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Sea Spiders (sea + spider)

Distribution by Scientific Domains

Life Sciences	50%

Selected Abstracts

A NEW SEA SPIDER (ARTHROPODA: PYCNOGONIDA) WITH A FLAGELLIFORM TELSON FROM THE LOWER DEVONIAN HUNSRÜCK SLATE, GERMANY

PALAEONTOLOGY, Issue 5 2006
MARKUS POSCHMANN
Abstract:, 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. [source]

Pycnogonid affinities: a review

JOURNAL OF ZOOLOGICAL SYSTEMATICS AND EVOLUTIONARY RESEARCH, Issue 1 2005
J. A. Dunlop
Abstract 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. Zusammenfassung 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. [source]

Phylogeny of the sea spiders (Arthropoda, Pycnogonida) based on direct optimization of six loci and morphology

CLADISTICS, Issue 3 2007
Claudia P. Arango
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. [source]