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15 results for “Xiphosura”
Segmentation in early Xiphosura and the evolution of the thoracetron
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F in The morphological differentiation of two horseshoe crab species, Tachypleus tridentatus and Carcinoscorpius rotundicauda (Xiphosura), in Hong Kong with a regional Asian comparison
F. 3. (A) Size–weight distributions of Tachypleus tridentatus males (&) and females (%) in Hong Kong. (B) Size–weight distribution of Carcinoscorpius rotundicauda males ($) and females (#) in Hong Kong.
F in The morphological differentiation of two horseshoe crab species, Tachypleus tridentatus and Carcinoscorpius rotundicauda (Xiphosura), in Hong Kong with a regional Asian comparison
F. 2. Various body parts of the horseshoe crabs measured to the nearest millimetre. a=length of prosoma; b=length of opisthosoma; c=length of telson; d=maximum width of prosoma; e=distance between the two compound eyes; f =maximum width of opisthosoma; g=width of telson; S1–S6=length of first to sixth opisthosomal, marginal spines.
Figure 7 in Revised systematics of Palaeozoic 'horseshoe crabs' and the myth of monophyletic Xiphosura
Figure 7. Summary cladogram of higher-level relationships retrieved from the phylogenetic analysis. Chelicerata consists of Pycnogonida and Euchelicerata, with megacheirans forming a polytomy on the node below. Trilobites and xenopods form a basal clade which is here considered to represent Antennata. For the full consensus tree see the Supporting information.
Figure 2 in Revised systematics of Palaeozoic 'horseshoe crabs' and the myth of monophyletic Xiphosura
Figure 2. Schematic of a generalized synziphosurine arthropod, showing the distinction between the preabdominal and postabdominal non-functional pseudotagmata and the true tagmata of the prosoma, mesosoma, and metasoma.
Figure 10 in Revised systematics of Palaeozoic 'horseshoe crabs' and the myth of monophyletic Xiphosura
Figure 10. Weinbergina opitzi Richter & Richter, 1929 from the Lower Devonian (Emsian) of Bundenbach, Germany. Holotype specimen SMF VIII 7a, showing a flat articulating facet at the anterior of each tergite. Scale bar = 10 mm.
Figure 5. Limulus polyphemus Linnaeus, 1758 in Revised systematics of Palaeozoic 'horseshoe crabs' and the myth of monophyletic Xiphosura
Figure 5. Limulus polyphemus Linnaeus, 1758 from the Recent of North America. A, ventral view of opisthosoma showing opercula. B, dorsal view of opisthosoma with prosoma and telson removed. C, schematic of opisthosoma in dorsal view with apodemes (shallow pits indicating sites of muscle attachment) marked in black and the insertion points of the opercula shown by grey ovals. D, ventral view of opisthosoma with prosoma, telson, and opercula removed. It can be clearly seen that the opercula are not attached to the lateral regions of the opisthosoma. Scale bars = 10 mm.
Figure 4. A in Revised systematics of Palaeozoic 'horseshoe crabs' and the myth of monophyletic Xiphosura
Figure 4. A, Pasternakevia podolica Selden & Drygant, 1987 from the late Silurian (Ludlow) of Zalissia, Ukraine. Specimen ISEA I - F/MP/3/1499/08, isolated opsithosoma with articulated microtergite of somite VII (arrowed). Image courtesy of Ewa Krzemin´ ska. Scale bar = 10 mm. B, undescribed chasmataspidid from the Lower Devonian (Emsian) of Siberia, Russia. Specimen PIN 5116-6, disarticulated buckler with microtergite of somite VII (arrowed) still firmly attached to its anterior margin. Image courtesy of Dave Marshall. Scale bar = 2 mm.
Figure 8 in Revised systematics of Palaeozoic 'horseshoe crabs' and the myth of monophyletic Xiphosura
Figure 8. Summary cladogram of the internal relationships of Prosomapoda. Arachnids, eurypterids, and chasmataspidids form a clade, here termed Dekatriata. Xiphosurans are paraphyletic with respect to Dekatriata, with xiphosurids forming a monophyletic clade of their own. All taxa outside the two labels are synziphosurines, which would here be polyphyletic. For the full consensus tree see the Supporting information.
Figure 3. A, Bunodes lunula d in Revised systematics of Palaeozoic 'horseshoe crabs' and the myth of monophyletic Xiphosura
Figure 3. A, Bunodes lunula d'Eichwald, 1854 from the late Silurian (Ludlow) of Oesel, Estonia. Specimen ELM G1:262:2, clearly showing the partially reduced pre-opercula tergite of somite VII (arrowed) between the downturned carapace and the hypertrophied tergite of somite VIII. Scale bar = 10 mm. B, 'Eurypterus' stoermeri Novojilov, 1959, a chasmataspidid from the early Devonian (Lochkovian) of Siberia, Russia. Specimen PIN 1138-1, exhibiting the microtergite of somite VII (arrowed) positioned between the carapace and the buckler. The microtergite curves anteriorly towards its lateral edges, and is particularly noticeable on the left-hand side. The occurrence of ridges associated with the lateral eye on the carapace is also labelled. Image courtesy of Dave Marshall. Scale bar = 2 mm.
Data from: A critical appraisal of the placement of Xiphosura (Chelicerata) with account of known sources of phylogenetic error
Horseshoe crabs (Xiphosura) are traditionally regarded as sister to the clade of terrestrial chelicerates (Arachnida). This hypothesis has been challenged by recent phylogenomic analyses, but the non-monophyly of Arachnida has consistently been disregarded as artifactual. We reevaluated the placement of Xiphosura among chelicerates using the most complete phylogenetic dataset to date, expanding outgroup sampling and including data from whole genome sequencing projects. In spite of uncertainty in theplacement of some arachnid clades, all analyses show Xiphosura consistently nested within Arachnida as the sister group to Ricinulei (hooded tick spiders). It is apparent that the radiation of Arachnids is an old one and occurred over a brief period of time, resulting in several consecutive short internodes, and thus is a potential case for the confounding effects of incomplete lineage sorting (ILS). We simulated coalescent gene trees to explore the effects of increasing levels of ILS on the placement of horseshoe crabs. In addition, common sources of systematic error were evaluated, as well as the effects of fast evolving partitions and the dynamics of problematic long branch orders. Our results indicated that the placement of horseshoe crabs can not be explained by missing data, compositional biases, saturation, or incomplete lineage sorting. Interrogation of the phylogenetic signal showed that the majority of loci favor the derived placement of Xiphosura over a monophyletic Arachnida. Our analyses support the inference that horseshoe crabs represent a group of aquatic arachnids comparable to aquatic mites, breaking a long standing paradigm in chelicerate evolution and altering previous interpretations of the ancestral transition to the terrestrial habitat. Future studies testing chelicerate relationships should approach the task with a sampling strategy where the monophyly of Arachnida is not held as the premise.
Data from: A critical appraisal of the placement of Xiphosura (Chelicerata) with account of known sources of phylogenetic error
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F in The morphological differentiation of two horseshoe crab species, Tachypleus tridentatus and Carcinoscorpius rotundicauda (Xiphosura), in Hong Kong with a regional Asian comparison
F. 1. A map of Hong Kong showing the locations where horseshoe crabs were collected for this study.
F in The morphological differentiation of two horseshoe crab species, Tachypleus tridentatus and Carcinoscorpius rotundicauda (Xiphosura), in Hong Kong with a regional Asian comparison
F. 4. Geographic variation in maximum prosomal width of (A) Tachypleus tridentatus and (B) Carcinoscorpius rotundicauda (in part after Yamasaki et al., 1988).
Figure 6 in Revised systematics of Palaeozoic 'horseshoe crabs' and the myth of monophyletic Xiphosura
Figure 6. Schematic of the xiphosuran ground pattern. Somites are labelled 0–XVII; 'tl' indicates the telson, 'gills' the position of gills on the five posterior opisthosomal appendages, 'm' the mouth, 'a' the anus, and 'g' the gonopores. The appendages are shaded with the light ramus representing the endopod, the light grey the basipod, and the dark grey the exopod. The appendage of somite VII is reduced in the majority of taxa into the chilaria. It is possible that appendage VII may also have possessed a reduced exopod as in appendage VI given its intermediate position between appendage VI and the biramous opisthosomal appendages, but there is currently no evidence for this. To the left the somite compositions of the fused xiphosurid cephalothorax, thoracetron, and pretelson are shown, along with the varying combinations of segments that can form the diplotergite in bunodids and pseudoniscids.
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