Utahcaris orion Conway Morris & Robison, 1988, from the lower middle Cambrian (Series 3, Stage 5) Spence Shale Member situated in Utah, USA, is redescribed based on a restudy of the original material. Newly recognized features , including... more
Utahcaris orion Conway Morris & Robison, 1988, from the lower middle Cambrian (Series 3, Stage 5) Spence Shale Member situated in Utah, USA, is redescribed based on a restudy of the original material. Newly recognized features , including ventral lateral eyes, trunk appendages, and a bulbous cephalic shield with subtriangular extension, reinforce similarities with Sanctacaris uncata from the middle Cambrian Burgess Shale. Both are assigned to the new family, Sanctacarididae. Sanctacaridids represent the oldest chelicer-ates. Their ecology and that of their nearest non-chelicerate relatives indicate that Chelicerata were plesiomorphically predatory.
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Exceptional preservation of non-biomineralized arthropods, sponges and vermiform taxa occurs in the Darriwilian (Middle Ordovician) Llanfallteg Formation of South Wales, UK. The assemblage contains elements typical of Ordovician... more
Exceptional preservation of non-biomineralized arthropods, sponges and vermiform taxa occurs in the Darriwilian (Middle Ordovician) Llanfallteg Formation of South Wales, UK. The assemblage contains elements typical of Ordovician communities juxtaposed with those more commonly associated with Cambrian Konservat-Lagerstätten. This assemblage is
preserved in rocks of a deep-marine succession dominated by fine-grained siliciclastic and volcaniclastic density-current
deposits. Non-biomineralized taxa of Cambrian aspect are preserved as pyritized carbonaceous compressions on the spectrum of Burgess Shale-type preservation. Trilobites with phosphatized digestive structures have also been recovered. The assemblage of the Llanfallteg Formation Konservat-Lagerstätte demonstrates that some Burgess Shale-type faunal elements survived into the Middle Ordovician within cool, deep-water refugia in the Welsh Basin, offshore from communities dominated by typical Ordovician taxa.
preserved in rocks of a deep-marine succession dominated by fine-grained siliciclastic and volcaniclastic density-current
deposits. Non-biomineralized taxa of Cambrian aspect are preserved as pyritized carbonaceous compressions on the spectrum of Burgess Shale-type preservation. Trilobites with phosphatized digestive structures have also been recovered. The assemblage of the Llanfallteg Formation Konservat-Lagerstätte demonstrates that some Burgess Shale-type faunal elements survived into the Middle Ordovician within cool, deep-water refugia in the Welsh Basin, offshore from communities dominated by typical Ordovician taxa.
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The Middle Ordovician Llanfalteg Formation has yielded remains of soft-bodied organisms previously known only from Cambrian Burgess Shale-type deposits. Here a new arthropod, Etania howellsorum gen. et sp. nov. is described, characterised... more
The Middle Ordovician Llanfalteg Formation has yielded remains of soft-bodied organisms previously known only from Cambrian Burgess Shale-type deposits. Here a new arthropod, Etania howellsorum gen. et sp. nov. is described, characterised by a semi-circular cephalon, clusters of spinose endites on the endopod, and exopods with ovoid distal lobes. These characters are consistent with xenopod affinities, a clade otherwise known exclusively from the Cambrian. The discovery of E. howellsorum demonstrates that a number of Burgess Shale-type taxa, including xenopods survived past the Cambrian, albeit within a restricted environment, and may have been outcompeted during the Great Ordovician Biodiversification Event (GOBE).
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The morphology of the arthropod Sanctacaris uncata, from the Middle Cambrian Burgess Shale of Canada, is reinterpreted based on a restudy of previously described material. Although originally considered a chelicerate-like arthropod, these... more
The morphology of the arthropod Sanctacaris uncata, from the Middle Cambrian Burgess Shale of Canada, is reinterpreted based on a restudy of previously described material. Although originally considered a chelicerate-like arthropod, these affinities were dismissed based primarily on interpretations of the anterior appendages, and hypotheses which considered the megacheirans (“great-appendage” arthropods as putative ancestors of chelicerates. The similarities between megacheirans and chelicerates appear to be overstated however, and this study instead reaffirms the identity of putative chelicerate feature in Sanctacaris uncata and similar arthropods such as Sidneyia and Emeraldella, both also from the Middle Cambrian Burgess Shale. Newly interpreted features, including the presence of pediform exites, multi-partite trunk exopods, and a trunk differentiated into an anterior limb-bearing area and a differentiated posterior limb-less abdomen, were coded into an extensive phylogenetic data set of fossil and Recent arthropods. In all analyses Sanctacaris resolved as the basal-most member of total-group Euchelicerata (the least inclusive group including horseshoe crabs and arachnids but not pycnogonids), thus making it the oldest chelicerate in the fossil record. The vicissicaudates (including Sidneyia, Emeraldella, aglaspidids, and cheloniellids – all of which have previously been allied to chelicerates) resolved as sister-taxon to crown-group Chelicerata. This topology indicates that many purported chelicerate features, such as lamellar gills, and a differentiated posterior abdomen evolved sequentially in the chelicerate stem-lineage.
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A new arthropod, Enalikter asphon gen. et sp. nov., is described from the Silurian (Wenlock Series) Herefordshire Lagerstatte of the UK. It belongs to the Megacheira (=short-great-appendage group), which is recognized here, from the first... more
A new arthropod, Enalikter asphon gen. et sp. nov., is described from the Silurian (Wenlock Series) Herefordshire Lagerstatte of the UK. It belongs to the Megacheira (=short-great-appendage group), which is recognized here, from the first time, in strata younger than mid-Cambrian age. Discovery of this new Silurian taxon allows us to identify a Devonian megacheiran representative, Bundebachiellus giganteus from the Hunsruck 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 to the most comprehensive morphological dataset available, a stem euarthropod position is supported, 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.
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The morphology of two new bivalved arthropods, Loricicaris spinocaudatus gen. et sp. nov. and Nereocaris briggsi sp. nov. from the Middle Cambrian (Series 3, Stage 5) Burgess Shale Formation (Collins Quarry locality on Mount Stephen, Yoho... more
The morphology of two new bivalved arthropods, Loricicaris spinocaudatus gen. et sp. nov. and Nereocaris briggsi sp. nov. from the Middle Cambrian (Series 3, Stage 5) Burgess Shale Formation (Collins Quarry locality on Mount Stephen, Yoho National Park, British Columbia, Canada), are described. The material was originally assigned to the genus Branchiocaris but exhibits distinctive character combinations meriting its assignment to other taxa. Loricicaris spinocaudatus possesses an elongate and spinose abdomen comparable to the contemporaneous Perspicaris and Canadaspis, as well as chelate second head appendages and sub-triangular exopods, comparable to Branchiocaris. Nereocaris briggsi possesses a laterally compressed carapace, elongate and delicate appendages and a medial eye located between a pair of lateral eyes on a rhomboidal eye stalk. Although undoubtedly congeneric with Nereocaris exilis from a slightly younger horizon of the Burgess Shale Formation, N. briggsi differs in overall proportions and segment number, warranting assignment to a new species. The newly described taxa were coded into an extensive cladistic analysis of 755 characters, and 312 extinct and extant panarthropods, including a variety of Cambrian bivalved arthropods from both the Burgess Shale and the Chengjiang Lagerstätten. Cambrian bivalved arthropods consistently resolved as a paraphyletic assemblage at the base of Arthropoda. Important innovations in arthropod history such as the specialization of the deutocerebral head appendages and a shift from a nekton-benthic deposit feeding habit to a benthic scavenging/predatory habit, the symplesiomorphic feeding condition of Euarthropoda (crown-group arthropods), were found to have occurred among basal bivalved arthropods.
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Cambrian lobopodians have attracted much attention in that they are considered to have close affinities with the origin of arthropods, and recent studies agree that they sample the stem-groups of both Arthropoda and Onychophora. A new... more
Cambrian lobopodians have attracted much attention in that they are considered to have close affinities with the origin of arthropods, and recent studies agree that they sample the stem-groups of both Arthropoda and Onychophora. A new lobopodian species, Diania cactiformis Liu et al., 2011, was recently reported from the Chengjiang Lagerstätte (south-west China; Cambrian Series 2). This animal was interpreted as bearing arthropod-like articulated appendages and was resolved as the sister taxon to Arthropoda in a cladistic analysis. A reanalysis of the published dataset did not reproduce the phylogenetic placement of D. cactiformis, and the interpretation of its appendage morphology is controversial. New observations on D. cactiformis are made using newly collected material, rejecting the evidence for sclerotized, segmented and articulated appendages; instead the appendages compare more closely to lobopods. The criteria for defining arthropodized appendages are further discussed through comparative studies among fossil lobopodians, anomalocaridids and arthropods. Our study challenges the previous description of anteroposterior orientation, terminal claws and trunk tagmosis as expressed by differentiated anterior and posterior appendages. A synthesis of four recent datasets for lobopodians and stem-group arthropods is used to revisit the phylogeny of stem arthropods. Irrespective of how characters are weighted, D. cactiformis is resolved at a much more basal position to arthropods, indicating that the morphology of this animal's appendages does not fundamentally inform on the evolution of arthropodization.
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"Phylogenomics reconstructs an arthropod tree in which a monophyletic Arthropoda splits into Pycnogonida + Euchelicerata and Myriapoda + Pancrustacea. The same chelicerate–mandibulate groups are retrieved with morphological data sets,... more
"Phylogenomics reconstructs an arthropod tree in which a monophyletic Arthropoda splits into Pycnogonida + Euchelicerata and Myriapoda + Pancrustacea. The same chelicerate–mandibulate groups are retrieved with morphological data sets, including those encompassing most taxa known from Palaeozoic Konservat-Lagerst€atten. With respect to the interrelationships of the three extant clades of Panarthropoda, a sister group relationship between Onychophora and Arthropoda is endorsed by transcriptomics and microRNAs, although this hypothesis forces homoplasy in characters of the segmental ganglia that are shared by tardigrades and arthropods. Cambrian lobopodians, dinocaridids, bivalved arthropods and fuxianhuiids document the successive appearance of characteristic arthropod features in the stem lineage of Euarthropoda (crown-group arthropods). Molecular dating suggests that arthropods had their origin and
initial diversification in the Ediacaran, but no convincing palaeontological evidence for Panarthropoda is available until
the earliest Cambrian."
initial diversification in the Ediacaran, but no convincing palaeontological evidence for Panarthropoda is available until
the earliest Cambrian."
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The taxon Arthropoda has been repeatedly cited with various incorrect authorship data. Here, we review the primary literature and show the correct citation to be 'Arthropoda Von Siebold, 1848'.
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The interrelationships of major arthropod clades have long been contentious, but refinements in molecular phylogenetics underpin an emerging consensus. Striking topologies from these studies include the position of hexapods (insects)... more
The interrelationships of major arthropod clades have long been contentious, but refinements in molecular phylogenetics underpin an emerging consensus. Striking topologies from these studies include the position of hexapods (insects) within a paraphyletic Crustacea, and a putative myriapod/chelicerate alliance. Although some morphological characters support these groupings, they have not been recovered in large-scale phylogenetic analyses based on morphology. An analysis herein of 753 morphological characters for 309 fossil and Recent panarthropods shows enhanced congruence with molecular phylogenies. We resolve hexapods within Crustacea, with remipedes their closest extant relatives. Selective taxon and character removal exposes the traditional myriapod/hexapod linkage as an artefact of convergent character acquisition during terrestrialisation. Fossils break up such long branches, their intermediate morphologies linking clades that have otherwise had their ancestral conditions overprinted. Pycnogonids are an exemplar, grouping with euchelicerates when fossils are included, and as sister-taxon to all other extant arthropods when fossils are removed.
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Isoxys is a cosmopolitan genus of bivalved arthropod known almost exclusively from Cambrian Konservat Lagerstätten. Despite its wide geographical distribution in such sites of exceptional preservation, little was known of its soft-part... more
Isoxys is a cosmopolitan genus of bivalved arthropod known almost exclusively from Cambrian Konservat Lagerstätten. Despite its wide geographical distribution in such sites of exceptional preservation, little was known of its soft-part anatomy until recently when remains of eyes and raptorial frontal appendages were discovered. This absence has precluded determination of affinities. The new discovery of soft-parts led to two important hypotheses: (1) that Isoxys was related to the “great-appendage” arthropods and (2) its contained species were not congeneric. Neither has been tested using a detailed cladistics analysis. The morphology of Isoxys is re-evaluated and coded into an extensive cladistics analysis. Our results indicate that Isoxys was indeed a monophyletic genus with all representatives united by the presence of an expansive dorsal shield with prominent antero- and posterolateral cardinal spines. It also indicates that Isoxys occupies a crucial role in arthropod evolution, resolving at the base of Arthropoda. The “great-appendages” of Isoxys, are interpreted as innovating from either the protocerebral, or deutocerebral somite, and are therefore not homologous to those of other “great-appendage” arthropods, which are interpreted as originating from the tritocerebral somite of the head.
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The enigmatic marrellomorph arthropod Furca bohemica from the Upper Ordovician Letná Formation, is redescribed. Based on existing museum specimens and new material collected from the southern slope of Ostrý Hill (Beroun, Czech Republic),... more
The enigmatic marrellomorph arthropod Furca bohemica from the Upper Ordovician Letná Formation, is redescribed. Based on existing museum specimens and new material collected from the southern slope of Ostrý Hill (Beroun, Czech Republic), the morphology and taphonomy of F. bohemica is reappraised and expanded to produce a new anatomical interpretation. The previously distinct taxa F. pilosa and Furca sp., are synonymised with F. bohemica, the latter being represented by a tapho-series in which decay has obscured some of the diagnostic features. A cladistic analysis indicates close affinities between F. bohemica and the Hunsrück Slate marrellomorph Mimetaster hexagonalis, together forming the Family Mimetasteridae, contrary to previous models for marrellomorph internal relationships. As with other representatives of the group, the overall anatomy of F. bohemica is consistent with a benthic, or possibly nektobenthic, mode of life. The depositional setting of the Letná Formation indicates that F. bohemica inhabited a shallow marine environment, distinguishing it palaeoecologically from all other known marrellomorphs, which have been reported from the continental shelf.
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A new arthropod, Kootenichela deppi gen. et sp. nov., is described from the Stanley Glacier exposure of the middle Cambrian (Series 3, Stage 5) Stephen Formation in Kootenay National Park (British Columbia, Canada). This taxon possesses a... more
A new arthropod, Kootenichela deppi gen. et sp. nov., is described from the Stanley Glacier exposure of the middle Cambrian (Series 3, Stage 5) Stephen Formation in Kootenay National Park (British Columbia, Canada). This taxon possesses a number of primitive arthropod features such as an elongate homonomous trunk consisting of at least 29 segments, poorly sclerotised trunk appendages, and large pedunculate eyes associated with an anterior (ocular) sclerite. The cephalon encompasses a possible antenna-like appendage and enlarged raptorial appendages with a bipartite peduncle and three spinose-distal podomeres, indicative of megacheiran (“great-appendage” arthropod) affinities. The relationships of these taxa are controversial with them generally considered as either stem-euarthropods, or a paraphyletic stem-lineage of chelicerates (pycnogonids, horseshoe crabs and arachnids). A comprehensive cladistic analysis resolved Kootenichela as sister-taxon to the enigmatic Worthenella cambria from the middle Cambrian (Series 3, Stage 5), Burgess Shale Formation situated in Yoho National Park (British Columbia, Canada), which is herein reinterpreted as a megacheiran arthropod. Based on this both taxa were placed in the new family Kootenichelidae, to which Pseudoiulia from the Chengjiang biota is also tentatively assigned. All taxa possess an elongate, multi-segmented body and subtriangular exopods. This family occupies a basal position within a paraphyletic Megacheira, the immediate outgroup of Euarthropoda (crown-group arthropods). The resultant topology indicates that analyses that have resolved megacheirans as stem-chelicerates have done so because they have rooted on inappropriate taxa, e.g. trilobitomorphs and marrellomorphs.
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The phylogenetic position of aglaspidids, a problematic group of Lower Palaeozoic arthropods of undetermined affinities, is re-examined in the context of the major Cambrian and Ordovician lamellipedian arthropod groups. A cladistic... more
The phylogenetic position of aglaspidids, a problematic group of Lower Palaeozoic arthropods of undetermined affinities, is re-examined in the context of the major Cambrian and Ordovician lamellipedian arthropod groups. A cladistic analysis of ten genera of aglaspidids sensu stricto, six aglaspidid-like arthropods and 42 Palaeozoic arthropod taxa indicates that Xenopoda, Cheloniellida, Aglaspidida sensu lato and Trilobitomorpha form a clade (Artiopoda Hou and Bergstro¨ m, 1997) nested within the mandibulate stem-lineage, thus discarding previous interpretations of these taxa as part ’of the chelicerate stem-group (Arachnomorpha Heider, 1913). The results confirm an aglaspidid identity for several recently described arthropods, including Quasimodaspis brentsae, Tremaglaspis unite, Chlupacaris dubia, Australaglaspis stonyensis and an unnamed Ordovician Chinese arthropod. The problematic Bohemian arthropod Kodymirus vagans was recovered as sister taxon to Beckwithia typa, and both form a small clade that falls outside Aglaspidida sensu stricto, thus discarding eurypterid affinities for the former. The analysis does not support the phylogenetic position of Kwanyinaspis maotianshanensis at the base of Conciliterga as proposed in recent studies, but rather occupies a basal position within Aglaspidida sensu lato. The results indicate a close association of aglaspidid arthropods with xenopods (i.e. Emeraldella and Sidneyia) and cheloniellids (e.g. Cheloniellon, Duslia); the new clade ‘‘Vicissicaudata’’ is proposed to encompass these arthropods, which are characterized by a differentiated posterior region. The phylogenetic position of aglaspidid arthropods makes them good outgroup candidates for analysing the internal relationships within the groups that form Trilobitomorpha. This work provides a much clearer picture of the phylogenetic relationships among Lower Palaeozoic lamellipedians.
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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... more
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).
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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... more
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.
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Fossil scorpion systematics – particularly that of the diverse assemblage found in Carboniferous Coal Measure deposits – are problematic, being characterised by a large number of frequently monotypic taxa, based on a limited set of... more
Fossil scorpion systematics – particularly that of the diverse assemblage found in Carboniferous Coal Measure deposits – are problematic, being characterised by a large number of frequently monotypic taxa, based on a limited set of characters. These Palaeozoic scorpions have the potential to elucidate the nature of the scorpion stemgroup and hence to inform long running debates regarding the origin of the crown group and relationships between extant lineages. Carboniferous taxa are often found in siderite concretions and can be preserved in three dimensions. This study investigates two such fossils of the species Compsoscorpius buthiformis (Pocock, 1911) with the aid of X-ray Micro-Tomography (XMT) and computer reconstruction. These scans reveal important new details of the anatomy, such as the walking legs, that allow more detailed comparison with coeval taxa. Many of these taxa are revealed here to be junior synonyms of C. buthiformis and previous taxonomic practice, as it relates to Coal Measures scorpions, is criticised. The ecology of C. buthiformis is also reviewed in light of the new evidence from XMT. In particular, aspects of its walking leg and metastomal anatomy are analogous to that of extant lithophilic and ‘errant’ leaf-litter scorpions.
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Liu et al.1 described an ‘armoured’ lobopodian, Diania cactiformis, from the Chengjiang Lagerstätte (China; Cambrian, stage 3); this fossil bears potentially arthropod-like articulated and possibly sclerotized appendages, but lacks a... more
Liu et al.1 described an ‘armoured’ lobopodian, Diania cactiformis, from the Chengjiang Lagerstätte (China; Cambrian, stage 3); this fossil bears potentially arthropod-like articulated and possibly sclerotized appendages, but lacks a sclerotized body. A cladistic analysis resolved Diania as sister-taxon to arthropods. From this phylogenetic position the authors tentatively inferred that arthropodization (sclerotization of limbs) may have preceded arthrodization (sclerotization of body elements) in arthropod evolution. Although we concur with the reasoning behind this inference, it rests on a phylogenetic placement that our analysis of the published data set does not reproduce.
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An isolated ramus of the pterygotid eurypterid Jaekelopterus cf. howelli from the Early Devonian (Pragian) Beartooth Butte Formation (Cottonwood Canyon, north-western Wyoming) is described. Pterygotid taxonomy and synonymy is briefly... more
An isolated ramus of the pterygotid eurypterid Jaekelopterus cf. howelli from the Early Devonian (Pragian) Beartooth Butte Formation (Cottonwood Canyon, north-western Wyoming) is described. Pterygotid taxonomy and synonymy is briefly discussed with the genera Pterygotus, Acutiramus and Jaekelopterus shown to be potential synonyms. The use of cheliceral denticulation patterns as a generic-level character is discouraged in light of variations within genera and its unsuitability as a major characteristic in the other eurypterid families.
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The euthycarcinoids are a mysterious group of extinct, possibly amphibious, arthropods. Like many groups of arthropods their affinities remain enigmatic.They comprise 15 species known mainly from the Late Carboniferous Coal Measures of... more
The euthycarcinoids are a mysterious group of extinct, possibly amphibious, arthropods. Like many groups of arthropods their affinities remain enigmatic.They comprise 15 species known mainly from the Late Carboniferous Coal Measures of Europe and North America, although they have been found in older rocks in Argentina and Australia. The oldest known euthycarcinoid is Apankura machu, from the Late Cambrian of Argentina, and the youngest is Euthycarcinus kessleri, from the Middle Triassic of France.
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The taxon Arthropoda has been repeatedly cited with various incorrect authorship data. Here, we review the primary literature and show the correct citation to be ‘Arthropoda VON SIEBOLD, 1848’.
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The interrelationships of the major extant arthropod groups are equivocal with various conflicting hypotheses being proposed. Recent large-scale molecular analyses have produced partly equivocal, sometimes novel and often surprising... more
The interrelationships of the major extant arthropod groups are equivocal with various conflicting hypotheses being proposed. Recent large-scale molecular analyses have produced partly equivocal, sometimes novel and often surprising hypotheses of relationships. Some proposed relationships, such as a sister-taxon relationship between chelicerates and myriapods, seemingly lacked morphological support, thus prompting a reevaluation of character homologies and a search for potential synapomorphies. Despite this the novel molecular clades remain elusive in large-scale morphological phylogenies, which tend to favor “traditional” groupings. The causes of this phenomenon are poorly understood. Here we present a comprehensive phylogeny of 307 edysozoans (96 extant, 211 extinct) representing all major panarthropod groups and two cycloneuralian outgroups and utilizing data from morphology, development, behavior and gene-order. We recovered strong support for the inclusion of hexapods within a paraphyletic Crustacea as sister-taxon to remipedes and the Silurian crustaceomorph Tanazios. A remipede-hexapod relationship is consistent with recent molecular analyses but had not been previously recovered in a morphological phylogenetic analysis. This relationship is shown to be the consequence of the inclusion of fossil data within our analysis, reducing the effect of long-branch attraction between extant myriapods and hexapods resulting from convergent adaptations to terrestrialisation. Fossil evidence also indicates that the arthropods have an extensive and diverse stem-group which demonstrates the gradual acquisition of typical arthropod features. The diversity of this long stem serves to better polarize characters within Euarthropoda thus providing a better picture of basal arthropod features and reducing the effects of long-branch attraction amongst extant arthropod clades.
We present a comprehensive cladistic analysis, including 10 genera of aglaspidids sensu stricto, 6 aglaspidid-like arthropods and 42 Palaeozoic arthropod taxa. The results of this work provide a much clearer picture of the phylogenetic... more
We present a comprehensive cladistic analysis, including 10 genera of aglaspidids sensu stricto, 6 aglaspidid-like arthropods and 42 Palaeozoic arthropod taxa. The results of this work provide a much clearer picture of the phylogenetic relationships among Lower Palaeozoic artiopodans.
The mid-Silurian Wenlock Series Herefordshire Lagerstätte preserves invertebrates in three-dimensions as calcitic void in-fills in carbonate nodules within a volcaniclastic horizon. A new arthropod from this fauna belongs to the ‘short... more
The mid-Silurian Wenlock Series Herefordshire Lagerstätte preserves invertebrates in three-dimensions as calcitic void in-fills in carbonate nodules within a volcaniclastic horizon. A new arthropod from this fauna belongs to the ‘short great appendage’ (megacheiran) group. Phylogenetic analysis, based on the most detailed panarthropod character matrix yet assembled, recovers the new Herefordshire genus with other short great appendage genera in a clade that is the most derived of the euarthropod stem. It thus provides new insights into the nature of this stem and the morphology of immediately pre-crown group forms; its phylogenetic position, as well as that of other short great appendage genera, contrasts with the position of this group as determined in many other studies, in which they have been considered to be part of the chelicerate stem lineage; and the new genus represents the first identification of a short great appendage form in strata younger than mid-Cambrian in age.
Furca bohemica Fritsch, 1908 is one of the most unusual arthropod from the Letná Formation (Czech Republic). Stratigraphy, taphonomy as well as phylogenetic affinities and its comparison to its marrellomorphs relatives is studied.
A remarkable new horseshoe crab from the the mid-Silurian of Herefordshire (~425 Myr) preserves biramous limbs in the prosoma that can be homologized with those of living Limulus. The fossil shows that a true biramous limb evolved in... more
A remarkable new horseshoe crab from the the mid-Silurian of Herefordshire (~425 Myr) preserves biramous limbs in the prosoma that can be homologized with those of living Limulus. The fossil shows that a true biramous limb evolved in chelicerates as well as in crustaceans, in contrast to the endopod with exite present in trilobites. The evidence of the new fossil supports models for the origin of the limb morphology of living horseshoe crabs through loss of gene expression.
Bivalved arthropods are a common constituent of many early Palaeozoic Konservat Lagerstätten; despite their ubiquity in such sites of exceptional preservation, however, their soft-parts are rarely well preserved. This has made determining... more
Bivalved arthropods are a common constituent of many early Palaeozoic Konservat Lagerstätten; despite their ubiquity in such sites of exceptional preservation, however, their soft-parts are rarely well preserved. This has made determining their affinities difficult. Often considered relatives of crustaceans due to their carapace morphology, their limb morphology is at odds with this interpretation. Common crustacean features identified in these taxa, including two pairs of antenna and gnathobasic mandibles are actually absent. Recent discoveries from the Burgess Shale (Cambrian, Stage 5) and Chengjiang biota (Cambrian, Stage 3) have provided a wealth of new information regarding the anatomy of these arthropods and indicate they play a key role in understanding some of the fundamental questions in arthropod palaeobiology such as the acquisition of key “arthropod” characteristics in the euarthropod stem-lineage including biramy and head-limb specialisation. They also help to resolve issues regarding the segmental nature of early arthropod appendages. Specifically a number of these arthropods have been shown to possess both frontal antennae and “great-appendages” and calls into question the homology of the “greatappendage” with the appendages of extant arthropods. If homologous then the antenna of such taxa would be considered protocerebral and therefore unique amongst arthropods. The implications of this are discussed.
Whilst progress has been made in recovering a stable lower euarthropod stem group, basal stem groups systematics within the euarthropod crown remain problematic. Current concepts include an expanded ‘Antennata’ encompassing the mostly... more
Whilst progress has been made in recovering a stable lower euarthropod stem group, basal stem groups systematics within the euarthropod crown remain problematic. Current concepts include an expanded ‘Antennata’ encompassing the mostly Cambrian arthropods possessing a single antenna. The stem group of the Chelicerata, conversely, is poorly populated, with no universally agreed members before the appearance of chasmataspid trace fossils towards the end of the Cambrian. In short, the older consensus about the ‘Arachnomorpha’ whereby trilobites and their allies were considered to be related to chelicerates has thoroughly broken down. A hypothesis in which the ‘great appendage’ arthropods were considered stem-group chelicerates based on their chelicera-like frontal appendages has in its stead gained considerable support. However, this reconstruction pays insufficient attention to older ideas of stem-group chelicerates possessing an antenniform, not cheliceriform first appendage. When the ‘xenopod’ taxa such as Sidneyia and Emeraldella are included in the analysis, an antennate first appendage is recovered as a chelicerate and thus euarthropod plesiomorphy. This reconstruction has significant implications for both the composition of the stems of the pancrustaceans and euarthropods more generally, as well as raising the old problem of functional transitions within arthropod frontal appendages once more.
Cambrian lobopodians have attracted much attention in that they are considered to have close affinities with the origin of arthropods. A new lobopodian species, Diania cactiformis Liu et al. 2011, was recently reported from the Chengjiang... more
Cambrian lobopodians have attracted much attention in that they are considered to have close affinities with the origin of arthropods. A new lobopodian species, Diania cactiformis Liu et al. 2011, was recently reported from the Chengjiang Lagerstätte. The authors suggested that this animal bears arthropod-like appendages and resolved D. cactiformis as sister-taxon to Arthropoda in their cladistic analysis. However, a reanalysis of the published dataset does not reproduce the phylogenetic placement of D. cactiformis. In our study, new observations on D. cactiformis are made using newly collected material, which rejects the existence of unequivocal evidence for arthropod-like articulated appendages. The criteria for defining arthropodized appendages are further discussed through comparative studies
among fossil lobopodians, anomalocaridids and arthropods. Our study also challenges the previous description of antero-posterior orientation, terminal claws and trunk tagmosis. A synthesis of recent datasets for lobopodians and stem-group arthropods is used to revisit the phylogeny of stem arthropods. Irrespective of how characters are weighted, D. cactiformis is resolved at a much more basal position to arthropods, and is actually one of the most basal Cambrian lobopodians, indicating that the morphology of this animal’s appendages does not fundamentally inform on the evolution of arthropodization.
among fossil lobopodians, anomalocaridids and arthropods. Our study also challenges the previous description of antero-posterior orientation, terminal claws and trunk tagmosis. A synthesis of recent datasets for lobopodians and stem-group arthropods is used to revisit the phylogeny of stem arthropods. Irrespective of how characters are weighted, D. cactiformis is resolved at a much more basal position to arthropods, and is actually one of the most basal Cambrian lobopodians, indicating that the morphology of this animal’s appendages does not fundamentally inform on the evolution of arthropodization.
With over 1,000,000 described species, arthropods are the dominant phylum on Earth today. The interrelationships between their constituent clades (subphyla) remain poorly understood however, with conflicts arising between different... more
With over 1,000,000 described species, arthropods are the dominant phylum on Earth today. The interrelationships between their constituent clades (subphyla) remain poorly understood however, with conflicts arising between different molecular and morphological data sources. The position of the pycnogonids (sea spiders) is particularly problematic, some analyses allying them to euchelicerates (horseshoe crabs and arachnids) and others placing them as sister-taxon to all other extant arthropods. This problem is largely the result of an uncertain root position rather than topological conflicts per se; phylogenetic analyses incorporating stem-group euarthropod taxa have the potential to resolve these issues. Cambrian lagerstätten contain many candidate stem-group arthropods, supplemented here by two new exceptionally-preserved taxa from the Cambrian of British Columbia: an elongate ‘great-appendage arthropod’ with multipodomerous trunk limbs; and a bivalved form lacking distinct tergal pleurae, a feature previously considered diagnostic of extant arthropods. A comprehensive phylogenetic analysis was undertaken incorporating these and many other fossil and Recent arthropods; this resolved the stemlineage (including the new taxa) in some detail, documenting many steps in the sequential acquisition of euarthropod characters. Pycnogonids resolved as sister-taxon to other euarthropods, implying that their chelicerate-like characters (e.g. chelate first appendage) are plesiomorphies rather than synapomorphies for euchelicerates.
Arthropods are a diverse group of animals including spiders, scorpions, centipedes, millipedes, insects and crustaceans. With over 1,000,000 described species they outnumber all other extant (living) phyla combined, and have been the... more
Arthropods are a diverse group of animals including spiders, scorpions, centipedes, millipedes, insects and crustaceans. With over 1,000,000 described species they outnumber all other extant (living) phyla combined, and have been the dominant contributor to species diversity for the last 520 million years. They are also commonly used as model organisms for evolutionary studies. Despite all this, we understand very little about their actual relationships, either to each other or other animal groups. Nearly every possible arrangement of the arthropod subgroups has been proposed and each is largely dependent on the type of evidence used, i.e. morphological vs. molecular (DNA) data. Recently a ―total evidence‖ approach has been utilised which includes all available data, including information from extant and extinct species. Fossil data has been shown to be of particular importance in
phylogenetic studies as they tend to resolve major character conflicts created by extant taxa and provide a ―snapshot‖ of evolution. The aim of this talk is to provide an overview of recent works attempting to resolve arthropod interrelations and will demonstrate the importance of fossil data for
resolving this issue.
phylogenetic studies as they tend to resolve major character conflicts created by extant taxa and provide a ―snapshot‖ of evolution. The aim of this talk is to provide an overview of recent works attempting to resolve arthropod interrelations and will demonstrate the importance of fossil data for
resolving this issue.
The marrellomorphs are a small group of Palaeozoic arthropods noted for their aberrant morphology, which has precluded resolution of their phylogenetic position. Marrellomorpha includes two clades, the marrellids and the acerostracans,... more
The marrellomorphs are a small group of Palaeozoic arthropods noted for their aberrant morphology, which has precluded resolution of their phylogenetic position. Marrellomorpha includes two clades, the marrellids and the acerostracans, allied by the shared possession of numerous (>25) trunk somites with appendages that reduce in size posteriorly. The marrellids are characterised by the possession of extensive anteriolateral head spines and typified by the Burgess Shale taxon Marrella; the acerostracans are characterised by the possession of a dorsal ―shield‖ that covers their entire body and appendages. Recent work on marrellomorph anatomy, including descriptions of unpublished material, has augmented our understanding of the morphology of the group, and enabled us to identify potential homologies with other arthropod taxa. A cladistic analysis was undertaken including all currently recognised marrellomorphs and a variety of extinct and extant arthropods. Preliminary results indicate that the marrellomorphs are monophyletic that includes: marrellids; acerostracans; and ―skaniids‖, a paraphyletic group of Cambrian arthropods that, like the acerostracans, possess a dorsal ―shield‖. This analysis also indicates that the marrellomorphs form part of a stem-mandibulate lineage that includes megacheirans, trilobites and various ―orsten‖ taxa.
Arthropod tagmosis is the differentiation of segments along the anterior/posterior axis, producing appendages specialized for sensation, feeding, and locomotion. Characters relating to segment and limb number and morphology are important... more
Arthropod tagmosis is the differentiation of segments along the anterior/posterior axis, producing appendages specialized for sensation, feeding, and locomotion. Characters relating to segment and limb number and morphology are important in constructing phylogenies that include fossil taxa. Chelicerates are a diverse group, including the extant spiders, mites, harvestmen, scorpions, xiphosurans, and the extinct trigonotarbids, eurypterids, chasmataspids, and synziphosurans. The extant pygnogonids (sea spiders) may also belong to this group. The chelicerate body is divided into two major tagmata: prosoma and opisthosoma. There is variation, however, within this bauplan (e.g. number of segments in a limb, fusion of dorsal tergites). Developmentally, patterns of tagmosis are associated with changes in the expression of Hox genes, segment polarity genes, and limb gap genes. The distribution of extant taxa for which Hox expression patterns are known is sparse. Therefore, we studied morphological traits for which the developmental basis is understood in extant chelicerates. We constructed a new phylogeny for chelicerates from over 400 morphological characters. Representative extant taxa with sequence or developmental gene expression data were included, as well as fossils with unique patterns of tagmosis or other morphological traits. Using this morphological topology, ancestral character states were inferred. Results under different models of character evolution (parsimony, likelihood, Bayesian) are compared.
Arthropod tagmosis is the differentiation of segments along the anterior/posterior axis, producing appendages specialized for sensation, feeding, and locomotion. Characters relating to segment and limb number and morphology are important... more
Arthropod tagmosis is the differentiation of segments along the anterior/posterior axis, producing appendages specialized for sensation, feeding, and locomotion. Characters relating to segment and limb number and morphology are important in constructing phylogenies that include fossil taxa. Chelicerates are a diverse group, including the extant spiders, mites, harvestmen, scorpions, xiphosurans, and the extinct trigonotarbids, eurypterids, chasmataspids, and synziphosurans. The extant pygnogonids (sea spiders) may also belong to this group. The chelicerate body is divided into two major tagmata: prosoma and opisthosoma. There is variation, however, within this bauplan (e.g., number of segments in a limb, fusion of dorsal tergites). Developmentally, patterns of tagmosis are associated with changes in the expression of Hox genes. The distribution of extant taxa for which Hox expression patterns are known is sparse. Therefore, we studied
morphological traits for which the developmental basis is understood in extant chelicerates. We constructed a new phylogeny for chelicerates from over 400 morphological characters. Representative extant taxa with sequence or developmental gene expression data were included, as well as fossils with unique patterns of tagmosis or other morphological traits. Using this morphological topology, ancestral character states were inferred. Results under different models of character evolution (parsimony, likelihood, Bayesian) are compared.
morphological traits for which the developmental basis is understood in extant chelicerates. We constructed a new phylogeny for chelicerates from over 400 morphological characters. Representative extant taxa with sequence or developmental gene expression data were included, as well as fossils with unique patterns of tagmosis or other morphological traits. Using this morphological topology, ancestral character states were inferred. Results under different models of character evolution (parsimony, likelihood, Bayesian) are compared.
With over 1,000,000 currently recognised species, arthropods are the most speciose phylum on Earth. Four distinct major extant clades are recognised: Chelicerata (spiders, scorpions, ticks), Myriapoda (centipedes and millipedes), Hexapoda... more
With over 1,000,000 currently recognised species, arthropods are the most speciose phylum on Earth. Four distinct major extant clades are recognised: Chelicerata (spiders, scorpions, ticks), Myriapoda (centipedes and millipedes), Hexapoda (insects), and Crustacea (crabs, ostracods, brine shrimp, etc). Although the monophyly of these clades is often well supported, their relationships to each other are hotly debated – with nearly every conceivable grouping proposed. In morphological analyses, the main problem is identifying homologous structures amongst the different clades. New evidence from molecular and fossil data is helping to resolve these problems and establish a consensus. The importance of these techniques for determining patterns of arthropod head segmentation is given as a case study and highlighted as an area for future research.
A restudy of the scorpion fauna of Glencartholm (Lower Carboniferous, Viséan), in Scotland, has revealed that many previously described species are invalid: most taxa represent ontogentic stages of a single taxon, Archaeoctonus glaber... more
A restudy of the scorpion fauna of Glencartholm (Lower Carboniferous, Viséan), in Scotland, has revealed that many previously described species are invalid: most taxa represent ontogentic stages of a single taxon, Archaeoctonus glaber (Peach, 1882). A new phylogeny of Palaeozoic scorpions indicates that the same sequences of changes that occur throughout A. glaber’s ontogeny also occur throughout Palaeozoic scorpion evolution, i.e. ontogeny recapitulates phylogeny. In particular, abdominal plate morphology, an important taxobase in many previous classifications, varies considerably with ontogeny, and is therefore useful for determining the age of fossil scorpions. Regression analysis indicates that many aspects of the anatomy of A. glaber grew isometrically. Isometric scaling indicates that the largest specimen represents the ninth moult and may have grown to a length of over 510 mm.
Acanthoscorpio mucronatus Kjellesvig-Waering was one of three species of fossil scorpion described from the Lower Devonian (Pragian?), Beartooth Butte Formation (Wyoming). A restudy of the only known specimen has revealed this taxon to be... more
Acanthoscorpio mucronatus Kjellesvig-Waering was one of three species of fossil scorpion described from the Lower Devonian (Pragian?), Beartooth Butte Formation (Wyoming). A restudy of the only known specimen has revealed this taxon to be a juvenile eurypterid. Previously unrecogonised features supporting this placement include a Dolichopterus-type swimming paddle and a type-A genital appendage. The overall size of the individual, limb proportions and eye placement indicate this specimen is a juvenile. Only two species of eurypterid have previously been described from the Beartooth Butte Formation: Strobilopterus princetonii (Ruedemann) and Jaekelopterus (?) howelli (Kjellesvig-Waering and Størmer). A. mucronatus is considered an early instar of S. princetonii based on swimming paddle morphology and allometric growth estimates. A. mucronatus is a junior synonym of S. princetonii and is thus suppressed under Article 23.1 of the ICZN.
A. mucronatus has previously been assigned to Mesoscorpionina, a clade of terrestrial scorpions. Its reassignment thus has implications for the understanding of scorpion systematics and terrestrialisation. A reanalysis of Jeram’s (1998) data matrix was undertaken with A. mucronatus removed. Greater levels of support were found and Mesoscorpionina became paraphyletic with regards to crown-group scorpions. Other problematic scorpion taxa are briefly discussed in this context.
A. mucronatus has previously been assigned to Mesoscorpionina, a clade of terrestrial scorpions. Its reassignment thus has implications for the understanding of scorpion systematics and terrestrialisation. A reanalysis of Jeram’s (1998) data matrix was undertaken with A. mucronatus removed. Greater levels of support were found and Mesoscorpionina became paraphyletic with regards to crown-group scorpions. Other problematic scorpion taxa are briefly discussed in this context.