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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. Here, the authors describe an early synziphosurine from the Lower Ordovician Fezouata Shale of Morocco, which exhibits traits that elucidate the long-contentious relationships between crown euchelicerates and their sister taxa, and also clarifies euchelicerate body plan evolution.

Specimens of Euproops sp. (Xiphosura, Chelicerata) from the Carboniferous Piesberg quarry near Osnabrück, Germany, represent a relatively complete growth series of 10 stages. Based on this growth sequence, morphological changes throughout the ontogeny can be identified. The major change affects the shape of the epimera of the opisthosoma. In earlier stages, they appear very spine-like, whereas in later stages the bases of these spine-like structures become broader; the broadened bases are then successively drawn out distally. In the most mature stage known, the epimera are of trapezoidal shape and approach each other closely to form a complete flange around the thoracetron (=fused tergites of the opisthosoma). These ontogenetic changes question the taxonomic status of different species of Euproops, as the latter appear to correspond to different stages of the ontogenetic series reconstructed from the Piesberg specimens. This means that supposed separate species could, in fact, represent different growth stages of a single species. It could alternatively indicate that heterochrony (=evolutionary change of developmental timing) plays an important role in the evolution of Xiphosura. We propose a holomorph approach, i.e., reconstructing ontogenetic sequences for fossil and extant species as a sound basis for a taxonomic, phylogenetic, and evolutionary discussion of Xiphosura.

Horseshoe crabs are one of the most studied invertebrates due to their remarkable innate immunity mechanism and biological processes. In this work, the proteins of the lipopolysaccharides (LPS)-stimulated and non-stimulated hemocytes of Malaysian Tachypleus gigas were profiled using LC-MS/MS. A total of 154 proteins were identified in both types of samples. Additionally, seventy-seven proteins were commonly found in both conditions, while 52 and 25 proteins were uniquely found in the LPS-stimulated and non-stimulated hemocytes, respectively. ATP-dependent energy-generating proteins such as actins and BLTX actin-related proteins were detected in both stimulated and non-stimulated T. gigas hemocytes, but more of such proteins were found in the former type. Proteins such as tachylectin-2, coagulogen, c-reactive proteins, histones, hemocyanin, and DNA polymerase, which play key roles in the organism's innate immunity, were differentially expressed in the hemocytes following LPS challenge. In conclusion, the proteins identified in the hemolymph of T. gigas are vital for the organism's molecular functions, biological processes, and activation of innate immunity.

Horseshoe crabs are archetypal marine chelicerates with a fossil record extending from the Lower Ordovician to today. The major horseshoe crab groups are thought to have arisen in the middle to late Palaeozoic. Here we present the oldest known limuloid from the lower Carboniferous (Tournaisian stage, c. 350 million years ago) of Scotland: Albalimulus bottoni gen. et sp. nov. A comprehensive phylogenetic analysis supports the placement of A. bottoni as a representative of the extant family Limulidae and 100 million years older than any other limulid taxon. The use of geometric morphometric analyses corroborate the erection of the new taxon and illustrates the exploitation of morphospace by xiphosurids. This new taxon highlights the complex evolutionary history of xiphosurids and the importance of documenting these unique Palaeozoic individuals.

The current study investigated the proangiogenic effects of fraction VII of perivitelline fluid (hscPVF-VEGF) obtained from the late-stage embryos of Indian horseshoe crab (Tachypleus gigas; Müller) using human umbilical cord tissue-derived MSCs (hUCMSCs). Angiogenic potential of hscPVF-VEGF was investigated by analyzing transcripts of signature angiogenic markers, key transcription factors and matrix metalloproteases. Molecular docking studies were performed to predict the binding site of hscPVF-VEGF with the VEGF receptor (VEGFR). hscPVF-VEGF significantly upregulated VEGF, vWF, and downregulated sFlt-1. Significant increase in transcriptional levels of HOXA7, HOXB3, HOXB5, CD31, MMP2, and MMP9 further elucidated the molecular mechanism underlying the angiogenic ability of hscPVF-VEGF. Wound healing assay revealed the migratory potential of hscPVF-VEGF. Molecular docking studies predicted that hscPVF-VEGF may modulate hVEGFR activity by binding in a pocket within the extracellular domains (D5, D6, and D7) distal to the VEGF binding site (D2 and D3). This study infers the potential and molecular mechanism of hscPVF-VEGF inducing angiogenic differentiation in hUCMSCs, suggesting clinical application of a recombinant form of hsPVF-VEGF in disorders with dysfunctional angiogenesis.

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.

Abstract Horseshoe crabs, considered living fossils with a stable morphotype spanning ∼445 million years, are evolutionarily, ecologically, and biomedically important species experiencing rapid population decline. Of the four extant species of horseshoe crabs, the Atlantic horseshoe crab, Limulus polyphemus, has become an essential component of the modern medicine toolkit. Here, we present the first chromosome-level genome assembly, and the most contiguous and complete assembly to date, for L. polyphemus using nanopore long-read sequencing and chromatin conformation analysis. We find support for three horseshoe crab-specific whole-genome duplications, but none shared with Arachnopulmonata (spiders and scorpions). Moreover, we discovered tandem duplicates of endotoxin detection pathway components Factors C and G, identify candidate centromeres consisting of Gypsy retroelements, and classify the ZW sex chromosome system for this species and a sister taxon, Carcinoscorpius rotundicauda. Finally, we revealed this species has been experiencing a steep population decline over the last 5 million years, highlighting the need for international conservation interventions and fisheries-based management for this critical species. Graphical Abstract Graphical Abstract

The biology of the American horseshoe crab, Limulus polyphemus , is well documented—including its dietary habits, particularly the ability to crush shell with gnathobasic walking appendages—but virtually nothing is known about the feeding biomechanics of this iconic arthropod. Limulus polyphemus is also considered the archetypal functional analogue of various extinct groups with serial gnathobasic appendages, including eurypterids, trilobites and other early arthropods, especially Sidneyia inexpectans from the mid-Cambrian (508 Myr) Burgess Shale of Canada. Exceptionally preserved specimens of S. inexpectans show evidence suggestive of durophagous (shell-crushing) tendencies—including thick gnathobasic spine cuticle and shelly gut contents—but the masticatory capabilities of this fossil species have yet to be compared with modern durophagous arthropods. Here, we use advanced computational techniques, specifically a unique application of 3D finite-element analysis (FEA), to model the feeding mechanics of L. polyphemus and S. inexpectans : the first such analyses of a modern horseshoe crab and a fossil arthropod. Results show that mechanical performance of the feeding appendages in both arthropods is remarkably similar, suggesting that S. inexpectans had similar shell-crushing capabilities to L. polyphemus . This biomechanical solution to processing shelly food therefore has a history extending over 500 Myr, arising soon after the first shell-bearing animals. Arrival of durophagous predators during the early phase of animal evolution undoubtedly fuelled the Cambrian ‘arms race’ that involved a rapid increase in diversity, disparity and abundance of biomineralized prey species.

The group Xiphosurida (horseshoe “crabs”) is today only represented by four species. However, in the fossil record, several dozen species have been described, especially from the Carboniferous (about 300 million years ago). Several species have been interpreted as representatives of Euproops or Belinurus, but there is ongoing discussion which of these species are valid and how they can be differentiated. Recent studies suggested that differences in the timing of individual development could provide information for species distinction, exemplified by studies on Euproops danae (Mazon Creek, USA) and Euproops sp. (“Piesproops”; Piesberg, Germany). For this study, we reinvestigated all Carboniferous xiphosurids from the British Coal Measures stored in the collections of the Natural History Museum London. Size comparisons of the specimens revealed nine size groups; the smaller specimens were originally labelled as Belinurus, the larger ones as Euproops. The nine size groups exhibit five different morphotypes differing in structures surrounding the posterior shield (= thoracetron): spines of different lengths and, in larger specimens, a more or less developed flange. Two of these morphotypes show significantly longer spines than the remaining specimens and could be conspecific as E. anthrax. The remaining specimens are interpreted as growth series of another species, presumably of E. rotundatus. An ontogenetic flange formation is also known from E. danae and the “Piesproops”, but the timing differs between all three species. In E. rotundatus, the flange develops rather late, but then comparably abruptly, which makes this development more metamorphic in relation to development in the other species.

A remarkable specimen of Mesolimulus from the Upper Jurassic (Kimmeridgian) of Nusplingen, Germany, preserves the musculature of the prosoma and associated microbes in three dimensions in calcium phosphate (apatite). The musculature of Mesolimulus conforms closely to that of modern horseshoe crabs. Associated with the muscles are patches of mineralized biofilm with spiral and coccoid forms. This discovery emphasizes the potential of soft-bodied fossils as a source for increasing our knowledge of the diversity of fossil microbes in particular settings.