rudkin et al., 2013, which is probably a pycnogonid moult from the Upper Ordovician William Lake Lagerst€ atte (c. 450 Ma; Manitoba, Canada), is distinct due to the serial segmentation of its head (Rudkin et al. 2013). Palaeozoic pycnogonids have been assigned to up to three fossil-specific orders (Bamber 2007) and possible affinities to Pantopoda have been suggested only for three Devonian species (F. ...
Sea spiders (Pycnogonida) are strange arthropods characterized by a unique morphology, including reduced body, egg-carrying appendages and a proboscis. This peculiar body plan dates at least as early as the first undoubted occurrence of the group, 425 million years ago in the Silurian. All extant species belong to the order Pantopoda, characterized by cylindrical legs and an unsegmented abdomen. Palaeozoic fossils are much more diversified and exhibit features very different from pantopod morphology such as a segmented abdomen, limbs specialized to swim, or even a flagellum. The few Mesozoic fossils from the single Jurassic palaeoenvironment of La Voulte-sur-Rhône (south-eastern France) instead have strong affinities to Pantopoda. Here, we investigate the morphology of nine sea spider fossils using a new photographic protocol to document morphology, combining focus stacking and differential colourization. We also describe two new species of fossil pycnogonids from the Late Jurassic of Solnhofen (southern Germany): Colossopantopodus nanus sp. nov. is closely allied to a large species from La Voulte-sur-Rhône, but distinct in its smaller size; the other, ?Eurycyde golem sp. nov., is the first known fossil representative of the extant family Ascorhynchidae. Seven additional specimens, too poorly preserved for taxonomic description, are illustrated. The comparisons reveal that the shallow lagoons of Solnhofen contained a diverse assemblage of pantopods. Altogether with the fauna of La Voulte-sur-Rhône, the results suggest that Pantopoda became the dominant pycnogonid taxon of both deep and shallow marine waters after the Jurassic.
http://zoobank.org/urn:lsid:zoobank.org:pub:3C7AB6BF-A161-435F-B85C-53220A833188
... Every leg exuviae is shed like a sleeve and the body cuticle splits into a ventral and a dorsal part. It seems that it was already the case in the fossil species Palaeomarachne granulata Rudkin, Cuggy, Young & Thompson † [20]. Some adult P. litorale may grow without even molting [19], as they periodically "desquamate" portions of damaged cuticle [20]. ...
... It seems that it was already the case in the fossil species Palaeomarachne granulata Rudkin, Cuggy, Young & Thompson † [20]. Some adult P. litorale may grow without even molting [19], as they periodically "desquamate" portions of damaged cuticle [20]. ...
Limb anomalies are widespread and diversified in arthropods. From trilobites to insects, they range from the loss to the addition or fusion of legs and may appear congenitally or be induced experimentally (e.g., amputation or injury). Basal chelicerates pycnogonids, or sea spiders, also show deformities. Despite being understudied compared to other arthropods, quite a high diversity of limb malformations has been reported in the literature. The present study reports the leg anomalies of two adult females Ammothea hilgendorfi (Böhm, 1879) observed with duplicated leg podomeres. Both individuals were described ethologically and morphologically. Although the current knowledge on pycnogonids is limited, the anomaly is likely due to a problem in the molting process; the specimens were unable to totally remove their old exuviae, which then stacked after the proximal leg segments. The second specimen also showed other leg deformities, hinting at a problem during the molting process itself. The discussion emphasizes that understanding how pycnogonids normally molt would not only help our understanding of how the abnormal patterns appeared but also put pycnogonids into perspective with other arthropods, a phylum in which they have a key taxonomic position.
... Without such characters, it is not possible to adequately confirm crown group affinity. Another fossil species, Palaeomarachne granulata Rudkin et al., 2013 from the Late Ordovician of Manitoba, is specifically noted as a stem pantopod due to its likely plesiomorphic head tagmosis. ...
... trilobites), while the possible crown group taxa that have been described, e.g. ostracods (Siveter et al., 2014;Williams et al., 2008), barnacles (Van Roy et al., 2010), pycnogonids (Rudkin et al., 2013), xiphosurans (Lamsdell, 2013;Rudkin et al., 2008;Van Roy et al., 2010), and acariform mites (Bernini et al., 2002) do not meet the rigorous standards employed herein for determining calibration points. From the Late Devonian through the Mississippian (382.7 to 323.2 Ma) our dataset has another large gap during which we have only one calibration point, which interestingly corresponds with one of the largest mass extinctions events known in the fossil record (McGhee, 2013). ...
... Other analyses recovered the pycnogonids nested among the Euchelicerata 38,39 . Aspects of the highly derived anatomy of pycnogonids, such as their uniramous appendages and four segments in the head tagma, cast doubt on this sister-group relationship (Pycnogonida+Euchelicerata) 39,40 . These doubts have been further increased by the emergence of Habeliida as a potential alternative to Pycnogonida as the closest sister group for Euchelicerata 12,41 . ...
Euchelicerata is a clade of arthropods comprising horseshoe crabs, scorpions, spiders, mites and ticks, as well as the extinct eurypterids (sea scorpions) and chasmataspidids. The understanding of the ground plans and relationships between these crown-group euchelicerates has benefited from the discovery of numerous fossils. However, little is known regarding the origin and early evolution of the euchelicerate body plan because the relationships between their Cambrian sister taxa and synziphosurines, a group of Silurian to Carboniferous stem euchelicerates with chelicerae and an unfused opisthosoma, remain poorly understood owing to the scarce fossil record of appendages. Here we describe a synziphosurine from the Lower Ordovician (ca. 478 Ma) Fezouata Shale of Morocco. This species possesses five biramous appendages with stenopodous exopods bearing setae in the prosoma and a fully expressed first tergite in the opisthosoma illuminating the ancestral anatomy of the group. Phylogenetic analyses recover this fossil as a member of the stem euchelicerate family Offacolidae, which is characterized by biramous prosomal appendages. Moreover, it also shares anatomical features with the Cambrian euarthropod Habelia optata, filling the anatomical gap between euchelicerates and Cambrian stem taxa, while also contributing to our understanding of the evolution of euchelicerate uniramous prosomal appendages and tagmosis.
... Indeed, such metamery is one of the distinguishing traits of Pycnogonida (Dunlop and Arango, 2005;Miyazaki and Makioka, 2011) and is unique among chelicerates. Although sea spiders are generally considered to be an ancient group which maintained its bauplan since the Palaeozoic (Bergstr€ om et al., 1980;Siveter et al., 2004;Poschmann and Dunlop, 2006;Kühl et al., 2013;Rudkin et al., 2013), it is likely they had also acquired a number of specific traits, which cannot be seen as plesiomorphic to all chelicerates. One such trait is metamery of the peripheral gonad elements (Miyazaki and Makioka, 2010), which formed secondarily from the unitary origin (Beklemishev, 1969), i.e., the trunk gonad. ...
... trilobites), while the possible crown group taxa that have been described, e.g. ostracods (Siveter et al., 2014;Williams et al., 2008), barnacles (Van Roy et al., 2010), pycnogonids (Rudkin et al., 2013), xiphosurans (Lamsdell, 2013;Rudkin et al., 2008;Van Roy et al., 2010), and acariform mites (Bernini et al., 2002) do not meet the rigorous standards employed herein for determining calibration points. From the Late Devonian through the Mississippian (382.7 to 323.2 Ma) our dataset has another large gap during which we have only one calibration point, which interestingly corresponds with one of the largest mass extinctions events known in the fossil record (McGhee, 2013). ...
Fossil age data and molecular sequences are increasingly combined to establish a timescale for the Tree of Life. Arthropods, as the most species-rich and morphologically disparate animal phylum, have received substantial attention, particularly with regard to questions such as the timing of habitat shifts (e.g. terrestrialisation), genome evolution (e.g. gene family duplication and functional evolution), origins of novel characters and behaviours (e.g. wings and flight, venom, silk), biogeography, rate of diversification (e.g. Cambrian explosion, insect coevolution with angiosperms, evolution of crab body plans), and the evolution of arthropod microbiomes. We present herein a series of rigorously vetted calibration fossils for arthropod evolutionary history, taking into account recently published guidelines for best practice in fossil calibration. These are restricted to Palaeozoic and Mesozoic fossils, no deeper than ordinal taxonomic level, nonetheless resulting in 80 fossil calibrations for 102 clades. This work is especially timely owing to the rapid growth of molecular sequence data and the fact that many included fossils have been described within the last five years. This contribution provides a resource for systematists and other biologists interested in deep-time questions in arthropod evolution.
ABBREVIATIONS
AMNH American Museum of Natural History
AMS Australian Museum, Sydney
AUGD University of Aberdeen
BGR Bundesanstalt fur Geowissenschaften und Rohstoffe, Berlin
BMNH The Natural History Museum, London
CNU Key Laboratory of Insect Evolutionary & Environmental Change, Capital Normal University, Beijing
DE Ulster Museum, Belfast
ED Ibaraki University, Mito, Japan
FMNH Field Museum of Natural History
GMCB Geological Museum of China, Beijing
GSC Geological Survey of Canada
IRNSB Institut Royal des Sciences Naturelles de Belgique, Brussels
KSU Kent State University
Ld Musee Fleury, Lodeve, France
LWL Landschaftsverband Westfalen-Lippe-Museum fur Naturkunde, Munster
MACN Museo Argentino de Ciencias Naturales, Buenos Aires
MBA Museum fur Naturkunde, Berlin
MCNA Museo de Ciencias Naturales de Alava, Vitoria-Gasteiz, Alava, Spain
MCZ Museum of Comparative Zoology, Harvard University
MGSB Museo Geologico del Seminario de Barcelona
MN Museu Nacional, Rio de Janeiro
MNHN Museum national d'Histoire naturelle, Paris
NHMUK The Natural History Museum, London
NIGP Nanjing Institute of Geology and Palaeontology
NMS National Museum of Scotland
OUM Oxford University Museum of Natural History
PBM Palaobotanik Munster
PIN Paleontological Institute, Moscow
PRI Paleontological Research Institution, Ithaca
ROM Royal Ontario Museum
SAM South Australian Museum, Adelaide
SM Sedgwick Museum, University of Cambridge
SMNK Staatliches Museum fur Naturkunde, Karlsruhe
SMNS Staatliches Museum fur Naturkunde, Stuttgart
TsGM F.N. Chernyshev Central Geologic Prospecting Research Museum, St. Petersburg
UB University of Bonn
USNM US National Museum of Natural History, Smithsonian Institution
UWGM University of Wisconsin Geology Museum
YKLP Yunnan Key Laboratory for Palaeobiology, Yunnan University
YPM Yale Peabody Museum
ZPAL Institute of Paleobiology, Polish Academy of Sciences, Warsaw.
... Regardless of this, the presence of paired ISNs between free trunk segments can be securely traced to the last common ancestor of the pycnogonid crown group (Fig. 18C). Owing to the interdependency of ISNs and free trunk segments in extant sea spiders, these nerves may be even tentatively extrapolated to stem lineage representatives with free trunk segments [83,84]. Also in mandibulate taxa, the VNC features paired ISNs [40,58,[85][86][87][88], which are known to encompass motoneuron projections targeting body wall muscles in several representatives [89][90][91][92]. ...
Background
Pycnogonida (sea spiders) is the sister group of all other extant chelicerates (spiders, scorpions and relatives) and thus represents an important taxon to inform early chelicerate evolution. Notably, phylogenetic analyses have challenged traditional hypotheses on the relationships of the major pycnogonid lineages (families), indicating external morphological traits previously used to deduce inter-familial affinities to be highly homoplastic. This erodes some of the support for phylogenetic information content in external morphology and calls for the study of additional data classes to test and underpin in-group relationships advocated in molecular analyses. In this regard, pycnogonid internal anatomy remains largely unexplored and taxon coverage in the studies available is limited.
Results
Based on micro-computed X-ray tomography and 3D reconstruction, we created a comprehensive atlas of in-situ representations of the central nervous system and midgut layout in all pycnogonid families. Beyond that, immunolabeling for tubulin and synapsin was used to reveal selected details of ganglionic architecture. The ventral nerve cord consistently features an array of separate ganglia, but some lineages exhibit extended composite ganglia, due to neuromere fusion. Further, inter-ganglionic distances and ganglion positions relative to segment borders vary, with an anterior shift in several families. Intersegmental nerves target longitudinal muscles and are lacking if the latter are reduced. Across families, the midgut displays linear leg diverticula. In Pycnogonidae, however, complex multi-branching diverticula occur, which may be evolutionarily correlated with a reduction of the heart.
Conclusions
Several gross neuroanatomical features are linked to external morphology, including intersegmental nerve reduction in concert with trunk segment fusion, or antero-posterior ganglion shifts in partial correlation to trunk elongation/compaction. Mapping on a recent phylogenomic phylogeny shows disjunct distributions of these traits. Other characters show no such dependency and help to underpin closer affinities in sub-branches of the pycnogonid tree, as exemplified by the tripartite subesophageal ganglion of Pycnogonidae and Rhynchothoracidae. Building on this gross anatomical atlas, future studies should now aim to leverage the full potential of neuroanatomy for phylogenetic interrogation by deciphering pycnogonid nervous system architecture in more detail, given that pioneering work on neuron subsets revealed complex character sets with unequivocal homologies across some families.
... Such sea spiders are also found in the families Pycnogonidae and Nymphonidae and formed independently [4,16]. Most of modern [6] and extinct [8,23,24,28] pycnogonids have three free legbearing segments, and only Pentapantopus vogteli, whose phylogenetic position is controversial, has four free leg-bearing segments [19]. Apparently, this proves the lability of the pycnogonid body plan, but not the primitiveness of polymeric forms. ...
... Haliestes Fig. 5n, o) is arguably the best preserved and most complete of just thirteen known species (Sabroux et al. 2019) of fossil pycnogonid. These include a putative larval pycnogonid, Cambropycnogon klausmuelleri, from the upper Cambrian Orsten of Scandinavia (Waloszek & Dunlop 2002), and a basal stem group pycnogonid, Palaeomarachne granulata, from the Ordovician William Lake Lagerstätte of Canada (Rudkin et al. 2013). The Herefordshire pycnogonid Haliestes shows reduction of the body to a small trunk projection beyond the posteriormost limbs. ...
The Herefordshire Lagerstätte ( c . 430 Ma) from the UK is a rare example of soft-tissue preservation in the Silurian. It yields a wide range of invertebrates in unparalleled three-dimensional detail, dominated by arthropods and sponges. The fossils are exceptionally preserved as calcitic void infills in early diagenetic carbonate concretions within a volcaniclastic (bentonite) horizon. The Lagerstätte occurs in an outer shelf/upper slope setting in the Welsh Basin, which was located on Avalonia in the southern subtropics. The specimens are investigated by serial grinding, digital photography and rendering in the round as ‘virtual fossils’ by computer. The fossils contribute much to our understanding of the palaeobiology and early history of the groups represented. They are important in demonstrating unusual character combinations that illuminate relationships; in calibrating molecular clocks; in variously linking with taxa in both earlier and later Paleozoic Lagerstätten; and in providing evidence of the early evolution of crown-group representatives of several groups.
... (formerly known as Pseudopallene sp.; see [21]; Fig. 1e) followed the neurogenic processes in the ventral nerve cord (VNC) during the postembryonic developmental phase [22]. Pycnogonida (sea spiders) is an old lineage of marine arthropods dating back at least to the Ordovician [23], if not even to the Cambrian [24,25]. Notably, it is currently considered the sister group to all other extant chelicerates [7,[26][27][28]. ...
Background:
Comparative studies of neuroanatomy and neurodevelopment provide valuable information for phylogenetic inference. Beyond that, they reveal transformations of neuroanatomical structures during animal evolution and modifications in the developmental processes that have shaped these structures. In the extremely diverse Arthropoda, such comparative studies contribute with ever-increasing structural resolution and taxon coverage to our understanding of nervous system evolution. However, at the neurodevelopmental level, in-depth data remain still largely confined to comparably few laboratory model organisms. Therefore, we studied postembryonic neurogenesis in six species of the bizarre Pycnogonida (sea spiders), which - as the likely sister group of all remaining chelicerates - promise to illuminate neurodevelopmental changes in the chelicerate lineage.
Results:
We performed in vivo cell proliferation experiments with the thymidine analogs 5-bromo-2'-deoxyuridine and 5-ethynl-2'-deoxyuridine coupled to fluorescent histochemical staining and immunolabeling, in order to compare ventral nerve cord anatomy and to localize and characterize centers of postembryonic neurogenesis. We report interspecific differences in the architecture of the subesophageal ganglion (SEG) and show the presence of segmental "ventral organs" (VOs) that act as centers of neural cell production during gangliogenesis. These VOs are either incorporated into the ganglionic soma cortex or found on the external ganglion surface. Despite this difference, several shared features support homology of the two VO types, including (1) a specific arrangement of the cells around a small central cavity, (2) the presence of asymmetrically dividing neural stem cell-like precursors, (3) the migration of newborn cells along corresponding pathways into the cortex, and (4) the same VO origin and formation earlier in development.
Conclusions:
Evaluation of our findings relative to current hypotheses on pycnogonid phylogeny resolves a bipartite SEG and internal VOs as plesiomorphic conditions in pycnogonids. Although chelicerate taxa other than Pycnogonida lack comparable VOs, they are a characteristic feature of myriapod gangliogenesis. Accordingly, we propose internal VOs with neurogenic function to be part of the ground pattern of Arthropoda. Further, our findings illustrate the importance of dense sampling in old arthropod lineages - even if as gross-anatomically uniform as Pycnogonida - in order to reliably differentiate plesiomorphic from apomorphic neurodevelopmental characteristics prior to outgroup comparison.
... A number of minor components of the William Lake biota are also absent from the Michigan Lagerstätte, including strophomenate and rhynchonellate brachiopods. The rare Big Hill bivalves are unknown from the Manitoba biotas, and the enigmatic arthropod with diplotergites has no obvious counterpart in Manitoba, although it exhibits some similarity to specimens from William Lake interpreted as pycnogonids (Rudkin et al. 2013). Overall, the Big Hill biota shows closest affinity to William Lake out of the Manitoba Lagerstätten, in that both preserve abundant medusae and linguloid brachiopods alongside ostracods, scarce gastropods and algae. ...
A new exceptionally preserved marginal marine biota is reported from the Late Ordovician Big Hill Formation of Stonington Peninsula in Michigan’s Upper Peninsula. The new Lagerstätte hosts a moderately diverse fauna of medusae, linguloid brachiopods, non-mineralized arthropods and orthocone nautiloids, alongside dasycladalean green algae. The biota is similar to those of Lagerstätten from the Late Ordovician of Canada, revealing an extensive distribution of a distinctive marginal marine palaeocommunity in Laurentia at this time. The Big Hill biota extends the geographical range of exceptionally preserved Late Ordovician faunas in Laurentia and indicates that further examples remain to be discovered.
... Although largely unnoticed due to their cryptic life habits and economic insignificance, sea spiders are common benthic animals occurring from the littoral zone to the deep sea, from tropical to polar waters. The fossil record dates back to the early Paleozoic Era, with the earliest unequivocal records dating back to the Ordovician and Silur [28,29]. It has even been hypothesized that Pycnogonida might date back to the Cambrian [30]. ...
The research field of connectomics arose just recently with the development of new three-dimensional-electron microscopy (EM) techniques and increasing computing power. So far, only a few model species (for example, mouse, the nematode Caenorhabditis elegans, and the fruit fly Drosophila melanogaster) have been studied using this approach. Here, we present a first attempt to expand this circle to include pycnogonids, which hold a key position for the understanding of arthropod evolution. The visual neuropils in Achelia langi are studied using a focused ion beam-scanning electron microscope (FIB-SEM) crossbeam-workstation, and a three-dimensional serial reconstruction of the connectome is presented.
The two eyes of each hemisphere of the sea spider’s eye tubercle are connected to a first and a second visual neuropil. The first visual neuropil is subdivided in two hemineuropils, each responsible for one eye and stratified into three layers. Six different neuron types postsynaptic to the retinula (R-cells) axons are characterized by their morphology: five types of descending unipolar neurons and one type of ascending neurons. These cell types are also identified by Golgi impregnations. Mapping of all identifiable chemical synapses indicates that the descending unipolar neurons are postsynaptic to the R-cells and, hence, are second-order neurons. The ascending neurons are predominantly presynaptic and sometimes postsynaptic to the R-cells and may play a feedback role.
Comparing these results with the compound eye visual system of crustaceans and insects – the only arthropod visual system studied so far in such detail – we found striking similarities in the morphology and synaptic organization of the different neuron types. Hence, the visual system of pycnogonids shows features of both chelicerate median and mandibulate lateral eyes.
The three‐dimensionally preserved Haliestes dasos from the Silurian (Wenlock) Lagerstätte is the most complete fossil sea spider and the oldest unambiguous pycnogonid known from the fossil record. The discovery of two new specimens to add to the holotype reveals new features including proximal annulations of the appendages and segmentation of the trunk end, critical details for comparison with pycnogonids from the Devonian (Emsian) Hunsrück Slate and for the interpretation of the evolutionary significance of Palaeozoic genera. There is some evidence of sexual dimorphism. Haliestes dasos was nektobenthic and its morphology indicates an unusual mode of feeding compared with living pycnogonids. The new morphological features of H. dasos are closely similar to those in Palaeoisopus problematicus from the Hunsrück Slate and it clearly belongs, together with that species, in stem Pycnogonida and not the crown group.
Three species of sea spider (Arthropoda, Pycnogonida) have been described from the Middle Jurassic (Callovian) Konservat‐Lagerstätte of La Voulte‐sur‐Rhône: Palaeopycnogonides gracilis , Colossopantopodus boissinensis and Palaeoendeis elmii . These fossils were initially attributed to three extant families or superfamilies, justifying their use as calibration points in a recent tree‐dating analysis. However, their taxonomic affinities are still debated. Our knowledge of the morphology of these Jurassic sea spiders is limited by prior investigation with only light microscopy and radiographs, such that further morphological details such as cephalic appendages (chelifores, palps, ovigers) remain poorly documented. Here, we reinvestigate the La Voulte‐sur‐Rhône fossils using x‐ray microtomography and reflectance transformation imaging. A new specimen, tentatively attributed to P . gracilis , was found using x‐ray microtomography, while another fossil initially interpreted as Palaeoendeis elmii may also be related to this species. We attribute all fossils to Pantopoda, crown‐group Pycnogonida. Palaeopycnogonides gracilis had ovigers in at least one sex and had chelifores and palps that were either reduced or absent. Together, the cephalic appendage set and the structure of the ovigers are unique among Pantopoda, and this species is reassigned to the new family Palaeopycnogonididae. Colossopantopodus boissinensis lacked chelifores but had palps and ovigers, the latter with the typical structure shown by extant Colossendeinae, to which we attribute the fossil. The absence of chelifores and palps in Palaeoendeis elmii and the structure of its ovigers indicate affinities with Endeidae. The impact of these new taxonomic assignments on the way Jurassic sea spiders can be used as fossil calibrations is discussed.
A relatively uncommon arthropod of the Waukesha lagerstätte, Parioscorpio venator, is redescribed as an arthropod bearing a combination of characters that defy ready classification. Diagnostic features include sub‐chelate ‘great appendages’, a lack of antennae, multiramous anterior trunk appendages, filamentous fan‐like rear trunk appendages, and apparently thin and poorly preserved pleural fields. Phylogenetic analysis resolves this organism as basal to crown‐group Mandibulata and Chelicerata, but its exact placement is inconclusive. Thus, we compare its morphology to several stem groups of arthropods in a discussion of its plausible taxonomic affinities. The examined specimens are probably carcasses and preserve a variety of soft‐tissue details, including muscle blocks in the head, eyes and eye facets, likely ventral nerve cords, a central gut tract and trunk legs with multiple filamentous elements organized into stiff bundles. The preservation habits of P. venator are characterized and compared to previous assessments of Waukesha lagerstätte taxa. Four preservation habits are observed: a phosphatized habit showing flattened to partly three‐dimensional mineralization in francolite; a mouldic habit largely left behind by removed francolite that shows no carbon enrichment despite a darkened colour; sheet‐like or speckled carbonaceous compressions; and scattered pyrite crystals. This redescription highlights both the palaeobiological value of ‘small’ lagerstätten typical of the middle Palaeozoic and the caution that must be taken when interpreting their more enigmatic constituents.
Reconstructing patterns of macroevolution has become a central endeavor in paleobiology, because it offers insight into evolutionary models shaping the history of life. As the most diverse and abundant animals since the Cambrian period, arthropods provide copious data to elucidate the emergence of body plans in metazoan lineages. However, information provided by fossils on the tempo and mode of this phenomenon has lacked a recent synthesis. Here, I investigate macroevolutionary patterns of morphological evolution in Euarthropoda using a combined extinct and extant dataset optimized for multivariate analyses. Overall ordination patterns between the main morphogroups are consistent with another, independently coded, extant-only dataset providing molecular and morphological rates of evolution. Based on a “deep split” phylogenetic framework, total-group Mandibulata and Arachnomorpha emerge as directional morphoanatomical lineages, with basal fossil morphogroups showing heterogeneously spread-out occupations of the morphospace. In addition to a more homogeneous morphological variation, new morphogroups arose by successive reductions of translation distances; this pattern was interrupted only by terrestrialization events and the origin of pancrustaceans. A displaced optimum type of model is proposed to explain the fast assembly of canalized body plans during the Cambrian, with basal fossil morphogroups fitting intermediate fitness peaks in a moving adaptive landscape. Given time constraints imposed by the paleontological evidence, and owing to the interplay between canalization and modularity, as well as a decoupling between molecular and morphological rates, the rise of euarthropods would support the view that the swiftness of the Cambrian explosion was mostly associated with the buildup of genetic regulatory networks.
An updated classification of the two orders of the phylum is provided up to family level, and numbers of genera and species described so far are specified. The phylum is composed of two orders: Macrodasyida, with, 9 families, 33 genera (+1 genus incertae sedis) and 338 species (+1 species incertae sedis), and Chaetonotida, with 8 families, 30 genera and 454 species. Current taxonomy is relatively stable for the order Macrodasyida, except for the presence of a monotypic genus which cannot yet be assigned with certainty to any of the existing families. On the contrary, the taxonomy of the order Chaetonotida has been repeatedly revised in the last decades and is still unstable. An integrate taxonomical approach on morphological and molecular bases appears necessary in order to revise the current classification according to phylogenetic relationships.
An updated classification of the two orders of the phylum is provided up to family level, and numbers of genera and species described so far are specified. The phylum is composed of two orders: Macrodasyida, with, 9 families, 33 genera (+1 genus incertae sedis ) and 338 species (+1 species incertae sedis), and Chaetonotida, with 8 families, 30 genera and 454 species. Current taxonomy is relatively stable for the order Macrodasyida, except for the presence of a monotypic genus which cannot yet be assigned with certainty to any of the existing families. On the contrary, the taxonomy of the order Chaetonotida has been repeatedly revised in the last decades and is still unstable. An integrate taxonomical approach on morphological and molecular bases appears necessary in order to revise the current classification according to phylogenetic relationships.
Arachnids are an important group of arthropods. They are: diverse and abundant; a major constituent of many terrestrial ecosystems; and possess a deep and extensive fossil record. In recent years a number of exceptionally preserved arachnid fossils have been investigated using tomography and associated techniques, providing valuable insights into their morphology. Here we use X-ray microtomography to reconstruct members of two extinct arachnid orders. In the Haptopoda, we demonstrate the presence of 'clasp-knife' chelicerae, and our novel redescription of a member of the Phalangiotarbida highlights leg details, but fails to resolve chelicerae in the group due to their small size. As a result of these reconstructions, tomographic studies of three-dimensionally preserved fossils now exist for three of the four extinct orders, and for fossil representatives of several extant ones. Such studies constitute a valuable source of high fidelity data for constructing phylogenies. To illustrate this, here we present a cladistic analysis of the chelicerates to accompany these reconstructions. This is based on a previously published matrix, expanded to include fossil taxa and relevant characters, and allows us to: cladistically place the extinct arachnid orders; explicitly test some earlier hypotheses from the literature; and demonstrate that the addition of fossils to phylogenetic analyses can have broad implications. Phylogenies based on chelicerate morphology-in contrast to molecular studies-have achieved elements of consensus in recent years. Our work suggests that these results are not robust to the addition of novel characters or fossil taxa. Hypotheses surrounding chelicerate phylogeny remain in a state of flux.
The Arthropoda is here estimated to have 1,302,809 described species, including 45,769 fossil species (the diversity of fossil taxa is here underestimated for many taxa of the Arthropoda). The Insecta (1,070,781 species) is the most successful group, and it alone accounts for over 80% of all arthropods. The most successful insect order, Coleoptera (392,415 species), represents over one-third of all species in 39 insect orders. Another major group in Arthropoda is the class Arachnida (114,275 species), which is dominated by the Acari (55,214 mite and tick species) and Araneae (44,863 spider species). Other diverse arthropod groups include Crustacea (73,141 species), Trilobitomorpha (20,906 species) and Myriapoda (12,010 species).
Konservat-Lagerstätten, deposits in which soft-bodied or lightly sclerotized fossils are preserved, are very rare in Ordovician strata. Three significant sites are known from Upper Ordovician rocks in Manitoba: at Cat Head - McBeth Point, William Lake, and Airport Cove. These sites are in two distinct sedimentary basins: the former two are in the Williston Basin, while the latter is in the Hudson Bay Basin. All three sites contain marine fossils, but each has a different assemblage that contributes a distinct piece of the diversity picture. Important groups represented at one or more of the sites include seaweeds (algae), sponges, cnidarian medusae (jellyfish), conulariids, trilobites, eurypterids, xiphosurids (horseshoe crabs), and pycnogonids ('sea spiders'). The different biotas reflect depositional conditions at each site. Many of the fossils are unknown elsewhere in the Ordovician at the family level or higher. The province of Manitoba therefore makes a significant contribution to knowledge of Late Ordovician biodiversity.
A fourth species of rock crawler (Notoptera: Mantophasmatodea: Mantophasmatidae) is described and figured from an individual preserved in middle Eocene (Lutetian) Baltic amber. Adicophasma grylloblattoides Arillo and Engel, new species, is distinguished from its close relative, A. spinosum Engel and Grimaldi (reinstated), by the reduced pedicel, absence of spines on the maxillae, absence of mesofemoral spination, and proportions of the thoracic segments. The fossil shares with A. spinosum the presence of profemoral spination (confirmed by a new photograph of the holotype) and absence of the dorsal profemoral carina, characters that differentiate Adicophasma from the monotypic Raptophasma; it shares with all Baltic amber Notoptera the absence of the setal fan on the arolium. As noted by previous authors, the former order Mantophasmatodea is related to modern Grylloblattodea, whereas Mesozoic and Paleozoic grylloblattodeans represent a stem group to both. As such, Grylloblattodea and Mantophasmatodea are considered suborders of a single order, Notoptera Crampton (sensu novum), following the recommendation of Engel and Grimaldi (2004). The names for three rock crawlers are emended in order that the specific epithet may match the gender of the generic name: A. spinosum, Mantophasma zephyrum Zompro et al., and Tanzaniophasma subsolanum (Zompro et al.) (nomina emendata). Raptophasmatinae and Ensiferophasmatidae are new synonyms of Mantophasmatidae, while Tanzaniophasmatidae and Austrophasmatidae are newly demoted in rank to a subfamily and tribe of Mantophasmatidae, respectively. A hierarchical classification of Polyneoptera is appended.
The Arachnomorpha or Arachnata concepts have resolved Trilobita as most closely related to Chelicerata amongst extant Arthropoda. An alternative position of trilobites in the stem lineage of Mandibulata is suggested by their pattern of head tagmosis. The antennae of trilobites and Mandibulata are considered non-homologous with the antennae of Onycho-phora and stem lineage Euarthropoda: they represent 'secondary' and 'primary antennae', respectively. In extant taxa, 'secondary antennae' are deutocerebral, post-ocular, and are connected to deutocerebral olfactory neuropils, whereas 'primary antennae' are pre-ocular and connected to protocerebral olfactory neuropils. In fossils, an insertion at the antero-lateral margin of the hypostome rather than more anteriorly on the head allows 'secondary antennae' to be identified. A deutocerebral mouthpart, of which the onychophoran jaw and the chelicera are examples, is regarded as plesiomorphic for Arthropoda. A loss of 'primary antennae' and modification of the deutocerebral mouthpart into a sensory antenna defines the Mandibulata. Trilobites share a 'secondary antenna' and a clearly-delimited head tagma with mandibulates. Given the extensive homoplasy forced by the Arachnata concept (reversals in pycnogonids and arachnids), a trilobite/mandibulate alliance may be better supported. Dedicated to Fred Schram on the occasion of his retirement. Our article challenges canonical views about arthropods, and we were forced to question ideas that we have long considered the best explanation of facts. In doing so, we venture into a territory from which Fred Schram has never shied away. Fred's synthesis of data from living and fossil arthropods, his efforts to integrate classical morphological and evo-devo perspectives, and his willingness to explore dangerous ideas have inspired our reappraisal of the trilobite problem.
Extant arthropods are diverse and ubiquitous, forming a major constituent of most modern ecosystems. Evidence from early Palaeozoic Konservat Lagerstätten indicates that this has been the case since the Cambrian. Despite this, the details of arthropod origins remain obscure, although most hypotheses regard the first arthropods as benthic predators or scavengers such as the fuxianhuiids or megacheirans ('great-appendage' arthropods). Here, we describe a new arthropod from the Tulip Beds locality of the Burgess Shale Formation (Cambrian, series 3, stage 5) that possesses a weakly sclerotized thorax with filamentous appendages, encased in a bivalved carapace, and a strongly sclerotized, elongate abdomen and telson. A cladistic analysis resolved this taxon as the basal-most member of a paraphyletic grade of nekto-benthic forms with bivalved carapaces. This grade occurs at the base of Arthropoda (panarthropods with arthropodized trunk limbs) and suggests that arthrodization (sclerotization and jointing of the exoskeleton) evolved to facilitate swimming. Predatory and fully benthic habits evolved later in the euarthropod stem-lineage and are plesiomorphically retained in pycnogonids (sea spiders) and euchelicerates (horseshoe crabs and arachnids).
The basic arrangement of limbs in euarthropods consists of a uniramous head appendage followed by a series of biramous appendages. The body is divided into functional units or tagmata which are usually distinguished by further differentiation of the limbs. The living horseshoe crabs are remnants of a much larger diversity of aquatic chelicerates. The limbs of the anterior and posterior divisions of the body of living horseshoe crabs differ in the loss of the outer and inner ramus, respectively, of an ancestral biramous limb. Here we report a new fossil horseshoe crab from the mid-Silurian Lagerstätte in Herefordshire, United Kingdom (approximately 425 Myr B.P.), a site that has yielded a remarkably preserved assemblage of soft-bodied fossils. The limbs of the new form can be homologized with those of living Limulus, but retain an ancestral biramous morphology. Remarkably, however, the two limb branches originate separately, providing fossil evidence to suggest that repression or loss of gene expression might have given rise to the appendage morphology of Limulus. Both branches of the prosomal limbs of this new fossil are robust and segmented in contrast to their morphology in Cambrian arthropods, revealing that a true biramous limb was once present in chelicerates as well as in the mandibulates.
Staples, D. A. 2007. A new species of Colossendeis (Pycnogonida: Colossendeidae) together with records from Australian and New Zealand waters. Memoirs of Museum Victoria 64: 79–94. Six species of the genus Colossendeis Jarzynsky, 1870 and one species of Hedgpethia Turpaeva, 1973 are reported from Australian and New Zealand waters; namely Colossendeis arcuata Milne-Edwards, 1885, C. colossea Wilson, 1881, C. tasmanica sp. nov, C. melancholicus Stock, 1975, C. spicula Child, 1994, C. mycterismos Bamber, 2004 and Hedgpethia dampieri (Child, 1975). Colossendeis melancholicus, C. spicula and C. mycterismos are recorded from the region for the fi rst time. Diagnoses for each species are provided. Type specimens of Hedgpethia dampieri have been re-examined.
Higher‐level phylogenetics of Pycnogonida has been discussed for many decades but scarcely studied from a cladistic perspective. Traditional taxonomic classifications are yet to be tested and affinities among families and genera are not well understood. Pycnogonida includes more than 1300 species described, but no systematic revisions at any level are available. Previous attempts to propose a phylogeny of the sea spiders were limited in characters and taxon sampling, therefore not allowing a robust test of relationships among lineages. Herein, we present the first comprehensive phylogenetic analysis of the Pycnogonida based on a total evidence approach and Direct Optimization. Sixty‐three pycnogonid species representing all families including fossil taxa were included. For most of the extant taxa more than 6 kb of nuclear and mitochondrial DNA and 78 morphological characters were scored. The most parsimonious hypotheses obtained in equally weighted total evidence analyses show the two most diverse families Ammotheidae and Callipallenidae to be non‐monophyletic. Austrodecidae + Colossendeidae + Pycnogonidae are in the basal most clade, these are morphologically diverse groups of species mostly found in cold waters. The raising of the family Pallenopsidae is supported, while Eurycyde and Ascorhynchus are definitely separated from Ammotheidae. The four fossil taxa are grouped within living Pycnogonida, instead of being an early derived clade. This phylogeny represents a solid framework to work towards the understanding of pycnogonid systematics, providing a data set and a testable hypothesis that indicate those clades that need severe testing, especially some of the deep nodes of the pycnogonid tree and the relationships of ammotheid and callipallenid forms. The inclusion of more rare taxa and additional sources of evidence are necessary for a phylogenetic classification of the Pycnogonida.
© The Willi Hennig Society 2006.
A remarkable new fossil horseshoe crab, Lunataspis aurora gen. et sp. nov., from recently discovered Upper Ordovician (c. 445 Ma) shallow marine Konservat-Lagerstätten deposits in Manitoba (Canada), is characterized by fusion of opisthosomal tergites into two sclerites. A broad mesosoma of six or seven fused segments, followed by a narrow metasoma of three reduced segments, represents an advanced transitional condition in the development of the xiphosurid thoracetron. Lunataspis further possesses a large crescentic prosomal shield bearing lateral compound eyes on weak ophthalmic ridges that flank a low cardiac lobe, and a keeled lanceolate telson. Lunataspis is much older than the proposed ‘synziphosurine’ stem lineage of Carboniferous and post-Palaeozoic Xiphosurida, yet is strikingly similar to crown group limuline horseshoe crabs, indicating that major features of the distinctive and highly conserved xiphosurid Bauplan evolved considerably earlier in the Palaeozoic than was previously suspected. In addition to establishing a new temporal benchmark for assessing hypotheses of early chelicerate relationships, the discovery of horseshoe crabs in a Late Ordovician marginal marine setting marks the earliest definitive record of this persistent ecological association.
While a unique origin of the euarthropods is well established, relationships between the four euarthropod classes-chelicerates, myriapods, crustaceans and hexapods-are less clear. Unsolved questions include the position of myriapods, the monophyletic origin of chelicerates, and the validity of the close relationship of euarthropods to tardigrades and onychophorans. Morphology predicts that myriapods, insects and crustaceans form a monophyletic group, the Mandibulata, which has been contradicted by many molecular studies that support an alternative Myriochelata hypothesis (Myriapoda plus Chelicerata). Because of the conflicting insights from published molecular datasets, evidence from nuclear-coding genes needs corroboration from independent data to define the relationships among major nodes in the euarthropod tree. Here, we address this issue by analysing two independent molecular datasets: a phylogenomic dataset of 198 protein-coding genes including new sequences for myriapods, and novel microRNA complements sampled from all major arthropod lineages. Our phylogenomic analyses strongly support Mandibulata, and show that Myriochelata is a tree-reconstruction artefact caused by saturation and long-branch attraction. The analysis of the microRNA dataset corroborates the Mandibulata, showing that the microRNAs miR-965 and miR-282 are present and expressed in all mandibulate species sampled, but not in the chelicerates. Mandibulata is further supported by the phylogenetic analysis of a comprehensive morphological dataset covering living and fossil arthropods, and including recently proposed, putative apomorphies of Myriochelata. Our phylogenomic analyses also provide strong support for the inclusion of pycnogonids in a monophyletic Chelicerata, a paraphyletic Cycloneuralia, and a common origin of Arthropoda (tardigrades, onychophorans and arthropods), suggesting that previous phylogenies grouping tardigrades and nematodes may also have been subject to tree-reconstruction artefacts.
Divergence times inferred for major lineages of Chelicerata (scorpions, spiders, mites, pycnogonids and xiphosurans) in a recent paper on mitochondrial phylogeny by Jeyaprakash and Hoy are compared to the known stratigraphical occurrences of these groups. Erroneous statements concerning fossil date estimates in the original study are corrected. We emphasize that the fossil record of chelicerates is more complete than is sometimes assumed, and that paleontology plays a key role in dating cladogenesis by setting minimum divergence times, which can and do falsify molecular clock estimates where the inferred divergence is substantially younger than the known fossil record. The oldest representatives of each chelicerate order are documented here, together with similar data for the major mite lineages down to family level. Through these, we hope to provide a robust framework and reference points for future molecular systematic studies of this nature.
The interrelationships of major clades within the Arthropoda remain one of the most contentious issues in systematics, which has traditionally been the domain of morphologists. A growing body of DNA sequences and other types of molecular data has revitalized study of arthropod phylogeny and has inspired new considerations of character evolution. Novel hypotheses such as a crustacean-hexapod affinity were based on analyses of single or few genes and limited taxon sampling, but have received recent support from mitochondrial gene order, and eye and brain ultrastructure and neurogenesis. Here we assess relationships within Arthropoda based on a synthesis of all well sampled molecular loci together with a comprehensive data set of morphological, developmental, ultrastructural and gene-order characters. The molecular data include sequences of three nuclear ribosomal genes, three nuclear protein-coding genes, and two mitochondrial genes (one protein coding, one ribosomal). We devised new optimization procedures and constructed a parallel computer cluster with 256 central processing units to analyse molecular data on a scale not previously possible. The optimal 'total evidence' cladogram supports the crustacean-hexapod clade, recognizes pycnogonids as sister to other euarthropods, and indicates monophyly of Myriapoda and Mandibulata.
Independent specialization of arthropod body segments has led to more than a century of debate on the homology of morphologically diverse segments, each defined by a lateral appendage and a ganglion of the central nervous system. The plesiomorphic composition of the arthropod head remains enigmatic because variation in segments and corresponding appendages is extreme. Within extant arthropod classes (Chelicerata, Myriapoda, Crustacea and Hexapoda--including the insects), correspondences between the appendage-bearing second (deutocerebral) and third (tritocerebral) cephalic neuromeres have been recently resolved on the basis of immunohistochemistry and Hox gene expression patterns. However, no appendage targets the first ganglion, the protocerebrum, and the corresponding segmental identity of this anterior region remains unclear. Reconstructions of stem-group arthropods indicate that the anteriormost region originally might have borne an ocular apparatus and a frontal appendage innervated by the protocerebrum. However, no study of the central nervous system in extant arthropods has been able to corroborate this idea directly, although recent analyses of cephalic gene expression patterns in insects suggest a segmental status for the protocerebral region. Here we investigate the developmental neuroanatomy of a putative basal arthropod, the pycnogonid sea spider, with immunohistochemical techniques. We show that the first pair of appendages, the chelifores, are innervated at an anterior position on the protocerebrum. This is the first true appendage shown to be innervated by the protocerebrum, and thus pycnogonid chelifores are not positionally homologous to appendages of extant arthropods but might, in fact, be homologous to the 'great appendages' of certain Cambrian stem-group arthropods.
Arthropod head segments offer a paradigm for understanding the diversification of form during evolution, as a variety of morphologically diverse appendages have arisen from them. There has been long-running controversy, however, concerning which head appendages are homologous among arthropods, and from which ancestral arrangement they have been derived. This controversy has recently been rekindled by the proposition that the probable ancestral arrangement, with appendages on the first head segment, has not been lost in all extant arthropods as previously thought, but has been retained in the pycnogonids, or sea spiders. This proposal was based on the neuroanatomical analysis of larvae from the sea spider Anoplodactylus sp., and suggested that the most anterior pair of appendages, the chelifores, are innervated from the first part of the brain, the protocerebrum. Our examination of Hox gene expression in another sea spider, Endeis spinosa, refutes this hypothesis. The anterior boundaries of Hox gene expression domains place the chelifore appendages as clearly belonging to the second head segment, innervated from the second part of the brain, the deutocerebrum. The deutocerebrum must have been secondarily displaced towards the protocerebrum in pycnogonid ancestors. As anterior-most appendages are also deutocerebral in the other two arthropod groups, the Euchelicerata and the Mandibulata, we conclude that the protocerebral appendages have been lost in all extant arthropods.
Understanding the head is one of the great challenges in the fields of comparative anatomy, developmental biology, and palaeontology of arthropods. Numerous conflicting views and interpretations are based on an enormous variety of descriptive and experimental approaches. The interpretation of the head influences views on phylogenetic relationships within the Arthropoda as well as outgroup relationships. Here, we review current hypotheses about head segmentation and the nature of head structures from various perspectives, which we try to combine to gain a deeper understanding of the arthropod head. Though discussion about arthropod heads shows some progress, unquestioned concepts (e.g., a presegmental acron) are still a source of bias. Several interpretations are no longer tenable based on recent results from comparative molecular developmental studies, improved morphological investigations, and new fossils. Current data indicate that the anterior arthropod head comprises three elements: the protocerebral/ocular region, the deutocerebral/antennal/cheliceral segment, and the tritocerebral/pedipalpal/second antennal/intercalary segment. The labrum and the mouth are part of the protocerebral/ocular region. Whether the labrum derives from a former pair of limbs remains an open question, but a majority of data support its broad homology across the Euarthropoda. From the alignment of head segments between onychophorans and euarthropods, we develop the concept of "primary" and "secondary antennae" in Recent and fossil arthropods, posit that "primary antennae" are retained in some fossil euarthropods below the crown group level, and propose that Trilobita are stem lineage representatives of the Mandibulata.
The pycnogonids (or sea spiders) are an enigmatic group of arthropods, classified in recent phylogenies as a sister-group of either euchelicerates (horseshoe crabs and arachnids), or all other extant arthropods. Because of their bizarre morpho-anatomy, homologies with other arthropod taxa have been difficult to assess. We review the main morphology-based hypotheses of correspondence between anterior segments of pycnogonids, arachnids and mandibulates. In an attempt to provide new relevant data to these controversial issues, we performed a PCR survey of Hox genes in two pycnogonid species, Endeis spinosa and Nymphon gracile, from which we could recover nine and six Hox genes, respectively. Phylogenetic analyses allowed to identify their orthology relationships. The Deformed gene from E. spinosa and the abdominal-A gene from N. gracile exhibit unusual sequence divergence in their homeodomains, which, in the latter case, may be correlated with the extreme reduction of the posterior region in pycnogonids. Expression patterns of two Hox genes (labial and Deformed) in the E. spinosa protonymphon larva are discussed. The anterior boundaries of their expression domains favour homology between sea spider chelifores, euchelicerates chelicerae and mandibulate (first) antennae, in contradistinction with previously proposed alternative schemes such as the protocerebral identity of sea spider chelifores or the absence of a deutocerebrum in chelicerates. In addition, while anatomical and embryological evidences suggest the possibility that the ovigers of sea spiders could be a duplicated pair of pedipalps, the Hox data support them as modified anterior walking legs, consistent with the classical views.
The diverse and exceptionally well-preserved pycnogonids described herein from the Middle Jurassic La Voulte Lagerstätte fill a 400 Myr gap of knowledge in the evolutionary history of this enigmatic group of marine arthropods. They reveal very close morphological and functional (locomotion, feeding) similarities with present-day pycnogonids and, by contrast, marked differences with all Palaeozoic representatives of the group. This suggests a relatively recent, possibly Mesozoic origin for at least three major extant lineages of pycnogonids (Ammotheidae, Colossendeidae, Endeidae). Combined evidence from depositional environment, faunal associates and recent analogues indicate that the La Voulte pycnogonids probably lived in the upper bathyal zone (ca 200 m). Our results point to a remarkable morphological and ecological stability of this arthropod group over at least 160 Myr and suggest that the colonization of the deep sea by pycnogonids occurred before the Jurassic.
Long-held ideas regarding the evolutionary relationships among animals have recently been upended by sometimes controversial hypotheses based largely on insights from molecular data. These new hypotheses include a clade of moulting animals (Ecdysozoa) and the close relationship of the lophophorates to molluscs and annelids (Lophotrochozoa). Many relationships remain disputed, including those that are required to polarize key features of character evolution, and support for deep nodes is often low. Phylogenomic approaches, which use data from many genes, have shown promise for resolving deep animal relationships, but are hindered by a lack of data from many important groups. Here we report a total of 39.9 Mb of expressed sequence tags from 29 animals belonging to 21 phyla, including 11 phyla previously lacking genomic or expressed-sequence-tag data. Analysed in combination with existing sequences, our data reinforce several previously identified clades that split deeply in the animal tree (including Protostomia, Ecdysozoa and Lophotrochozoa), unambiguously resolve multiple long-standing issues for which there was strong conflicting support in earlier studies with less data (such as velvet worms rather than tardigrades as the sister group of arthropods), and provide molecular support for the monophyly of molluscs, a group long recognized by morphologists. In addition, we find strong support for several new hypotheses. These include a clade that unites annelids (including sipunculans and echiurans) with nemerteans, phoronids and brachiopods, molluscs as sister to that assemblage, and the placement of ctenophores as the earliest diverging extant multicellular animals. A single origin of spiral cleavage (with subsequent losses) is inferred from well-supported nodes. Many relationships between a stable subset of taxa find strong support, and a diminishing number of lineages remain recalcitrant to placement on the tree.
Pycnogonids ("sea spiders') are exclusively marine invertebrates numbering approx 1000 species in 84 genera. This cosmopolitan group is largely epibenthic, but a few are interstitial, and some bathypelagic. There are a number of commensal and parasitic species associated with coelenterate, poriferan, molluscan and echinoderm hosts. Littoral pycnogonids tend to be small; deeper water and polar species tend towards gigantism. This review considers: morphology, anatomy and classification; physiology and function; life cycle; interrelationships with other organisms; zoogeography; and palaeontologzy and systematic affinities.-P.J.Jarvis
The Pycnogonida or “sea spiders” are exclusively marine invertebrates numbering, to-date, about 1000 species in 84 genera. In the past they were referred to as the Podosomata or Pantopoda, a name persisting in some European literature, and now surviving as the extant order of the Class (or Subphylum) Pycnogonida. As they rarely occur in prolific density, they are often cryptic and have little economic significance, they have long been regarded as a “minor” group among the marine fauna, receiving little more than superficial coverage in general zoology texts. While the majority of species are epibenthic, a few are interstitial, some are bathypelagic, and recently increasing numbers of commensal and parasitic species have been described, associated with coelenterate, poriferan, molluscan, and echinoderm hosts. This chapter attempts to collate and summarize the current knowledge on all known aspects of pycnogonid biology, drawing from both the latest published and unpublished researches in this field, and is aimed at both specialists and other marine biologists.
The internal and external relationships within the Class Pycnogonida are rationalized based on the entire history of whole-animal understanding, detailed morphometrics, the fossil record, larval structure, mathematical multivariate analyses, and molecular analyses (electrophoretic protein and DNA). The pre-Jurassic fossils are placed in separate orders, with the Lower Silurian Haitestes giving the clearest indications of the form of a hypothetical "protopycnogonid". Living forms, together with some Lower Devonian and the Jurassic fossils, are retained in the Order Pantopoda. No relation-ships to other Classes of the Arthropoda are yet indicated, and the concept of the Pycnogonida as a sister group to the Euarthropoda is maintained. The Pantopoda are divided into two suborders, isolating the Austrodecidae. The remaining taxa are classified into six superfamilies, on a consensus of overall morphological trends, larval forms, and the findings of the only previous comprehensive morphological multivariate analysis, and recent molecular analyses. The Colossendeidae, Pycnogonidae and Rhynchothoracidae are isolated within their own superfamilies. The Ammotheidae sensu lato is subdivided (within one superfamily), unfortunately leaving a number of genera incertae sedis. The Nymphonidae, Callipallenidae and Pallenopsidae are associated within another superfamily. The Jurassic fossils are placed within the Endeidae within a super-family together with the Phoxichilidiidae, while some Lower Devonian fossils remain incertae sedis. Diagnoses are given as appropriate where possible.
Pycnogonids or sea spiders are a group of marine arthropods whose relations to the chelicerates have been an issue of controversy. Higher-level phylogenetic relationships among the lineages of sea spiders are investigated using 36 morphological characters from 37 species from all extant families and a Devonian pycnogonid fossil. This is one of the first attempts to analyze the higher-level relationships of the Pycnogonida using cladistic techniques. Character homoplasy (implied weights) is taken into account to construct a polytomous, most-parsimonious tree in which two major clades within Pycnogonida are obtained. Clade A includes Ammotheidae paraphyletic with Colossendeidae, Austrodecidae and Rhynchothoracidae, and clade B is formed by Nymphonidae, Callipallenidae (apparently paraphyletic), Pycnogonidae and Phoxichilidiidae. The analysis of equally weighted data is presented and helps to identify those characters less consistent. The reduction of the chelifores, palps and ovigers — shown independently within each of the clades as parallel evolution events — challenges the assumption of a gradual mode of reduction within the group, according to analysis of unordered vs ordered characters. Most of the phylogenetic affinities proposed here are compatible with traditional classifications. However, traditional taxonomic characters need to be complemented by sets of anatomical, molecular and developmental data, among others, to produce more robust phylogenetic hypotheses on the higher- and lower-level relationships of the sea spiders.
Kurzfassung
Palaeoisopus problematicus und Palaeopantopus maucheri werden neu beschrieben und als urtümliche Pycnogoniden aufgefßt, die ausgestorbenen Ordnungen angehoren.Palaeoisopus ist besonders ursprünglich hinsichtlich des langen Abdomens mit einem Telson. Er unterscheidet sich von anderen Formen durch
abgeflachte Extremitäten, die auf eine schwimmende Lebensweise hindeuten, und durch anders angeordnete Ocellen.Palaeothea devonica nov. gen. et nov. spec. ist ein winziger Pycnogonide, der einzige bisher bekannte fossile Vertreter der rezenten Ordnung
Pantopoda, der darauf hinweist, daß im frühen Devon durchaus schon moderne Formen vorhanden waren. Eine Abstammung der Pycnogonida
von frühen Merostomen wird als wahrscheinlich betrachtet.
Thirty-three species of tropical shallow-water pycnogonids are reported from the Queensland coast, and coral reef habitats of the Great Barrier Reef and the Coral Sea. New species in the genera Austrodecus, Anoplodactylus and Pycnogonum are described, and 14 species not previously recorded for Australia are reported. All the families except Rhynchothoracidae are represented in this collection and rare species such as Rhopalorhynchus tenuissimum and Nymphopsis acinacispinata among others are reported for tropical Queensland for the first time. Anoplodactylus appears as highly diverse with eight species found in the area. The intertidal green alga Cladophora prolifera supports a rich pycnogonid fauna shown by a total of 14 species found in the alga at two different sites. On coral reefs, microhabitats such as coral rubble, macroalgae and zoanthids seem to shelter conspicuous pycnogonid populations. Pycnogonid fauna from tropical Queensland is related to the Indo-west Pacific zoogeographical region also including pantropical species. This study highlights the diversity of pycnogonids in shallow-water habitats and contributes to the knowledge of tropical faunae. It includes illustrations and descriptions for relevant records and emphasizes the strong need of taxonomic and phylogenetic revision of some of the major taxa.
There are few body fossil biotas known from early Paleozoic accretionary shorelines, and very few examples of Ordovician soft-bodied assemblages. This study documents two recently discovered biotas from separate sedimentary basins in Manitoba, Canada, that provide unique information about tropical shoreline communities shortly before the Late Ordovician extinction event. Each site represents a distinct depositional environment, but they share biotic elements, including eurypterids, xiphosurids, and large problematic tubes. The William Lake biota, representing more restricted conditions, includes jellyfish that are among the best hydromedusan body fossils known. Rocks at the Airport Cove site, deposited under more open circulation, contain scolecodonts and noncalcified algae. These biotas have some parallels with the recently described Middle Ordovician Winneshiek Lagerstätte, but are also similar to some Late Silurian assemblages. Considered together, early Paleozoic marginal marine deposits are a rich but as yet poorly known source of paleobiodiversity data.
Post embryonic development of a pycnogonid, Propallene longiceps, is described from observations of the development of an individual egg to a sexually mature adult. A fertilized egg hatches after 6 to 7 days as a second instar larva, because it completes the first moulting at the time of hatching. No protonymphon larval stage was observed. The larva of P. longiceps undergoes 3 moults during its attaching larval stage and 6 moults during its free-swimming larval stage. With the 9th moult, the larva becomes an adult. It took about 5 months for a fertilized egg to become an adult in P. longiceps. Some aspects of the intraclass relationship of the Family Pallenidae are discussed.
Among a set of small, secondarily phosphatised larval arthropods from the Upper Cambrian ‘Orsten’ of Sweden, described by Müller and Walossek in 1986, one form bears a remarkable resemblance to the hatching protonymph larva of extant Pantopoda. This ‘larva D’ shares with protonymphs their gross body form, the anteroventral mouth on a slightly off-set forehead region, the cheliceral morphology, two homeomorphic pairs of post-cheliceral limbs, and further detailed similarities. It is described herein as Cambropycnogon klausmuelleri gen. et sp. nov. and is proposed as the oldest unequivocal record of both Pycnogonida and Chelicerata. Plesiomorphic features such as a pair of rudimentary pre-cheliceral limbs and the gnathobasic basipods of the two post-cheliceral limbs distinguish it from all known larvae of extant Pantopoda and lead us to propose a phylogeny of the Pycnogonida of the form (Cambropycnogon klausmuelleri+ (Palaeoisopus+ (Palaeopantopus+ Pantopoda))). The fossil may help to resolve the long debate about the relationships of Pycnogonida to other Arthropoda and supports a (Pycnogonida + Euchelicerata) relationship within the Chelicerata. The pre-cheliceral limbs in this fossil support traditional morphological studies in which the chelicera represent the second (a2) head appendage, corresponding to the crustacean ‘second antennae’, and contradict recent data based on homeobox genes implying that the chelicerae are the first (a1) head appendages homologous with crustacean first antennae.
A new Lower Devonian sea spider (Arthropoda: Pycnogonida) from the Hunsrück Slate, Germany, is described as Flagellopantopus blocki gen. et sp. nov. This is only the sixth fossil pycnogonid species to be described. Its most remarkable and unique aspect is the long, flagelliform telson. Although our fossil apparently lacks chelifores (an apomorphy), the retained telson and the segmented trunk end behind the last pair of legs resolve F. blocki to a fairly basal position in the pycnogonid stem lineage. It probably lies between Palaeoisopus problematicus Broili, which has a lanceolate telson and the most trunk segments of any sea spider, and all other Silurian–Recent Pycnogonida. Our new material shows that at least two fossil pycnogonids retained a telson, albeit with very different morphologies, and further supports the idea that a greater diversity of body plans existed among the Palaeozoic pycnogonid taxa.
The present paper is a critical review of data and hypotheses on the head segmental composition in chelicerates and in extinct non-mandibulate arthropods. It successively takes into account data from morphology and embryology, from the structure of the nervous system, from palaeontology and from developmental genetics. Discussion focuses on possible homologies between the head segments and appendages in arachnomorphs and those in mandibulates. The comparative anatomical and ontogenetic data, especially those concerning the central nervous system, its connections with the stomatogastric system, and head innervation, show many similarities between the head organization of chelicerates and that of mandibulates, and lead to conclusions that contradict some of the hypotheses deduced from recent studies on developmental biology, but favour more traditional views. In particular they support the presence of a deutocerebral segment in the head region of the ground pattern of arthropods and its loss in all extant chelicerates. They also support the homology of the cheliceral ganglia with the tritocerebral ganglia of mandibulates. The possible existence of a precheliceral segment and of a presegmental acron remains open to question.
Pycnogonids or sea spiders are a group of marine arthropods whose relations to the chelicerates have been an issue of controversy. Higher-level phylogenetic relationships among the lineages of sea spiders are investigated using 36 morphological characters from 37 species from all extant families and a Devonian pycnogonid fossil. This is one of the first attempts to analyze the higher-level relationships of the Pycnogonida using cladistic techniques. Character homoplasy (implied weights) is taken into account to construct a polytomous, most-parsimonious tree in which two major clades within Pycnogonida are obtained. Clade A includes Ammotheidae paraphyletic with Colossendeidae, Austrodecidae and Rhynchothoracidae, and clade B is formed by Nymphonidae, Callipallenidae (apparently paraphyletic), Pycnogonidae and Phoxichilidiidae. The analysis of equally weighted data is presented and helps to identify those characters less consistent. The reduction of the chelifores, palps and ovigers — shown independently within each of the clades as parallel evolution events — challenges the assumption of a gradual mode of reduction within the group, according to analysis of unordered vs ordered characters. Most of the phylogenetic affinities proposed here are compatible with traditional classifications. However, traditional taxonomic characters need to be complemented by sets of anatomical, molecular and developmental data, among others, to produce more robust phylogenetic hypotheses on the higher- and lower-level relationships of the sea spiders.
Recurrent patterns of disarticulation and dislocations of the eurypterid exoskeleton reflect biological processes and can be used to distinguish moults from carcases. This distinction is a prerequisite to understanding the palaeobiology of the Eurypterida. Taphonomic patterns indicate that eurypterid ecdysis was largely similar to that in Recent chelicerate orders, involving opening of sutures along the carapace margin and on ventral plates prior to emergence anteriorly. The diverse morphology of eurypterids resulted in a range of constrictions or ‘bottlenecks’ in the exoskeleton through which parts of the body passed during ecdysis. The requirement to moult highly spiniferous, dentate or pectinate appendages, broad swimming legs, a broad preabdomen and postabdominal epimera may have limited eurypterid disparity. The fossil record of certain eurypterid clades is confined largely to a few Lagerstätten. Some of the best-known eurypterid occurrences (i.e., in New York and Ontario) represent conditions favourable for ecdysis and the preservation of non-mineralized cuticle rather than settings normally inhabited by the arthropods, although the eurypterid locality at Kokomo, Indiana appears to represent a mass mortality. The reduction in suitable ecdysial refugia close to the end of the Silurian may have contributed to the decline and extinction of some eurypterid genera.
Early authors regarded Pycnogonida (sea spiders) either as aquatic arachnids, ‘degraded’ crustaceans or as some sort of intermediate form between the two. Subsequently, pycnogonids were either placed among the Chelicerata or considered as an isolated group, unrelated to other arthropods. The latter model is untenable under phylogenetic systematics and recent cladistic studies have supported one of two alternative hypotheses. The first is the traditional Chelicerata s.lat. concept, i.e. (Pycnogonida + Euchelicerata). This, however, has only one really convincing synapomorphy: chelate chelicerae. The second hypothesis recognizes (Pycnogonida + all other Euarthropoda) and has been recovered in various ‘total evidence’ studies. Morphologically some characters – the presence of gonopores on the trunk and absence of a labrum, nephridia and intersegmental tendons – support Cormogonida (Euarthropoda excluding pycnogonids). Advances in developmental biology have proposed clear interpretations of segmentation homologies. However, so far there is also a confrontation of the two hypotheses depending on whether the last walking leg segment is considered part of the prosoma. In this case pycnogonids have too many prosomal segments compared with Euchelicerata; perhaps implying they are not sister groups. Alternatively, if part of the postprosomal region, the last leg pair could correspond to the chilarial segment in euchelicerates and its uniramous state could be apomorphic with respect to other euarthropods. Molecular phylogenies need to be more rigorously analysed, better supported by data from different sources and technique-sensitive aspects need to be explored. Chelicerata s.lat. may emerge as the more convincing model, yet even the putative autapomorphy of chelicerae needs to be treated with caution as there are fossil ‘great appendage’ arthropods in the early Palaeozoic which also have a robust, food-gathering, pair of head limbs and which may lie on the chelicerate, or even the euarthropod, stem lineage.
Frühere Autoren betrachteten die Pycnogonida (Asselspinnen) entweder als wasserbewohnende Spinnentiere oder als rückgebildete Krebstiere oder als eine Zwischenform zwischen den beiden Gruppen. Später wurden die Pycnogonida entweder den Chelicerata zugeordnet oder als eine isolierte Gruppe, die mit den anderen Arthropoden in keiner Verwandtschftsbeziehung steht, betrachtet. Die letztere Annahme ist unter den Aspekten der phylogenetischen Systematik unbrauchbar. Neue cladistische Untersuchungen unterstützen zwei verschiedene, alternative Hypothesen: die erste Hypothese entspricht dem traditionellen Chelicerata s. lat.-Konzept, d.h. Pycnogonida + Euchelicerata bilden eine Gruppe. Hier gibt es aber nur eine einzige überzeugende Synapomorphie: klauenartige Cheliceren. Die zweite Hypothese anerkennt eine Gruppierung (Pycnogonida + alle anderen Euarthropoda), entsprechend den Ergebnissen verschiedener ‘‘Total Evidence-Analysen’’. Einige morphologische Merkmale, wie das Auftreten von Gonoporen am Rüssel, das Fehlen des Labrums, der Nephridien und der intersegmentalen Sehnen, unterstützt das Taxon Cormogonida (alle Euarthropoda mit Ausschluß der Pycnogonida). Fortschritte in der Entwicklungsbiologie erlauben jetzt eine klare Homologisierung der Segmente. Es besteht jedoch weiterhin ein Konflikt zwischen den beiden Hypothesen bezüglich der Frage, ob das letzte Laufbeinsegment ein Teil des Prosomas ist. In letzterem Fall hätten die Pycnogonida im Vergleich mit den Euchelicerata zu viele prosomale Segmente; vielleicht kann das als Hinweis angesehen werden, daß die beiden Taxa keine Schwestergruppen sind. Alternativ, wenn das letzte Laufbeinsegment ein Teil der post-prosomatischen Region ist, könnte es dem chilarialen Segment der Euchelicerata in seinem einstrahligen Zustand entsprechen und eine Apomorphie gegenüber den anderen Euarthropoden darstellen. Die molekularen Phylogeniestudien müssen strenger analysiert, besser durch Daten von anderer Seite unterstützt und die Aspekte der Empfindlichkeit der technischen Methoden besser untersucht werden. Das Chelicerata s. lat.- Modell mag als das mehr überzeugende erscheinen, doch müssen die vermutlichen Autoapomorphien der Chelicerata mit Vorsicht behandelt werden, denn es gibt da die fossilen Gliederfüßler aus dem frühen Paläozooikum, die sogenannten ‘‘great appendage’’ - Euarthropoden mit einem großen robusten Paar von Kopfgliedern als Fangapparat, die nahe bei den Chelicerata stehend oder sogar als Stammgruppe der Euarthropoden angesehen werden können.
The phylogenetic position of the enigmatic Pycnogonida (sea spiders) is still controversial. This is in part due to a lack of detailed data about the morphology and ontogenesis of this, in many aspects, aberrant group. In particular, studies on the embryonic development of pycnogonids are rare and in part contradictory. Here, we present the first embryological study of a pycnogonid species using scanning electron microscopy (SEM). We describe the late embryogenesis of Pycnogonum litorale from the first visible appendage anlagen to the hatchling in 11 embryonic stages. The three pairs of appendage anlagen gain in length by growth, as well as by extension of furrows into the embryo. The opening of the stomodaeum is located far in front of the anlagen of the chelifores and has a Y-shaped lumen from the onset. During further embryogenesis, the position of the mouth shifts ventrally, until it is located between the chelifores. The proboscis anlage grows out as a circumoral wall-like structure, which is initially more pronounced ventrally. Hypotheses about the evolution of the proboscis by fusion of originally separated components are critically discussed, because the proboscis anlage of P. litorale shows no indications of a composite nature. In particular, a participation of post-cheliforal elements in proboscis formation is rejected by our data. Further, no preoral structure and no stage in proboscis formation was found, which could plausibly be homologized with the labrum of othereuarthropods. Thus, our study supports the assumption of a complete lack of a labrum in Pycnogonida.
Chelicerata probably appeared during the Cambrian period. Their precise origins remain unclear, but may lie among the so-called great appendage arthropods. By the late Cambrian there is evidence for both Pycnogonida and Euchelicerata. Relationships between the principal euchelicerate lineages are unresolved, but Xiphosura, Eurypterida and Chasmataspidida (the last two extinct), are all known as body fossils from the Ordovician. The fourth group, Arachnida, was found monophyletic in most recent studies. Arachnids are known unequivocally from the Silurian (a putative Ordovician mite remains controversial), and the balance of evidence favours a common, terrestrial ancestor. Recent work recognises four principal arachnid clades: Stethostomata, Haplocnemata, Acaromorpha and Pantetrapulmonata, of which the pantetrapulmonates (spiders and their relatives) are probably the most robust grouping. Stethostomata includes Scorpiones (Silurian-Recent) and Opiliones (Devonian-Recent), while Haplocnemata includes Pseudoscorpiones (Devonian-Recent) and Solifugae (Carboniferous-Recent). Recent works increasingly favour diphyletic mite origins, whereby Acaromorpha comprises Actinotrichida (Devonian-Recent), Anactinotrichida (Cretaceous-Recent) and Ricinulei (Carboniferous-Recent). The positions of the Phalangiotarbida (Devonian-Permian) and Palpigradi (Neogene-Recent) are poorly resolved. Finally, Pantetrapulmonata includes the following groups (listed here in their most widely recovered phylogenetic sequence): Trigonotarbida (Silurian-Permian), Uraraneida (Devonian-Permian), Araneae (Carboniferous-Recent), Haptopoda (Carboniferous), Amblypygi (?Devonian-Recent), Thelyphonida (Carboniferous-Recent) and Schizomida (Paleogene-Recent).
Within the last decade, gene expression patterns and neuro-anatomical data have led to a new consensus concerning the long-debated association between anterior limbs and neuromeres in the arthropod head. According to this new view, the first appendage in all extant euarthropods is innervated by the second neuromere, the deutocerebrum, whereas the anterior-most head region bearing the protocerebrum lacks an appendage. This stands in contrast to the clearly protocerebrally targeted "antennae" of Onychophora and to some evidence for protocerebral limbs in fossil euarthropod representatives. Yet, the latter "frontal appendages" or "primary antennae" have most likely been reduced or lost in the lineage, leading to extant taxa. Surprisingly, a recent neuro-anatomical study on a pycnogonid challenged this evolutionary scenario, reporting a protocerebral innervation of the first appendages, the chelifores. However, this interpretation was soon after questioned by Hox gene expression data. To re-evaluate the unresolved controversy, we analyzed neuro-anatomy and neurogenesis in four pycnogonid species using immunohistochemical techniques. We clearly show the postprotocerebral innervation of the chelifores, which is resolved as the plesiomorphic condition in pycnogonids when evaluated against a recently published comprehensive phylogeny. By providing direct morphological support for the deutocerebral status of the cheliforal ganglia, we reconcile morphological and gene expression data and argue for a corresponding position between the anterior-most appendages in all extant euarthropods. Consequently, other structures have to be scrutinized to illuminate the fate of a presumptive protocerebral appendage in recent euarthropods. The labrum and the "frontal filaments" of some crustaceans are possible candidates for this approach.
Pycnogonids (sea spiders) are marine arthropods numbering some 1,160 extant species. They are globally distributed in depths of up to 6,000 metres, and locally abundant; however, their typically delicate form and non-biomineralized cuticle has resulted in an extremely sparse fossil record that is not accepted universally. There are two opposing views of their phylogenetic position: either within Chelicerata as sister group to the euchelicerates, or as a sister taxon to all other euarthropods. The Silurian Herefordshire Konservat-Lagerstatte in England (approximately 425 million years (Myr) bp) yields exceptionally preserved three-dimensional fossils that provide unrivalled insights into the palaeobiology of a variety of invertebrates. The fossils are preserved as calcitic void in-fills in carbonate concretions within a volcaniclastic horizon, and are reconstructed digitally. Here we describe a new pycnogonid from this deposit, which is the oldest adult sea spider by approximately 35 Myr and the most completely known fossil species. The large chelate first appendage is consistent with a chelicerate affinity for the pycnogonids. Cladistic analyses place the new species near the base of the pycnogonid crown group, implying that the latter had arisen by the Silurian period.
Aspects of pantopod ontogeny have been known for a long time, but specific information is available for only a few species. Our account of the postembryonic development of Pycnogonum litorale is based on laboratory-reared individuals and SEM studies. We documented particularly all early developmental stages, with emphasis on morphogenetic changes of head structures and appendages. In P. litorale the protonymphal limbs, the chelicerae and two more uniramous legs, degenerate already during the larval phase; only the third one, the ovigers, reappears in male juveniles. Other Pantopoda vary in this aspect from retention of all three protonymphal appendages to their complete reduction, as in P. litorale. Accordingly, the two post-cheliceral larval appendages are separate legs in front of the walking legs in the adults, the 'parapalps' and the 'ovigers', but they do not occur in all pantopods. The scarcity of studies of the ontogeny of Pantopoda prevents us from a more conclusive picture, but our data are promising to state that additional such studies will increase the usability of ontogenetic data for a phylogenetic analysis of Pantopoda, the crown group of the Pycnogonida. We also discuss the phylogenetic implications of our data in the light of new information from Hox genes and developmental-biological data on body segmentation and tagmosis of the Chelicerata. These suggest the homology of chelicerae and antenn(ul)ae of other euarthropods. Accepting this, we conclude that the adult pycnogonid/pantopod head, the cephalosoma, corresponds to the euarthropod head and that the protonymph with three appendage-bearing segments may represent an even shorter, possibly phylogenetically older larval type than the euarthropod 'head larva' bearing four pairs of appendages. In further consequence, the fourth walking legs of Pycnogonida/Pantopoda should correspond to the first opisthosomal appendages, the chilaria, of euchelicerates. This implies that within Pycnogonida the post-prosomal region became compacted during evolution to a single leg-bearing segment plus a tubular end piece. Accordingly, neither the anterior nor the posterior functional boundaries of the walking-leg region correspond to the original tagma borders.
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