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An unusual euchelicerate linking horseshoe crabs and eurypterids, from the Lower Devonian (Lochkovian) of Yunnan, China
Abstract
Two new specimens of the xiphosuran Kasibelinurus yueya Lamsdell, Xue & Selden, 2013 are described, from the same horizon and locality as the type. The new specimens are conspecific with the type, but show ventral morphology, which considerably alters interpretation of the species. It can no longer be referred to Kasibelinurus Pickett, 1993, and a new genus, Houia n. gen., is erected. Houia shows a unique combination of xiphosurid and chasmataspidid/eurypterid characteristics such as lack of opisthosomal pleura and possession of a large, ventral metastomal plate. Phylogenetic analysis of all the major chelicerate orders suggests that Houia branched from the main euchelicerate lineage prior to the divergence of the three constituent clades of the Dekatriata Lamsdell, 2013 (Eurypterida, Arachnida, Chasmataspidida). Together with bunodids and pseudoniscids, Houia provides evidence for basal dekatriatans persisting into the middle Palaeozoic alongside eurypterids and arachnids and that the morphological diversity of these basal forms was greater than previously thought.
... comparisons with fossil chelicerates Further support for this migration hypothesis comes from examining non-eurypterid fossil chelicerates. Weinbergina opitzi Richter and Richter, 1929-an exceptionally preserved synziphosurine from the Lower Devonian Hunsrück Slate-is placed phylogenetically either at the base of the clade containing synziphosurines, Xiphosurida (true horseshoe crabs), arachnids, eurypterids, and chasmataspids (Lamsdell 2013;Selden et al. 2015) or sister to the Xiphosurida (Briggs et al. 2012) within the Xiphosura. In W. opitzi, appendage VII has been interpreted as a fully functioning walking leg (Stürmer and Bergström 1981;Moore et al. 2005), and the reduction of this appendage to chilaria is an apomorphy of Xiphosurida (Lamsdell 2013;Dunlop and Lamsdell 2017). ...
... In W. opitzi, appendage VII has been interpreted as a fully functioning walking leg (Stürmer and Bergström 1981;Moore et al. 2005), and the reduction of this appendage to chilaria is an apomorphy of Xiphosurida (Lamsdell 2013;Dunlop and Lamsdell 2017). However, this interpretation was questioned in Selden et al. (2015), who proposed that W. opitzi prosomal appendages may be biramous. This would have produced an incorrect interpretation of the number of appendages, that is, appendage VII may be appendage VI (Dunlop and Lamsdell 2017). ...
... 32h). However, there is no indication of the presence of these endites on the sixth appendage in either xiphosurans or synziphosurines (Sutton et al. 2002;Briggs et al. 2012) Another option is based on the observation of a biramous sixth appendage in Offacalus (Sutton et al. 2002) and possibly in Weinbergina (Selden et al. 2015). In this scenario, the metastoma is derived from the exopod of this appendage. ...
The endostoma of eurypterids is a small ventral structure previously known from only 11 specimens and in detail from only a single species, Eurypterus tetraganophthalmus. Four previously undescribed eurypterid specimens from the Samuel J. Ciurca Jr. Collection, Yale Peabody Museum of Natural History, likely belonging to Eurypterus, and a previously documented specimen of the pterygotid Acutiramus cummingsi in the New York State Museum, are considered here to reinterpret the morphology and homologies of the endostoma. We demonstrate that the endostoma is a bilateral, flaplike appendage, with distal margins bearing anteriorly facing setae. An anterior embayment and posterior medial notch are noted. The latter represents the point where the endostoma articulates with an oval sclerite. Here we propose that, contrary to previous interpretations, the endostoma is an appendage, likely of the seventh body segment, and is probably homologous to the xiphosuran chilaria. This has direct implications for the hypotheses of homology for the metastoma, a ventral structure common to eurypterids and chasmataspids.
... However, little modelling has been presented recently. This is striking, because sea scorpions are bracketed phylogenetically by two extant clades that can be used to test functional hypotheses, namely horseshoe crabs (Xiphosura) and Arachnida (Legg et al., 2013;Brandt & McCoy, 2014;Selden et al., 2015). Indeed, aspects of eurypterid ecology are commonly compared with extant xiphosurans, in particular Limulus polyphemus (Linnaeus, 1758) (Andrews et al., 1974;Dalingwater, 1975Dalingwater, , 1985Selden, 1981;Anderson et al., 2014;McCoy et al., 2015;Bicknell et al., 2018c). ...
... Reconstructions were informed by examining fossils, high-resolution published images of appendages, proposed reconstructions and, where possible, identifying specimens that demonstrate different preservation orientations (Holm, 1898;Selden, 1981;Lamsdell et al., 2015;Haug, 2020). The scale of podomeres relative to each other was also determined from fossils. ...
... This inflation would not impact the modelling, because overall morphology primarily influences the kinematic motion. The eurypterid material from the Viita Formation and the Winneshiek Lagerstätte is exceptionally preserved and represents two rare situations where original eurypterid cuticle is preserved (Selden, 1981;Lamsdell et al., 2015;Bicknell et al., 2018c;Haug, 2020). These specimens have also been successfully removed from their rock matrix. ...
Sea scorpions (Euchelicerata: Eurypterida) explored extreme limits of the aquatic euchelicerate body plan, such that the group contains the largest known marine euarthropods. Inferences on eurypterid life modes, in particular walking and eating, are commonly made by comparing the group with horseshoe crabs (Euchelicerata: Xiphosura). However, no models have been presented to test these hypotheses. Here, we reconstruct prosomal appendages of two exceptionally well-preserved eurypterids, Eurypterus tetragonophthalmus and Pentecopterus decorahensis, and model the flexure and extension of these appendages kinematically in three dimensions (3D). We compare these models with 3D kinematic models of Limulus polyphemus prosomal appendages. This comparison highlights that the examined eurypterid prosomal appendages could not have moved prey items effectively to the gnathal edges and would therefore not have emulated the motion of an L. polyphemus walking leg. It seems that these eurypterid appendages were used primarily to walk or grab prey, and other appendages would have moved prey for mastication. Such 3D kinematic modelling highlights how eurypterid appendage morphologies placed substantial limits on their function, suggesting a high degree of specialization, especially when compared with horseshoe crabs. Such three-dimensional kinematic modelling of these extinct groups therefore presents an innovative approach to understanding the position of these animals within their respective palaeoecosystems.
... However, little modelling has been presented recently. This is striking, because sea scorpions are bracketed phylogenetically by two extant clades that can be used to test functional hypotheses, namely horseshoe crabs (Xiphosura) and Arachnida (Legg et al., 2013;Brandt & McCoy, 2014;Selden et al., 2015). Indeed, aspects of eurypterid ecology are commonly compared with extant xiphosurans, in particular Limulus polyphemus (Linnaeus, 1758) (Andrews et al., 1974;Dalingwater, 1975Dalingwater, , 1985Selden, 1981;Anderson et al., 2014;McCoy et al., 2015;Bicknell et al., 2018c). ...
... Reconstructions were informed by examining fossils, high-resolution published images of appendages, proposed reconstructions and, where possible, identifying specimens that demonstrate different preservation orientations (Holm, 1898;Selden, 1981;Lamsdell et al., 2015;Haug, 2020). The scale of podomeres relative to each other was also determined from fossils. ...
... This inflation would not impact the modelling, because overall morphology primarily influences the kinematic motion. The eurypterid material from the Viita Formation and the Winneshiek Lagerstätte is exceptionally preserved and represents two rare situations where original eurypterid cuticle is preserved (Selden, 1981;Lamsdell et al., 2015;Bicknell et al., 2018c;Haug, 2020). These specimens have also been successfully removed from their rock matrix. ...
Sea scorpions (Euchelicerata: Eurypterida) explored extreme limits of the aquatic euchelicerate body plan, such that the group contains the largest known marine euarthropods. Inferences on eurypterid life modes, in particular walking and eating, are commonly made by comparing the group with horseshoe crabs (Euchelicerata: Xiphosura). However, no models have been presented to test these hypotheses. Here, we reconstruct prosomal appendages of two exceptionally well-preserved eurypterids, Eurypterus tetragonophthalmus and Pentecopterus decorahensis, and model the flexure and extension of these appendages kinematically in three dimensions (3D). We compare these models with 3D kinematic models of Limulus polyphemus prosomal appendages. This comparison highlights that the examined eurypterid prosomal appendages could not have moved prey items effectively to the gnathal edges and would therefore not have emulated the motion of an L. polyphemus walking leg. It seems that these eurypterid appendages were used primarily to walk or grab prey, and other appendages would have moved prey for mastication. Such 3D kinematic modelling highlights how eurypterid appendage morphologies placed substantial limits on their function, suggesting a high degree of specialization, especially when compared with horseshoe crabs. Such three-dimensional kinematic modelling of these extinct groups therefore presents an innovative approach to understanding the position of these animals within their respective palaeoecosystems.
... More recently, expanded phylogenetic analyses have been proposed (Lamsdell, 2016(Lamsdell, , 2020(Lamsdell, , 2021, and the composition of Xiphosura is constrained to solely Lunataspis, Kasibelinuridae and Xiphosurida. As for synziphosurines, formerly suggested as stem Xiphosura, in recent analyses they have been excluded from the xiphosurans, and considered as a polyphyletic group comprising stem euchelicerates and stem dekatriatans (Dekatriata = Chasmataspidida + Eurypterida + Arachnida; Lamsdell, 2016Lamsdell, , 2020Selden, Lamsdell, & Liu, 2015). The discovery of Houia yueya (Lamsdell, Xue, & Selden, 2013) and Winneshiek youngae (Lamsdell, Briggs, Liu, Witzke, & McKay, 2015), synziphosurines with characteristics of both xiphosurans and eurypterids, has not only augmented phylogenetic analysis, supporting that most synziphosurines do not resolve within the xiphosuran clade, but also provided important evidence for potentially higher morphological diversity and geographical distribution of basal dekatriatans (Selden et al., 2015). ...
... As for synziphosurines, formerly suggested as stem Xiphosura, in recent analyses they have been excluded from the xiphosurans, and considered as a polyphyletic group comprising stem euchelicerates and stem dekatriatans (Dekatriata = Chasmataspidida + Eurypterida + Arachnida; Lamsdell, 2016Lamsdell, , 2020Selden, Lamsdell, & Liu, 2015). The discovery of Houia yueya (Lamsdell, Xue, & Selden, 2013) and Winneshiek youngae (Lamsdell, Briggs, Liu, Witzke, & McKay, 2015), synziphosurines with characteristics of both xiphosurans and eurypterids, has not only augmented phylogenetic analysis, supporting that most synziphosurines do not resolve within the xiphosuran clade, but also provided important evidence for potentially higher morphological diversity and geographical distribution of basal dekatriatans (Selden et al., 2015). However, the fossil record of these forms is scarce. ...
... nov., from the Lower Devonian Cangwu Formation (Pragian-Emsian) of South China (Figure 1). This discovery significantly extends the geographical distribution of Houia, supporting its range in the Early Devonian (Selden et al., 2015), providing evidence of potential relationships between shallow-water animals on the Yangtze and Cathaysia blocks. ...
Houia is a possible annectent form between horseshoe crabs and eurypterids (sea scorpions). However, fossils are rare, previously only known from a single locality. Here, we report a new species of this genus, Houia guangxiensis sp. nov., from the Lower Devonian Cangwu Formation of Guangxi Province, South China. This discovery extends the geographical distribution of Houia from the Yangtze to the Cathaysia Block, supporting the living age of this genus in the Early Devonian (Lochkovian–Emsian). It not only adds to the record of this rare ‘intermediate’ form but also suggests potential relationships of shallow‐water species in the Yangtze and Cathaysia blocks. Houia guangxiensis sp. nov. from the Lower Devonian of the Cathaysia Block, South China.
... Dunlop & Lamsdell, 2017). Reports of LCEs have become more frequent in recent times (Stürmer & Bergström, 1981), with taxa such as Bunodes sp. in Bergström (1975), Cyamocephalus loganensis Currie, 1927, Legrandella lombardii Eldredge, 1974, Limuloides limuloides (Woodward, 1865, Pseudoniscus clarkei Ruedemann, 1916 and Pseudoniscus falcatus (Woodward, 1868) having been reported to possess lateral ocular features ( Fig. 1; Størmer, 1934;Eldredge, 1974;Bergström, 1975;Dunlop & Selden, 1998;Rudkin & Young, 2009;Selden et al., 2015). Other forms may have possessed putative LCEs, including Drabovaspis complexa (Barrande, 1872), Pasternakevia podolica Selden & Drygant, 1987, Weinbergina opitzi Richter & Richter, 1929 and Willwerathia laticeps (Størmer, 1936) (Lehmann, 1956;Chlupáč, 1963Chlupáč, , 1965Stürmer & Bergström, 1981;Anderson et al., 1998;Krzemiński et al., 2010;Ortega-Hernández et al., 2010). ...
... The original description of Houia yueya from the Early Devonian-aged (Lochkovian) Xishancun Formation in Yunnan, China reported no conclusive evidence for LCEs . On reconsideration of the taxon by Selden et al. (2015), ovate LCEs were noted on both sides of the prosomal shield, a statement that we reaffirm here (Fig. 6A, B). Lateral compund eyes are 8 mm from the lateral, 10 mm from the posterior and 4 mm from the anterior boarders and 4 mm from the prosomal midline. ...
... Our revision of the fossil record of synziphosurine LCEs carries direct implications for the evolution of these visual structures in Euchelicerata. Recent reconstructions of chelicerate phylogeny indicate that synziphosurines and other early Palaeozoic xiphosurans are not monophyletic (see Lamsdell, 2013Lamsdell, , 2016Selden et al., 2015;contra Eldredge, 1974), but rather a paraphyletic assemblage of taxa that occupy positions as stem-group euchelicerates (e.g. Legrandella lombardii), stem-group xiphosurans (e.g. ...
Vision allows animals to interact with their environment. Aquatic chelicerates dominate the early record of lateral compound eyes among non-biomineralizing crown-group euarthropods. Although the conservative morphology of lateral eyes in Xiphosura is potentially plesiomorphic for Euarthropoda, synziphosurine eye organization has received little attention despite their early diverging phylogenetic position. Here, we re-evaluate the fossil evidence for lateral compound eyes in the synziphosurines Bunodes sp., Cyamocephalus loganensis, Legrandella lombardii, Limuloides limuloides, Pseudoniscus clarkei, Pseudoniscus falcatus and Pseudoniscus roosevelti. We compare these data with lateral eyes in the euchelicerates Houia yueya, Kasibelinurus amicorum and Lunataspis aurora. We find no convincing evidence for lateral eyes in most studied taxa, and Pseudoniscus roosevelti and Legrandella lombardii are the only synziphosurines with this feature. Our findings support two scenarios for euchelicerate lateral eye evolution. The elongate-crescentic lateral eyes of Legrandella lombardii might represent the ancestral organization, as suggested by the phylogenetic position of this taxon in stem-group Euchelicerata. Alternatively, the widespread occurrence of kidney-shaped lateral eyes in stem-group Xiphosura and stem-group Arachnida could represent the plesiomorphic condition; Legrandella lombardii eyes would therefore be derived. Both evolutionary scenarios support the interpretation that kidney-shaped lateral eyes are ancestral for crown-group Euchelicerata and morphologically conserved in extant Limulus polyphemus.
... Pickettia carteri is shown in bold. The topology of Chasmataspidida, Eurypterida and Arachnida clades were collapsed as they are unchanged from Selden et al. (2015) and were not the focus our study. Fig. 2. Résultats de l'analyse phylogénétique. ...
... Picketta carteri figure en gras. La topologie des clades de Chamataspidida, Eurypterida et Arachnida est inchangée selon Selden et al. (2015) ; ces clades sont mis ensemble et ne sont pas l'objet de cette étude. and articulated with the posterior opisthosoma margin. ...
... The phylogenetic analysis resulted in 18 most parsimonious trees of length 738 (CI: 0.472; RI: 0.879). The topology of the strict consensus tree is similar to that presented in Lamsdell (2016) and Selden et al. (2015); see Fig. 2) in hypothesising a monophyletic Xiphosurida, with synziphosurines represented as polyphyletic grade (sensu Lamsdell, 2013) forming the basal members of the sister taxon of, Xiphosurida + stem xiphosurids. Pickettia carteri resolves within a polytomy containing "Kasibelinurus" randalli (Beecher, 1902) and Lunataspis aurora Rudkin et al., 2008 and immediately crownward of K. amicorum Pickett, 1993). ...
Horseshoe crabs are an iconic group of marine chelicerates that have an impressive fossil record extending back to at least the Lower Ordovician. Despite their long fossil record and associated palaeontological interest, a range of fossil horseshoe crab taxa erected in the 19th and 20th centuries have remained understudied. Recent phylogenetic hypotheses have led to improvements in the understanding of xiphosuran origins and evolutionary history; however, the resolution among the basal-most Devonian-aged members remains poor. Here, the type specimen of “Bellinurus” carteri Eller, 1940 from the Late Devonian of Pennsylvania is reconsidered. Based on a revised morphological description and comparison, we conclude that the species is not referable to the genus Bellinurus and erected a new genus: Pickettia gen. nov. A phylogenetic analysis resolves Pickettia carteri within a polytomy containing taxa previously considered to comprise the group Kasibelinuridae, but which is currently a paraphyletic assemblage. We discuss P. carteri within the context of other stem xiphosurids and conclude that the diversity of this assemblage has been overstated. The redescription of P. carteri highlights the need for more inclusive studies to resolve the evolutionary relationships of stem xiphosurids.
... Chelicerate phylogeny indicates that the group is sister to a clade comprising eurypterids and arachnids [2][3][4], although poor preservation of a number of species has hindered attempts at resolving chasmataspidid internal relationships. Currently, chasmataspidids are divided between two family groups: Chasmataspididae, comprising only the species Chasmataspis laurencii from the Early Ordovician of Tennessee [5,6]; and Diploaspididae, encompassing all other known species from the Silurian to Devonian of Europe, North America, and Russia [1,7]. ...
... A broader revision of the known chasmataspidid species is required before a full phylogenetic framework is developed; however, preliminary analysis of a few species as part of a broader phylogeny of chelicerates retrieved Dvulikiaspis as part of a polytomy with the Middle Devonian Achanarraspis and a clade of the remaining Devonian diploaspidids [1,3]. Loganamaraspis resolves as the sister taxon to all diploaspidids, based predominantly on its supposed possession of a pediform appendage VI [7], although restudy of the only known specimen has been unable to confirm the morphology of the appendages (DJ Marshall pers. ...
... 8a, d and 9a), suggesting that the anteriorly-positioned, medially curving paired incisions described from the buckler plate of Chasmataspis may in fact represent the anterior of a partially displaced plate [5]. The ventral plate of Hoplitaspis bears a strong similarity to a structure in the Devonian chelicerate Houia that has been interpreted as an enlarged metastoma [3]. Originally considered a horseshoe crab [42], the discovery of a broad ventral plate with an anterior medial notch led to the interpretation of Houia as the sister taxon to a clade Table 3 Hoplitaspis hiawathai prosomal appendage II & III measurements Specimen Appendage Podomeres Coxa 2 3 4 5 6 7 8 UWGM 1840A ...
Background
Chasmataspidids are a rare group of chelicerate arthropods known from 12 species assigned to ten genera, with a geologic range extending from the Ordovician to the Devonian. The Late Ordovician (Richmondian) fauna of the Big Hill Lagerstätte includes a new species of chasmataspidid represented by 55 specimens. This taxon is only the second chasmataspidid described from the Ordovician and preserves morphological details unknown from any of the previously described species.
Results
The new chasmataspidid species is described as Hoplitaspis hiawathai gen. et sp. nov.. Comparison with all other known chasmataspidids indicates that Hoplitaspis occupies an intermediate morphological position between the Ordovician Chasmataspis and the Silurian-Devonian diploaspidids. While the modification of appendage VI into a broad swimming paddle allies Hoplitaspis to the Diploaspididae, the paddle lacks the anterior ‘podomere 7a’ found in other diploaspidids and shows evidence of having been derived from a Chasmataspis-like chelate appendage. Other details, such as the large body size and degree of expression of the first tergite, show clear affinities with Chasmataspis, providing strong support for chasmataspidid monophyly.
Conclusions
The large body size and well-developed appendage armature of Hoplitaspis reveals that chasmataspidids occupied a greater breadth of ecological roles than previously thought, with the abundance of available specimens indicating that Hoplitaspis was an important component of the local community. The miniaturization and ecological limiting of diploaspidids potentially coincides with the major radiation of eurypterids and may suggest some degree of competition between the two groups. The geographic distribution of chasmataspidid species suggests the group may have originated in Laurentia and migrated to the paleocontinents of Baltica and Siberia as tectonic processes drew the paleocontinents into close proximity.
... The prevalence of cuticular UV fluorescence has not been explored among chelicerate orders other than scorpions, and its phylogenetic significance is unknown. Chelicerate phylogeny is controversial (Jones et al., 2007;Shultz, 2007;Dunlop, 2010;Pepato, da Rocha & Dunlop, 2010;Arabi et al., 2012;Legg, Sutton & Edgecombe, 2013;Dunlop, Borner & Burmester, 2014;Garwood & Dunlop, 2014;Sharma et al., 2014;Lamsdell et al., 2015a;Selden, Lamsdell & Qi, 2015;Lamsdell, 2016). Nevertheless, sea scorpions (Eurypterida), an extinct chelicerate order that appeared in the Ordovician and disappeared with the Permian extinction (Lamsdell et al., 2015b), are consistently retrieved as the sister group to Arachnida, with horseshoe crabs (Xiphosura) forming the sister group to this clade (Shultz, 2007;Lamsdell et al., 2015a;Selden et al., 2015;Lamsdell, 2016). ...
... Chelicerate phylogeny is controversial (Jones et al., 2007;Shultz, 2007;Dunlop, 2010;Pepato, da Rocha & Dunlop, 2010;Arabi et al., 2012;Legg, Sutton & Edgecombe, 2013;Dunlop, Borner & Burmester, 2014;Garwood & Dunlop, 2014;Sharma et al., 2014;Lamsdell et al., 2015a;Selden, Lamsdell & Qi, 2015;Lamsdell, 2016). Nevertheless, sea scorpions (Eurypterida), an extinct chelicerate order that appeared in the Ordovician and disappeared with the Permian extinction (Lamsdell et al., 2015b), are consistently retrieved as the sister group to Arachnida, with horseshoe crabs (Xiphosura) forming the sister group to this clade (Shultz, 2007;Lamsdell et al., 2015a;Selden et al., 2015;Lamsdell, 2016). The hyaline layer is commonly preserved in the cuticle of fossil scorpions (Bartram, Jeram & Selden, 1987). ...
... The presence of the hyaline layer within horseshoe crabs and sea scorpions suggests further that cuticular UV fluorescence evolved in the common ancestor of chelicerates or possibly even earlier. Chelicerate phylogeny is controversial (Jones et al., 2007;Shultz, 2007;Dunlop, 2010;Pepato et al., 2010;Arabi et al., 2012;Legg et al., 2013;Garwood & Dunlop, 2014;Sharma et al., 2014;Lamsdell et al., 2015b;Selden et al., 2015;Lamsdell, 2016). Although there is broad agreement that Eurypterida are the sister group to Arachnida and that Xiphosura are the sister group to euchelicerates, the position of scorpions is debatable, with analyses retrieving them either as the sister group of all other arachnids (Jones et al., 2007;Legg et al., 2013;Garwood & Dunlop, 2014;Lamsdell et al., 2015b;Selden et al., 2015;Lamsdell, 2016), as part of a Dromopoda clade, comprising camel spiders, harvestmen and pseudoscorpions, nested within Arachnida (Pepato et al., 2010;Sharma et al., 2014), or in an unresolved position (Shultz, 2007;Dunlop, 2010;Arabi et al., 2012). ...
The cuticle of scorpions (Chelicerata: Arachnida) fluoresces under long-wave ultraviolet (UV) light due to the presence of beta-carboline and 7-hydroxy-4-methylcoumarin in the hyaline layer of the exocuticle. The adaptive significance of cuticular UV fluorescence in scorpions is debated. Although several other chelicerate orders (e.g. Opiliones and Solifugae) have been reported to fluoresce on exposure to UV light, the prevalence of cuticular UV fluorescence has not been confirmed beyond scorpions. A systematic study of living chelicerates revealed that UV fluorescence of the unsclerotized integument is ubiquitous across Chelicerata, whereas only scorpions and horseshoe crabs (Xiphosura) exhibit cuticular UV fluorescence. Scanning electron microscopy and histological sectioning confirmed the presence of a hyaline layer in taxa exhibiting cuticular fluorescence. The hyaline layer is absent in all other chelicerates except sea scorpions (Eurypterida) in which a taphonomically altered hyaline layer, that may have fluoresced under UV light, was observed in exceptionally preserved cuticle. Cuticular UV fluorescence appears to be associated with the presence of a hyaline layer, as has long been recognized in scorpions, and may be plesiomorphic among chelicerates. The presence of a hyaline layer in horseshoe crabs and sea scorpions suggests that several putative functions for cuticular UV fluorescence in scorpions can be discounted.
... These two species possess elongate chelicerae and a peculiar limb arrangement in the prosoma with biramous appendages comprising stenopodous exopods. Since their description, they have been consistently retrieved as basal euchelicerates in phylogenetic analyses 3,4,6,12,18,19 . ...
... The second set of Bayesian phylogenetic analyses ( Fig. 6 and Supplementary Fig. 6 and 11) were performed on the data matrix from Lamsdell 4 , modified by removing Fuxianhuia protensa and Willwerathia laticeps from the data matrix (given its uncertain status as basal euarthropod in the first case 55 and as euchelicerates in the second 56,57 ), using Yohoia tenuis as outgroup, and adding Setapedites abundantis, Dibasterium durgae (based on 3,17 ), Habelia optata (based on 12 ), and Mollisonia plenovenatrix (based on 6 ). Two versions of this matrix have been analysed to test different characters codings for Mollisonia plenovenatrix (see Supplementary Notes for list of modified codings and Supplementary Data 4 and 5 for phylogenetic characters and matrix). ...
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.
... The Xiaxishancun Formation is c. 51 m thick and the bottom conformably overlies the black fissile shale of the Yulongsi Formation; its top is also conformable with a purple sandstone of the Xitun Formation. The Xiaxishancun Formation consists mainly of continental deposits characterized by yellow sandstone and green shale, which yield abundant fish remains (Lu et al. 2017), some primary plant fossils (Xue, 2012) and euchelicerates (Lamsdell et al. 2013b;Selden et al. 2015) Based on palynological data and carbon isotope (δ 13 C org ) analyses, the Xiaxishancun Formation is considered to be of Lochkovian age (Hao et al. 2007;Zhao et al. 2011Zhao et al. , 2015Zhao et al. , 2021. The miospore assemblage identified is the Streelispora newportensis -Chelinospora cassicula Assemblage Zone (Fang et al. 1994;Hao et al. 2007) which approximately corresponds to the Emphanisporites micrornatus -Streelispora newportensis Assemblage Zone of the Lochkovian age (Richardson & McGregor, 1986;Fang et al. 1994;Hao et al. 2007). ...
... Like the Willwerath Lagerstätte, beside the eurypterids Pterygotus wanggaii n.sp. and Parahughmilleria fuea n. sp., the euchelicerate Houia yueya Selden et al. 2015) and plant Zosterophyllum xishanense (Hao et al. 2007;Xue, 2012) are also present in the Xiaxishancun Formation. The Willwerath Lagerstätte is characterized by grey silty mudstones and muddy siltstones interbedded with fine sandstones, and the palaeoenvironment is considered as marginal marine; the lithological combination and marine tidal flat habitat of the Xiaxishancun Formation concurs with this. ...
Two new eurypterids, a pterygotid Pterygotus wanggaii n. sp. and an adelophthalmoid Parahughmilleria fuea n. sp., are described from the Early Devonian (Lochkovian) Xiaxishancun Formation of Yunnan province, southwest China. This discovery represents the first record of Parahughmilleria from Gondwana and the first Pterygotus from China. Pterygotus wanggaii n. sp. is characterized by the first primary denticles (d1,d1′) being located on the middle part of the cheliceral ramus and third primary denticles (d3,d3′) elongate, even longer than the first primary denticles. Parahughmilleria fuea n. sp. is differentiated by being a large Parahughmilleria with strongly developed lateral epimera from tergites T4 to T12. These discoveries not only extend the geographical extent of the genera Pterygotus and Parahughmilleria from Euramerica to SW China, but also give insight into the similarity of ecosystem structures across the Early Devonian world. In addition, based on previous studies, the new discoveries further support the hypothesis that eurypterids underwent a crisis during the Silurian–Devonian boundary interval.
... The so-called true horseshoe crabs (Order Xiphosurida), are one of two extant marine chelicerates and have an exceptional fossil record ranging from the Lower Ordovician to today; as such have been a source of ongoing biological and palaeontological research (Størmer, 1955;Eldredge, 1974;Shuster Jr., 1982;Dunlop and Selden, 1998;Shultz, 2001;Rudkin and Young, 2009;Lamsdell, 2016;Bicknell et al., 2018aBicknell et al., , 2018bBicknell et al., , 2019c. Palaeontological research has focused on their exceptionally long fossil record (Van Roy et al., 2010, 2015 and apparent morphological conservatism from the Jurassic onwards (Fisher, 1984;Kin and Błażejowski, 2014;Błażejowski, 2015;Bicknell et al., 2018cBicknell et al., , 2019aBicknell et al., , 2019d. However, as horseshoe crab fossils are represented by mostly single observations (outside of Coal Measures formations and the Solnhofen-like deposits, Anderson, 1994;Kin and Błażejowski, 2014;Haug and Rötzer, 2018;Haug and Haug, 2020), fossil horseshoe crabs have been examined sporadically over the last two centuries (Bicknell et al., 2019b;Bicknell and Pates, 2020). ...
... 3-7), it seems that no combination of features can unite the group beyond the overall Bauplan of a prosoma, opisthosoma (a thoracetron in crown group taxa) and telson. The unfused opisthosoma and freely articulating opisthosomal segments is a unifying feature of most stem xiphosurids (Selden et al., 2015), but this is not observed in Lunataspis aurora and possibly Elleria morani. ...
Xiphosura, the so-called horseshoe crabs, are a group of extant marine chelicerates that have a fossil record extending back to the Lower Ordovician. Their temporal range, coupled with a possible record of bradytelic evolution has made them a focus of palaeontological consideration for over two centuries. As such, there are an array of taxa erected during the 20 th century that have remained unexamined in light of new material and revised phylogenetic theories. Stem xiphosurids in particular require further research to uncover the diversity and disparity of non-crown group horseshoe crabs. To align with the recent increase in mid to late Paleozoic horseshoe crab research, we reconsider the stem xiphosurids-Kasibelinurus‖ randalli and-Bellinurus‖ alleganyensis from the Late Devonian of Pennsylvania and New York State. We conclude that they are synonyms and belong in neither Kasibelinurus nor Bellinurus, and the new genus, Patesia, is erected to accommodate them. Reconsidering stem xiphosurids highlights that particularly in the Late Devonian there was a high disparity of form. One such taxon may represent the morphological stock that gave rise to the Carboniferous taxa and the first xiphosurid radiation.
... Cyamocephalus, Limuloides, Pasternakevia, and Pseudoniscus have been placed into Planaterga (Figures 6-9; Lamsdell, 2013). Synziphosurines are characterized by large prosomal shields, unfused opisthosoma with nine to 11 segmented and expressed tergites (Størmer, 1934(Størmer, , 1955Rudkin et al., 2008;Lamsdell, 2013;Selden et al., 2015). In extreme cases, the three most posterior tergites form a narrow postabdominal (pretelson) section leading to a styliform telson. ...
... Synziphosurines inhabited marine to marginal marine environments, and the general lack of thick prosomal margin suggests that the group may not have burrowed, and instead potentially moved above the substrate (Størmer, 1952;Bergström, 1975;Stürmer and Bergström, 1981;Lamsdell et al., 2013). Affinities of synziphosurines are actively debated due to the few useful synapomorphies that have been identified to date (Anderson et al., 1998), which has resulted in an unnatural grouping of assorted stem euchelicerates (Krzeminski et al., 2010;Lamsdell, 2013Lamsdell, , 2016Lamsdell and Mckenzie, 2015;Selden et al., 2015). To build on the phylogenetic work presented in Lamsdell (2013), in which Lamsdell highlighted that Eichwald, 1854;Woodward, 1866Woodward, , 1867Zittel, 1881;Vogdes, 1917;Eldredge, 1974;Bergström, 1975;Novozhilov, 1991; Figure 6 Limuloides horridus (Woodward, 1872) Bunodidae Leintwardine Formation, England, UK Silurian Marine (sensu Gladwell, 2018) Woodward, 1872 Figure 7H Limuloides limuloides (Woodward, 1865) Bunodidae Leintwardine Formation, England, UK Silurian Marine (sensu Gladwell, 2018) Woodward, 1865, 1866, 1867Zittel, 1881;Gaskell, 1908;Vogdes, 1917;Størmer, 1955;Bergström, 1975;Novozhilov, 1991; Figures 7A-C "Bunaia" heintzi Størmer, 1934a Unspecified Ringerike Sandstone, Norway Silurian Marine Størmer, 1934Størmer, , 1955Novozhilov, 1991 Figure 8E Bunaia woodwardi Clarke, 1919 Unspecified Vernon Formation, New York, USA Silurian Marine Clarke, 1919;Eldredge, 1974;Selden and Nudds, 2008;Rudkin and Young, 2009 Nieszkowski, 1858;Woodward, 1866Woodward, , 1867Vogdes, 1917;Eldredge, 1974;Bergström, 1975 ...
Horseshoe crabs are an iconic group of extant chelicerates, with a stunning fossil record that extends to at least the Lower Ordovician (~480 million years ago). As such, the group has retained significant biological and palaeontological interest. The sporadic nature of descriptive and systematic research into fossil horseshoe crabs over the last two centuries has spread information on the group across more than 200 texts dating from the early nineteenth century to the present day. We present the most comprehensive pictorial atlas of horseshoe crabs to date to pool these important data together. This review highlights taxa such as Bellinurus lacoei and Limulus priscus that have never been documented with photography. Furthermore, key morphological features of the true horseshoe crab (Xiphosurida) families—Austrolimulidae, Belinuridae, Limulidae, Paleolimulidae, and Rolfeiidae—are described. The evolutionary history of horseshoe crabs is reviewed and the current issues facing any possible biogeographic work are presented. Four major future directions that should be adopted by horseshoe crab researchers are outlined. We conclude that this review provides the basis for innovative geographic and geometric morphometric studies needed to uncover facets of horseshoe crab evolution.
... Cyamocephalus, Limuloides, Pasternakevia, and Pseudoniscus have been placed into Planaterga (Figures 6-9; Lamsdell, 2013). Synziphosurines are characterized by large prosomal shields, unfused opisthosoma with nine to 11 segmented and expressed tergites (Størmer, 1934(Størmer, , 1955Rudkin et al., 2008;Lamsdell, 2013;Selden et al., 2015). In extreme cases, the three most posterior tergites form a narrow postabdominal (pretelson) section leading to a styliform telson. ...
... Synziphosurines inhabited marine to marginal marine environments, and the general lack of thick prosomal margin suggests that the group may not have burrowed, and instead potentially moved above the substrate (Størmer, 1952;Bergström, 1975;Stürmer and Bergström, 1981;Lamsdell et al., 2013). Affinities of synziphosurines are actively debated due to the few useful synapomorphies that have been identified to date (Anderson et al., 1998), which has resulted in an unnatural grouping of assorted stem euchelicerates (Krzeminski et al., 2010;Lamsdell, 2013Lamsdell, , 2016Lamsdell and Mckenzie, 2015;Selden et al., 2015). To build on the phylogenetic work presented in Lamsdell (2013), in which Lamsdell highlighted that Eichwald, 1854;Woodward, 1866Woodward, , 1867Zittel, 1881;Vogdes, 1917;Eldredge, 1974;Bergström, 1975;Novozhilov, 1991; Figure 6 Limuloides horridus (Woodward, 1872) Bunodidae Leintwardine Formation, England, UK Silurian Marine (sensu Gladwell, 2018) Woodward, 1872 Figure 7H Limuloides limuloides (Woodward, 1865) Bunodidae Leintwardine Formation, England, UK Silurian Marine (sensu Gladwell, 2018) Woodward, 1865, 1866, 1867Zittel, 1881;Gaskell, 1908;Vogdes, 1917;Størmer, 1955;Bergström, 1975;Novozhilov, 1991; Figures 7A-C "Bunaia" heintzi Størmer, 1934a Unspecified Ringerike Sandstone, Norway Silurian Marine Størmer, 1934Størmer, , 1955Novozhilov, 1991 Figure 8E Bunaia woodwardi Clarke, 1919 Unspecified Vernon Formation, New York, USA Silurian Marine Clarke, 1919;Eldredge, 1974;Selden and Nudds, 2008;Rudkin and Young, 2009 Nieszkowski, 1858;Woodward, 1866Woodward, , 1867Vogdes, 1917;Eldredge, 1974;Bergström, 1975 ...
... We present new information on the microstructure of gnathobasic spines in Limulus polyphemus to determine whether the gnathobases of the Silurian eurypterid Eurypterus tetragonophthalmus Fischer, 1839 from Estonia, and the Cambrian artiopodan Sidneyia inexpectans from Canada have similar structure and function. Eurypterus tetragonophthalmus was chosen because of the threedimensional preservation of appendage specimens that have been isolated from the carbonate host rock, allowing the spines to be sectioned (Holm, 1898;Selden, 1981), and the relatively close phylogenetic relationship between eurypterids and xiphosurans (e.g., Legg et al., 2013;Lamsdell, 2013Lamsdell, , 2016Legg, 2014;Garwood and Dunlop, 2014;Selden et al., 2015;Aria and Caron, 2017, in press). While some studies (see Dalingwater, 1975Dalingwater, , 1987Selden, 1981) have detailed certain cuticular microstructures of the gnathobasic spines of E. tetragonophthalmus, no study has sectioned the spines and imaged the internal features using scanning electron microscopy. ...
... 5) produced a similar topology using implied character weighting, but equal character weighting placed S. inexpectans as a stem-group euchelicerate. Other phylogenetic analyses have also demonstrated a close relationship between xiphosurans and eurypterids, but typically show S. inexpectans to be in a more basal position outside of the chelicerates (e.g., Lamsdell, 2013;Selden et al., 2015;Aria and Caron, 2017, in press). Although these analyses differ in terms of taxon sampling, character selection, coding, and methodology, the relatively consistent phylogenetic positions of these three taxa suggests that perhaps gnathobasic spines with thickened fibrous cuticle (as seen in Limulus polyphemus and S. inexpectans) is the plesiomorphic condition for chelicerates and more basal sister taxa within Arachnomorpha (sensu Aria and Caron, in press). ...
Gnathobasic spines are located on the protopodal segments of the appendages of various euarthropod taxa, notably chelicerates. Although they are used to crush shells and masticate soft food items, the microstructure of these spines are relatively poorly known in both extant and extinct forms. Here we compare the gnathobasic spine microstructures of the Silurian eurypterid Eurypterus tetragonophthalmus from Estonia and the Cambrian artiopodan Sidneyiainexpectans from Canada with those of the Recent xiphosuran chelicerate Limulus polyphemus to infer potential variations in functional morphology through time. The thickened fibrous exocuticle in L. polyphemus spine tips enables effective prey mastication and shell crushing, while also reducing pressure on nerve endings that fill the spine cavities. The spine cuticle of E. tetragonophthalmus has a laminate structure and lacks the fibrous layers seen in L. polyphemus spines, suggesting that E. tetragonophthalmus may not have been capable of crushing thick shells, but a durophagous habit cannot be precluded. Conversely, the cuticle of S. inexpectans spines has a similar fibrous microstructure to L. polyphemus, suggesting that S. inexpectans was a competent shell crusher. This conclusion is consistent with specimens showing preserved gut contents containing various shelly fragments. The shape and arrangement of the gnathobasic spines is similar for both L. polyphemus and S. inexpectans, with stouter spines in the posterior cephalothoracic or trunk appendages, respectively. This differentiation indicates that crushing occurs posteriorly, while the gnathobases on anterior appendages continue mastication and push food towards and into the mouth. The results of recent phylogenetic analyses that considered both modern and fossil euarthropod clades show that xiphosurans and eurypterids are united as crown-group euchelicerates, with S. inexpectans placed within more basal artiopodan clades. These relationships suggest that gnathobases with thickened fibrous exocuticle, if not homoplasious, may be plesiomorphic for chelicerates and deeper relatives within Arachnomorpha. This study shows that the gnathobasic spine microstructure best adapted for durophagy has remained remarkably constant since the Cambrian.
... Despite previous doubts as to the validity of the taxon Chasmataspidida (Bergström, 1980;Tetlie & Braddy, 2004), it is now considered to be a monophyletic group diagnosed by the possession of an ankylosed four-segmented 'buckler'. This group resolves phylogenetically as sister to a clade comprising eurypterids and arachnids Selden, Lamsdell & Liu, 2015;Lamsdell, 2016). Two families of chasmataspidid are recognized: Chasmataspididae, comprising the single genus Chasmataspis from the Ordovician of Tennessee (Dunlop, Anderson & Braddy, †Author for correspondence: jlamsdell@amnh.org ...
... This morphology is almost certainly the derived condition, as a long postabdomen is present in the older more xiphosurid-like Ordovician Chasmataspis (Dunlop, Anderson & Braddy, 2004). Phylogenetic analyses (summarized in Fig. 4) Selden, Lamsdell & Liu (2015) with ranges correlated to the geologic timescale. The hypothesized phylogenetic position of Diploaspis praecursor is indicated with a dashed line. ...
A single specimen of a new species of the chasmataspidid Diploaspis Størmer, 1972 is described from the upper Silurian (Pridoli) Phelps Member of the Fiddlers Green Formation (Bertie Group) in Herkimer County, New York State, USA. Diploaspis praecursor sp. nov. is distinguished by the shape of the posterolateral margins of the buckler, which are drawn out into angular epimera, and by the lack of elongate tubercles on the postabdomen. This discovery increases the taxonomic diversity of the Bertie Group by extending the geographic extent of Diploaspididae into North America. D. praecursor pre-dates previously known species of Diploaspis by more than 10 million years.
... None of these legs were demonstrably biramous in the available descriptions, and Moore et al. (2005a) also commented on the fact that the podomere count of the legs was more like the eurypterid/arachnid condition than the legs of modern horseshoe crabs. It should, however, be noted that Selden et al. (2015) suggested that the limbs of Weinbergina may in fact be biramous; referring to several published figures in support of this interpretation. Further study of the available specimens of Weinbergina is needed to ascertain the veracity of these suggestions, however if they are accurate this would suggest that Weinbergina has a closer relationship to Offacolus and Dibasterium than to Prosomapoda, and would be considered a derivative of the euchelicerate stem lineage. ...
... However, we should note that all other arachnid orders have, in this scenario, reduced their counts to 12 or less (see below). Recently, Lamsdell et al. (2015b) described an intriguing fossil, Winneshiekia youngae Lamsdell et al., 2015, from the Ordovician Winneshiek Lagerst€ atte of the USA as a member of the Dekatriata clade. It preserves a combination of xiphosuran-like features, such as a semicircular head shield with ophthalmic ridges, together with a 13-segmented opisthosoma. ...
... Lamsdell pers. obs.), which bracket eurypterids phylogenetically [34,35], is known to do so. This lack of fluorescence may be original or due to diagenetic change: previous studies of eurypterid cuticle have found it to be almost identical in structure to that of xiphosurids [36], although we know of no other attempts to determine whether eurypterid cuticle fluoresces. ...
... The occurrence of derived eurypterid clades in the Middle Ordovician indicates that Eurypterida either have a longer evolutionary history than previously recognized, extending back into the Cambrian, or underwent an explosive radiation following an Ordovician origin. This would also push back the origin of arachnids, which likely have a sister-group relationship with eurypterids [34,35] from the early Silurian to at least the Middle Ordovician. ...
Eurypterids are a diverse group of chelicerates known from ~250 species with a sparse Ordovician record currently comprising 11 species; the oldest fully documented example is from the Sandbian of Avalonia. The Middle Ordovician (Darriwilian) fauna of the Winneshiek Lagerstätte includes a new eurypterid species represented by more than 150 specimens, including some juveniles, preserved as carbonaceous cuticular remains. This taxon represents the oldest described eurypterid, extending the documented range of the group back some 9 million years.
The new eurypterid species is described as Pentecopterus decorahensis gen. et sp. nov.. Phylogenetic analysis places Pentecopterus at the base of the Megalograptidae, united with the two genera previously assigned to this family by the shared possession of two or more pairs of spines per podomere on prosomal appendage IV, a reduction of all spines except the pair on the penultimate podomere of appendage V, and an ornamentation of guttalate scales, including angular scales along the posterior margin of the dorsal tergites and in longitudinal rows along the tergites. The morphology of Pentecopterus reveals that the Megalograptidae are representatives of the derived carcinosomatoid clade and not basal eurypterids as previously interpreted.
The relatively derived position of megalograptids within the eurypterids indicates that most eurypterid clades were present by the Middle Ordovician. Eurypterids either underwent an explosive radiation soon after their origination, or earlier representatives, perhaps Cambrian in age, remain to be discovered. The available instars of Pentecopterus decorahensis suggest that eurypterids underwent extreme appendage differentiation during development, a potentially unique condition among chelicerates. The high degree of appendage specialization in eurypterids is only matched by arachnids within chelicerates, supporting a sister taxon relationship between them.
... [Color figure can be viewed at wileyonlinelibrary.com] thus, the independent recovery of Poecilophysidea across disparate data types initially appeared to herald a rare moment of consilience in chelicerate phylogeny. However, morphological data sets, rather than supporting any of the above phylogenetic positions, have instead tended to support a sister group relationship of Solifugae with Pseudoscorpiones, forming the clade Haplocnemata ( Figure 1c) (Bicknell et al., 2019;Selden et al., 2015;Shultz, 1990Shultz, , 2007Weygoldt & Paulus, 1979;Wheeler & Hayashi, 1998). This grouping alternatively appears as Apatellata in the literature, based on the putative absence of the patella in this pair of orders (van der Hammen, 1977). ...
Recent advances in higher-level invertebrate phylogeny have leveraged shared features of genomic architecture to resolve contentious nodes across the tree of life. Yet, the interordinal relationships within Chelicerata have remained recalcitrant given competing topologies in recent molecular analyses. As such, relationships between topologically unstable orders remain supported primarily by morphological cladistic analyses. Solifugae, one such unstable chelicerate order, has long been thought to be the sister group of Pseudoscorpiones, forming the clade Haplocnemata, on the basis of eight putative morphological synapomorphies. The discovery, however, of a shared whole genome duplication placing Pseudoscorpiones in Arachnopulmonata provides the opportunity for a simple litmus test evaluating the validity of Haplocnemata. Here, we present the first developmental transcriptome of a solifuge (Titanopuga salinarum) and survey copy numbers of the homeobox genes for evidence of systemic duplication. We find that over 70% of the identified homeobox genes in T. salinarum are retained in a single copy, while representatives of the arachnopulmonates retain orthologs of those genes as two or more copies. Our results refute the placement of Solifugae in Haplocnemata. Subsequent reevaluation of putative interordinal morphological synapomorphies among chelicerates reveals a high incidence of homoplasy, reversals, and inaccurate coding within Haplocnemata and other small clades, as well as Arachnida more broadly, suggesting existing morphological character matrices are insufficient to resolve chelicerate phylogeny.
... 13 iScience Article (STAR Methods), the latter of which has been considered to live in marine habitats. [67][68][69] It can be suggested that the fossil-bearing horizon represents a shallow-water marine environment (Figure 4). Our fossils, then, would play a significant role in extending the record of marine VSMs into the Early Devonian, during which the earliest freshwater testate amoebae occurred. ...
Testate amoebae, a polyphyletic protist group inhabiting a wide variety of extant ecosystems, have evolved as far back as early Neoproterozoic. However, their fossil record is discontinuous and biased towards empty shells. Here, we report an arcellinid testate amoeba species, Cangwuella ampulliformis gen. nov., sp. nov., from a shallow-marine community in the Early Devonian of Guangxi, southwestern China. With the aid of scanning electron microscopy and X-ray micro-tomography, we find that the shell of our testate amoeba contains some acetabuliform structures. Although such configuration does not match exactly with the known internal structures in extant testate amoebae, our fossils highlight the potential of exploring the ecological relationships between fossil testate amoebae and their associated organisms, and increase our knowledge on the diversity of testate amoebae in Early Devonian environments.
... Below the Xiaxishancun Fm. is the Yulongsi (Yulungssu) Fm., which contains rich marine fossils indicative of a Pridoli age. The previously described fossils from the Xiaxishancun Fm. include: abundant fishes including galeaspids, placoderms and sarcopterygians (Zhao and Zhu, 2010;Meng and Gai, 2022); the early vascular plant Zosterophyllum xishanense Hao et al., 2007(Hao et al., 2007; mollusks ; crustaceans, including leperditiids and ostracodes (beyrichiids and cryptophyllus) (Wang et al., 1992;Wang, 1994); another euchelicerate arthropod (Houia yueya Selden et al., 2015)); etc. On the basis of the lithology and fossil content, it is considered that the Xiaxishancun Fm. was deposited in a foreshore-offshore environment (Zhao and Zhu, 2010). ...
Eurypterids constituted an important component of Paleozoic marine ecosystems, but their fossil record has been mainly reported from North America and Europe, and the evolution of this group in other regions such as East Asia remains poorly understood. A new eurypterid, Malongia mirabilis nov. gen., nov. sp., is described from the Lower Devonian (Lochkovian) Xiaxishancun Formation of Qujing, Yunnan, southwestern China. The holotype shows a ventral prosoma with appendages II–VI and part of the opisthosoma. The characters indicating a dolichopterid affinity of the new taxon include: metastoma anterior cordate and basally truncated; appendage III bearing multiple enlarged spines; appendage V nonspiniferous; long appendage VI with expanded distal podomere and narrow triangular VI-7a; subrhomboidal coxa of appendage VI with a rounded principal tooth. Malongia nov. gen. is the fourth genus described within the Dolichopteridae Kjellesvig-Waering and Størmer, 1952, other members of which are Dolichopterus Hall, 1859, Clarkeipterus Kjellesvig-Waering, 1966, and Ruedemannipterus Kjellesvig-Waering, 1966; it represents the only record of this family in South China.
... The assignment of the traces to Chasmataspida is based on impressions of plate-like opisthosomal opercula, one of the characters used to define Euchelicerata (Dunlop et al., 2004) Furthermore, recent phylogenetic analyses of morphology place chasmataspid body fossil species within Euchelicerata, as sister group to eurypterids Legg et al., 2013) or sister group to a clade composed of eurypterids and arachnids (Lamsdell, 2013;Selden et al., 2015). ...
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.
... The phylogenetic position of Sidneyia within Euarthropoda has been revised (e.g. Bruton, 1981;Boxshall, 2004;Selden et al., 2015) and, recent phylogenetic analyses resolved it as the most basal Vicissicaudata within Artiopoda (based on its well-developed tergopleurae and terminal trunk segment bearing a pair of non-walking appendages) (Lerosey-Aubril et al., 2017), or a possible chelicerate (based on the similarities between its exopod morphology and the gill opercula of euchelicerates) (Legg, 2014;Aria and Caron, 2017). ...
Sidneyia inexpectanswas first excavated in Laurentia, and since then Sidneyia and Sidneyia-like euarthropods have been reported from different continents. Here we describe a new Sidneyia-like euarthropod from the Guanshan biota and, its preserved dorsal exoskeleton and appendages resemble those of S. inexpectans. The discovery in the Guanshan biota provides a new data point of Sidneyia-like euarthropod in Cambrian Stage 4 and a second occurrence on the South China plate.
... The phylogenetic position of Sidneyia within Euarthropoda has been revised (e.g. Bruton, 1981;Boxshall, 2004;Selden et al., 2015) and, recent phylogenetic analyses resolved it as the most basal Vicissicaudata within Artiopoda (based on its well-developed tergopleurae and terminal trunk segment bearing a pair of non-walking appendages) (Lerosey-Aubril et al., 2017), or a possible chelicerate (based on the similarities between its exopod morphology and the gill opercula of euchelicerates) (Legg, 2014;Aria and Caron, 2017). ...
Sidneyia inexpectans was first excavated in Laurentia, and since then Sidneyia and Sidneyia-like euarthropods have been reported from different continents. Here we describe a new Sidneyia-like euarthropod from the Guanshan biota and, its preserved dorsal exoskeleton and appendages resemble those of S. inexpectans. The discovery in the Guanshan biota provides a new data point of Sidneyia-like euarthropod in Cambrian Stage 4 and a second occurrence on the South China plate.
... Shpineviolimulus jakovlevi is shown in bold and underlined. Pycnogonida, Synziphosurines, Chasmataspidida, Eurypterida, and Arachnida were collapsed and the phylogeny ofSelden et al. (2015) (seeSupplementary Fig. 1for the full tree)ConclusionRe-examination of specimens from historically important collections is a salient approach for uncovering new morphological and taxonomic information on long-lived groups. Redescribing three horseshoe crab specimens from the Chernyshev Central Museum of Geological Exploration here has uncovered more morphological information on one of the oldest xiphosurids and prompted the naming of a new limuloid genus. ...
Xiphosura are extant marine chelicerates that have displayed apparent morphological conservatism and remarkable survivorship across their~480 Ma fossil record. The easily recognisable features that are known to even the earliest xiphosurans-a crescentic prosoma and often trapezoidal thoracetron (opisthosoma)-have generated debate surrounding their origins and taxonomic significance. This interest resulted in the description of numerous horseshoe crab species during the early to mid-twentieth century, particularly in Russia, that have remained unrevised since their original publications and unconsidered in the light of recent phylogenetic hypotheses. Here, we reexamine the non-belinurid taxa housed within the Chernyshev Central Museum for Geological Exploration in Saint Petersburg. We present the first formal diagnosis of Bellinuroopsis rossicus, erect Shpineviolimulus jakovlevi (Glushenko and Ivanov, 1961) comb. nov., to contain the species formerly described as 'Paleolimulus' jakovlevi and refer Paleolimulus juresanensis to Paleolimulidae incertae sedis. Phylogenetic analysis places S. jakovlevi at the base of Limulina. This position, coupled with a prosomal shield that is notably larger than the thoracetron, and lack of hypertrophied genal spines, suggests that this morphology may represent the ancestral austrolimulid shape. As an extension of this revision, we assessed the general austrolimulid morphological characters and uncovered two possible groups of these bizarre xiphosurids.
... In Euchelicerata the systematic affinities are still in a certain state of flux and the feeding apparatus cannot be properly reconstructed for all early fossil representatives. The feeding apparatus and in general the tagmosis pattern can be reconstructed to a certain extent for the species Offacolus kingi (Sutton et al., 2002), Dibasterium durgae (Briggs et al., 2012) and Weinbergina opitzi (Stürmer & Bergström, 1981;Moore, Briggs & Bartels, 2005), successively splitting off the evolutionary lineage towards the remaining representatives of Euchelicerata (= Prosomapoda sensu Lamsdell, 2013, though unclear if also including W. opitzi depending on the presence of exopods on the walking appendages; see also Selden, Lamsdell & Qi, 2015;Lamsdell, 2016;Dunlop & Lamsdell, 2017). ...
When the evolution of Euarthropoda is discussed, often the lineage of Chelicerata s. str. is assumed to be the more ‘primitive’ or ‘basal’ part of the tree, especially when compared to the other major lineage, Mandibulata. This claimed primitiveness is (at least partly) based on the assumption that different morphological structures are still in an ancestral state and did not evolve any further. One of these sets of structures is the feeding apparatus, which has been stated to be highly advanced in Mandibulata, but not ‘properly’ developed, or at least not to such a high degree, within Chelicerata s. str. In this study, I reinvestigate the feeding apparatus of different ingroups of Euchelicerata, with a focus on assumed ‘primitive’ groups such as Eurypterida and Trigonotarbida. The basis of this study is a large amount of material from different museum collections, with fossils with the entire feeding apparatuses being exceptionally well preserved. Based on high-resolution micro-photography and three-dimensional imaging, it is possible to resolve fine details of the feeding apparatuses. The results make clear that the feeding apparatuses of different ingroups of Euchelicerata are highly specialised and often possess morphological structures comparable to those of the feeding apparatuses of representatives of Mandibulata, apparently convergently evolved. Though the reconstruction of the evolution of the feeding apparatus within Euchelicerata is to a certain degree hampered by unclear phylogenetic relationships, there was clearly a shortening of the feeding apparatus from posterior (i.e. only the anterior appendages being involved in the feeding apparatus), probably linked to the colonisation of land in Arachnida.
... , which itself draws on matrices presented inLamsdell (2013),Selden et al. (2015),Lamsdell and McKenzie (2015), and.The resulting matrix comprises 256 characters coded for 153 taxa and is available in the online MorphoBank database (O'Leary and Kaufman 2012) under the project code p2606 (accessible from http://morphobank.org/permalink/? P2606) as well as in the Supplementary Material. ...
The occupation of new environments by evolutionary lineages is frequently associated with morphological changes. This covariation of ecotype and phenotype is expected due to the process of natural selection, whereby environmental pressures lead to the proliferation of morphological variants that are a better fit for the prevailing abiotic conditions. One primary mechanism by which phenotypic variants are known to arise is through changes in the timing or duration of organismal development resulting in alterations to adult morphology, a process known as heterochrony. While numerous studies have demonstrated heterochronic trends in association with environmental gradients, few have done so within a phylogenetic context. Understanding species interrelationships is necessary to determine whether morphological change is due to heterochronic processes; however, research is hampered by the lack of a quantitative metric with which to assess the degree of heterochronic traits expressed within and among species. Here I present a new metric for quantifying heterochronic change, expressed as a heterochronic weighting, and apply it to xiphosuran chelicerates within a phylogenetic context to reveal concerted independent heterochronic trends. These trends correlate with shifts in environmental occupation from marine to nonmarine habitats, resulting in a macroevolutionary ratchet. Critically, the distribution of heterochronic weightings among species shows evidence of being influenced by both historical, phylogenetic processes and external ecological pressures. Heterochronic weighting proves to be an effective method to quantify heterochronic trends within a phylogenetic framework and is readily applicable to any group of organisms that have well-defined morphological characteristics, ontogenetic information, and resolved internal relationships.
... A phylogenetic analysis resulted in nine most parsimonious trees (CI: 0.468, RI: 0.877, tree length 743). The overall topology of a strict consensus tree produced from these trees (Fig. 8) reflects previous phylogenies that used the same matrix (see 17,18,21,62 ). Our phylogenetic analysis placed Albalimulus bottoni within Limulidae, in a polytomy close to the base of the group. ...
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 phylogenetic analysis resulted in five most parsimonious trees (CI: 0.473, RI: 0.880). The topology of the strict consensus tree is similar to those in Selden et al. (2015), Lamsdell (2016) and Bicknell et al. (2019d) (Figure 6). Tasmaniolimulus patersoni gen. ...
... Even accounting for specimens retained under open nomenclature, xiphosurids have a low standing diversity over the majority of their evolutionary history. Xiphosurids (and xiphosurans, inclusive of supposed horseshoe crabs without a fused thoracetron, in general) attained the peak of their diversity during the Carboniferous when bellinurines invaded and diversified within non-marine coal swamp habitats (Lamsdell, 2016), a trend that has persisted even with the realisation that Palaeozoic synziphosurines are not true horseshoe crabs (Lamsdell, 2013;Selden et al., 2015) and that a number of bellinurine species represent different ontogenetic stages of a single species and thus are synonyms (Haug et al., 2012). Nevertheless, throughout the rest of the Palaeozoic, and into the recent, it has been rare for more than three or four xiphosurid species to co-occur globally. ...
Two specimens of a new species of horseshoe crab, Mesolimulus tafraoutensis sp. nov., are described from the Late Cretaceous (Cenomanian–Turonian) Gara Sbaa Lagerstätte of southeast Morocco. These most likely represent juveniles, as suggested by their small size and possession of a number of characteristics, such as short genal spines, that are characteristic of modern juvenile horseshoe crabs. Despite this, the development of the prosomal keel into a broader cardiac ridge and the scalloped lateral margins of the cardiac lobe clearly place these specimens within Mesolimulus. A further characteristic, the occurrence of only two tubercles on the thoracetron pleural ridges, marks Mesolimulus tafraoutensis sp. nov. as a distinct species. As Mesolimulus resolves phylogenetically as a total group limulid outside of the crown group, the new discovery indicates that stem-lineage limulids persisted into the Cretaceous and co-existed with crown-limulids as they underwent their major radiation.
... Arachnomorpha is considered here as a potential equivalent to total-group Chelicerata (Fig. 1). Chelicerate relationships follow Selden et al. (2015) and Aria and Caron (2017b), with pycnogonids tentatively placed as basalmost chelicerates, considered here as the earliest taxa to possess chelicerae (called idiosyncratically 'chelifores'). Chelicerata forms with habeliidans a clade provisionally called 'Panchelicerata' (sensu Aria & Caron, 2017b), which would include all taxa part of the chelicerate lineage that are not within Artiopoda. ...
As the most diverse animal phylum, Arthropoda expectedly has a complex nomenclatural history. Fossil stem groups scattering diagnostic traits of extant clades further complicate the matter. There have been some recent attempts at reorganizing higher-level arthropod taxonomy based on new fossil interpretations and phylogenetic results. However, I argue that this proposed terminology has introduced unnecessary confusion both for semantic reasons and because the core of these interpretations is being falsified. In this paper, I defend the rightful use of Arthropoda and Euarthropoda as key terms in organizing the major branches of the arthropod evolutionary tree and emend Euarthropoda based on the most recent findings in this field. To help with the description of the tree when dealing with euarthropods that belong outside of the main radiative clade including extant taxa, I propose the name Cenocondyla nom. nov., which represents the least inclusive group containing both Mandibulata and Chelicerata.
... Expressed in a simplified phylogenetic view, Xiphosura (sensu stricto) is the sister group to all remaining euchelicerates (the group which is called Metastomata; see also Shultz 2007;Dunlop 2010;Garwood and Dunlop 2014). In a more complex view, many fossil species traditionally classified as xiphosurans might well be early representatives of the lineage below the split of Xiphosura sensu stricto and Metastomata (e.g., Lamsdell 2013Lamsdell , 2016Lamsdell et al. 2015;Selden et al. 2015). ...
In recent years, methods for investigating the exo-morphology of zoological specimens have seen large improvements. Among new approaches, auto-fluorescence imaging offers possibilities to document specimens under high resolution without introducing additional artifacts as, for example, seen in scanning electron microscopy (SEM) imaging. Additionally, while SEM imaging is restricted to the outer morphology of the current instar, auto-fluorescence imaging can be used to document changes of the outer morphology of the next instar underneath the cuticle of the current instar. Thus, reinvestigating seemingly well known species with these methods may lead to interesting new insights. Here we reinvestigate the late embryonic development of the xiphosuran (“sword tail”) Limulus polyphemus, which is often treated as a proxy for early eucheliceratan evolution. In addition to entire specimens, the appendages of the embryos were dissected off and documented separately with composite-autofluorescence microscopy. Based on these data, we can distinguish six developmental stages. These stages do not match exactly the formerly described stages, as these were largely based on SEM investigation. Our stages appear to represent earlier or later phases within what has in other studies been identified as one stage. This finer subdivision is visible as we can see the developing cuticle under the outer cuticle. In comparison to data from fossil xiphosurans, our results and those of other studies on the ontogeny of L. polyphemus point to a derived mode of development in this species, which argues against the idea of L. polyphemus as a “living fossil.”
... This enforced grouping was causing a broad inversion of polarity for the chelicerate clade, with habeliids being derived within a monophyletic "Xiphosura" and Chasmataspis and Megalograptus retrieved as derived arachnids, sister taxa to scorpions. Although we rejected this topological arrangement as a result of a clear polarization bias, it is worth noting that the phylogenetic position of Scorpiones has a strong impact of the overall topology of Chelicerata, and in particular that morphology seems to support a sister-group relationship of scorpions to other arachnids (as in ref. (5); but see ref. (6)). ...
... This enforced grouping was causing a broad inversion of polarity for the chelicerate clade, with habeliids being derived within a monophyletic "Xiphosura" and Chasmataspis and Megalograptus retrieved as derived arachnids, sister taxa to scorpions. Although we rejected this topological arrangement as a result of a clear polarization bias, it is worth noting that the phylogenetic position of Scorpiones has a strong impact of the overall topology of Chelicerata, and in particular that morphology seems to support a sister-group relationship of scorpions to other arachnids (as in ref. (5); but see ref. (6)). ...
Background:
Chelicerata represents a vast clade of mostly predatory arthropods united by a distinctive body plan throughout the Phanerozoic. Their origins, however, with respect to both their ancestral morphological features and their related ecologies, are still poorly understood. In particular, it remains unclear whether their major diagnostic characters were acquired early on, and their anatomical organization rapidly constrained, or if they emerged from a stem lineage encompassing an array of structural variations, based on a more labile "panchelicerate" body plan.
Results:
In this study, we reinvestigated the problematic middle Cambrian arthropod Habelia optata Walcott from the Burgess Shale, and found that it was a close relative of Sanctacaris uncata Briggs and Collins (in Habeliida, ord. nov.), both retrieved in our Bayesian phylogeny as stem chelicerates. Habelia possesses an exoskeleton covered in numerous spines and a bipartite telson as long as the rest of the body. Segments are arranged into three tagmata. The prosoma includes a reduced appendage possibly precursor to the chelicera, raptorial endopods connected to five pairs of outstandingly large and overlapping gnathobasic basipods, antennule-like exopods seemingly dissociated from the main limb axis, and, posteriorly, a pair of appendages morphologically similar to thoracic ones. While the head configuration of habeliidans anchors a seven-segmented prosoma as the chelicerate ground pattern, the peculiar size and arrangement of gnathobases and the presence of sensory/tactile appendages also point to an early convergence with the masticatory head of mandibulates.
Conclusions:
Although habeliidans illustrate the early appearance of some diagnostic chelicerate features in the evolution of euarthropods, the unique convergence of their cephalons with mandibulate anatomies suggests that these traits retained an unusual variability in these taxa. The common involvement of strong gnathal appendages across non-megacheirans Cambrian taxa also illustrates that the specialization of the head as the dedicated food-processing tagma was critical to the emergence of both lineages of extant euarthropods-Chelicerata and Mandibulata-and implies that this diversification was facilitated by the expansion of durophagous niches.
... The assignment of the traces to Chasmataspida is based on impressions of plate-like opisthosomal opercula, one of the characters used to define Euchelicerata (Dunlop et al., 2004) Furthermore, recent phylogenetic analyses of morphology place chasmataspid body fossil species within Euchelicerata, as sister group to eurypterids Legg et al., 2013) or sister group to a clade composed of eurypterids and arachnids (Lamsdell, 2013;Selden et al., 2015). ...
Vermicularia Lamarck, 1799 is a clade of Miocene–Recent gastropods with an unusual
uncoiled shell morphology. Like other “worm-snails,” they are taxonomically troublesome, and although earlier work affirmed the turritellid affinity of one species, their systematic relationships have not otherwise been previously examined. Here, we present a molecular phylogeny of turritellids, including members of other previously named genera [Mesalia Gray, 1847; Zaria Gray, 1847; Torcula Gray, 1847; Protomella Thiele, 1929; Maoricolpus Finlay, 1926; and the recently named Turritellinella Harzhauser & Landau, 2019, represented by Turritellinella tricarinata (Brocchi, 1814) = “Turritella communis” Risso, 1826], to establish the monophyly of Vermicularia and determine higher level relationships. We provide a revised taxonomy with detailed diagnoses for all known living and fossil species of Vermicularia, including the synonymy of Vermicularia lumbricalis (Linnaeus, 1758) and the commonly used junior synonym Vermicularia knorrii (Deshayes, 1843). The new Plio–Pleistocene species Vermicularia katiae n. sp. is described from the Dominican Republic. The phylogenetic relationships within living and fossil Vermicularia are also established based on morphological characters and with close outgroup turritelliform species chosen based on the molecular phylogeny. Callostracum gracile (Maltzan, 1883) was found to be well nested among the other species and is reassigned to Vermicularia herein as Vermicularia gracilis (Maltzan, 1883). The reconstructed phylogeny indicates that new species arose via cladogenesis much more frequently than by anagenesis, with most lineages originating in the Pliocene, but prior to the complete closure of the Central American Seaway ~ 3.5 million years ago.
Animals breaking away from the sea was a revolutionary event in the evolution of life. Arthropods were the earliest metazoans to move onto land, and although a few Silurian freshwater and/or terrestrial arthropods have been found so far, these records are all from Laurussia. Here, we describe a new freshwater arthropod, Maldybulakia saierensis sp. nov., from the western Junggar, northwest China. Evidence from co-occurring spores and body fossils of plants is presented in support of a Silurian (Pridoli) age for this new Maldybulakia species, alongside palaeosalinity data in support of our interpretation of it having lived in a freshwater environment. The discovery of this species brings forward the earliest appearance of the Maldybulakia, previously known from theDevonian of Kazakhstan and eastern Australia, to the late Silurian. It is the oldest body fossil record of a putatively freshwater arthropod outside Laurussia, and greatly expands their
palaeogeographical distribution. In the middle and late Silurian, the discovery of freshwater arthropods on multiple plates/terranes, as well as their morphological diversity during
this period, suggests that arthropods had left the marine environment by the early Silurian or even earlier.
Xiphosurans are aquatic chelicerates with a fossil record extending into the Early Ordovician and known from a total of 88 described species, four of which are extant. Known for their apparent morphological conservatism, for which they have gained notoriety as supposed ‘living fossils’, recent analyses have demonstrated xiphosurans to have an ecologically diverse evolutionary history, with several groups moving into non-marine environments and developing morphologies markedly different from those of the modern species. The combination of their long evolutionary and complex ecological history along with their paradoxical patterns of morphological stasis in some clades and experimentation among others has resulted in Xiphosura being of particular interest for macroevolutionary study. Phylogenetic analyses have shown the current taxonomic framework for Xiphosura—set out in the Treatise of Invertebrate Paleontology in 1955—to be outdated and in need of revision, with several common genera such as Paleolimulus Dunbar, 1923 and Limulitella Størmer, 1952 acting as wastebasket taxa. Here, an expanded xiphosuran phylogeny is presented, comprising 58 xiphosuran species as part of a 158 taxon chelicerate matrix coded for 259 characters. Analysing the matrix under both Bayesian inference and parsimony optimisation criteria retrieves a concordant tree topology that forms the basis of a genus-level systematic revision of xiphosuran taxonomy. The genera Euproops Meek, 1867, Belinurus König, 1820, Paleolimulus , Limulitella , and Limulus are demonstrated to be non-monophyletic and the previously synonymized genera Koenigiella Raymond, 1944 and Prestwichianella Cockerell, 1905 are shown to be valid. In addition, nine new genera ( Andersoniella gen. nov. , Macrobelinurus gen. nov. , and Parabelinurus gen. nov. in Belinurina; Norilimulus gen. nov. in Paleolimulidae; Batracholimulus gen. nov. and Boeotiaspis gen. nov. in Austrolimulidae; and Allolimulus gen. nov., Keuperlimulus gen. nov., and Volanalimulus gen. nov. in Limulidae) are erected to accommodate xiphosuran species not encompassed by existing genera. One new species, Volanalimulus madagascarensis gen. et sp. nov., is also described. Three putative xiphosuran genera— Elleria Raymond, 1944, Archeolimulus Chlupáč, 1963, and Drabovaspis Chlupáč, 1963—are determined to be non-xiphosuran arthropods and as such are removed from Xiphosura. The priority of Belinurus König, 1820 over Bellinurus Pictet, 1846 is also confirmed. This work is critical for facilitating the study of the xiphosuran fossil record and is the first step in resolving longstanding questions regarding the geographic distribution of the modern horseshoe crab species and whether they truly represent ‘living fossils’. Understanding the long evolutionary history of Xiphosura is vital for interpreting how the modern species may respond to environmental change and in guiding conservation efforts.
The putative xiphosuran Kiaeria Størmer, 1934, from the Late Silurian (Ludlow) of Ringerike, Norway, is redescribed from the holotype and only known specimen as a chasmataspidid chelicerate arthropod. Morphological features such as the presence of a fused buckler of three opisthosomal segments clearly indicate a chasmataspidid affinity, while the size of Kiaeria, along with the occurrence of a raised axial region and expanded anterior articulation, suggest a close phylogenetic relationship to the Ordovician Chasmataspis. As such, Kiaeria represents the first indication that the Chasmataspis-type chasmataspidid morphology persisted alongside the radiation of diploaspidids. This is also only the third chasmataspidid species recorded from the Silurian.
Horseshoe crabs are classic “living fossils”, supposedly slowly evolving, conservative taxa, with a long fossil record back to the Ordovician. The evolution of their exoskeleton is well documented by fossils, but appendage and soft-tissue preservation is extremely rare. Here we analyse details of appendage and soft-tissue preservation in Yunnanolimulus luopingensis, a Middle Triassic (ca. 244 million years old) horseshoe crab from Yunnan Province, SW China. The remarkable preservation of anatomical details including the chelicerae, five pairs of walking appendages, opisthosomal appendages with book gills, muscles, and fine setae permits comparison with extant horseshoe crabs. The close anatomical similarity between the Middle Triassic horseshoe crabs and their recent analogues documents anatomical conservatism for over 240 million years, suggesting persistence of lifestyle. The occurrence of Carcinoscorpius-type claspers on the first and second walking legs in male individuals of Y. luopingensis indicates that simple chelate claspers in males are plesiomorphic for horseshoe crabs, and the bulbous claspers in Tachypleus and Limulus are derived.
The compound eyes of three taxa of Rhenish Lower Devonian eurypterids are examined and compared with those known from other eurypterids and the extant horseshoe crab Limulus polyphemus. The lateral eyes of the small species Rhenopterus diensti, a phylogenetically basal representative of the stylonurine clade, are characterized by a comparatively low number of lenses and high interommatidial angle Δφ (2.8°). The comparatively limited visual capacities of R. diensti are more similar to L. polyphemus than to its closer relatives of the eurypterine clade and perhaps this reflects a progression of lateral eye structure in the evolution of eurypterids as a whole. The number of eye facets in Adelophthalmus sievertsi is higher than that in the supposed ambush predator Acutiramus cummingsi, but lower than that in other ‘swimming’ eurypterids (Eurypterina). Due to poor preservation, no other eye parameters could be analysed in this species, but further morphological attributes and geographical distribution designate the mid-sized A. sievertsi as an able swimmer. A low interommatidial angle Δφ of less than 1° confirms that the visual capacities of Jaekelopterus rhenaniae are in line with an interpretation of this giant species as an active high-level predator. The inferred lifestyles of adult individuals of these three, co-occurring Rhenish eurypterids indicate niche differentiation avoiding to some degree the competition for food in their marginal marine to delta plain transitional habitats.
Xiphosurids are an archaic group of aquatic chelicerate arthropods, generally known by the colloquial misnomer of 'horseshoe crabs'. Known from marine environments as far back as the early Ordovician, horseshoe crabs are generally considered 'living fossils' - descendants of a bradytelic lineage exhibiting little morphological or ecological variation throughout geological time. However, xiphosurids are known from freshwater sediments in the Palaeozoic and Mesozoic; furthermore, the contention that xiphosurids show little morphological variation has never been tested empirically. Attempts to test this are hampered by the lack of a modern phylogenetic framework with which to explore different evolutionary scenarios. Here, I present a phylogenetic analysis of Xiphosurida and explore patterns of morphospace and environmental occupation of the group throughout the Phanerozoic. Xiphosurids are shown to have invaded non-marine environments independently at least five times throughout their evolutionary history, twice resulting in the radiation of major clades - bellinurines and austrolimulids - that occupied novel regions of morphospace. These clades show a convergent ecological pattern of differentiation, speciation and subsequent extinction. Horseshoe crabs are shown to have a more dynamic and complex evolutionary history than previously supposed, with the extant species representing only a fraction of the group's past ecological and morphological diversity.
Euchelicerates were a major component of Palaeozoic faunas, but their basal relationships are uncertain: it has been suggested that Xiphosura-xiphosurids (horseshoe crabs) and similar Palaeozoic forms, the synziphosurines-may not represent a natural group. Basal euchelicerates are rare in the fossil record, however, particularly during the initial Ordovician radiation of the group. Here, we describe Winneshiekia youngae gen. et sp. nov., a euchelicerate from the Middle Ordovician (Darriwilian) Winneshiek Lagerstätte of Iowa, USA. Winneshiekia shares features with both xiphosurans (a large, semicircular carapace and ophthalmic ridges) and dekatriatan euchelicerates such as chasmataspidids and eurypterids (an opisthosoma of 13 tergites). Phylogenetic analysis resolves Winneshiekia at the base of Dekatriata, as sister taxon to a clade comprising chasmataspidids, eurypterids, arachnids, and Houia. Winneshiekia provides further support for the polyphyly of synziphosurines, traditionally considered the stem lineage to xiphosurid horseshoe crabs, and by extension the paraphyly of Xiphosura. The new taxon reveals the ground pattern of Dekatriata and provides evidence of character polarity in chasmataspidids and eurypterids. The Winneshiek Lagerstätte thus represents an important palaeontological window into early chelicerate evolution.
A workflow for digital photography of fossil specimens is presented, in the hope that it will provide helpful for those wishing to set up a facility or enhance their existing setup for illustrations of geological specimens....
Chelicerata is one of the major arthropod groups, characterized by a body divided into two tagmata, prosoma and opisthosoma, while the name Chelicerata refers to the chelicerae, the chelate first pair of appendages. Chelicerates comprise the arachnids (spiders, scorpions, mites, etc.), the extinct eurypterids (sea scorpions) and the xiphosurans (horseshoe crabs). Among Recent groups the limits of the Chelicerata are well defined, i.e. arachnids and xiphosurans, though the position of pycnogonids (sea spiders) remains uncertain (King, 1973). Phylogenetic relationships within the Chelicerata remain unresolved (Weygoldt and Paulus, 1979; van der Hammen, 1989; Shultz, 1990). When fossil taxa are considered, the limits of the Chelicerata become less well constrained. This is especially true of various problematic arthropods: fossils such as aglaspidids (Raasch, 1939), chasmataspids (Caster and Brooks, 1956), Sanctacaris Briggs and Collins, 1988 and other Burgess Shale-type arthropods (see Conway Morris, 1992 for a review).
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 S. 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 limbless 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.
Stoermeropterus conicus (Eurypterida: Eurypterina), from the Telychian (Upper Llandovery, Silurian) of Pentland Hills near Edinburgh, Scotland, is described from material originally assigned to three different species (Nanahughmilleria conica, Drepanopterus bembycoides and Drepanopterus lobatus). Two other existing eurypterid species, Hughmilleria lata from the Wenlock of Norway and Drepanopterus nodosus from the Pridoli of North America are recognized as being congeneric with S. conicus, united principally by their possession of moveable mesosomal spines on the mesosoma, genital spatulae and a bulbous telson ‘boss’. Several characters support the assignment Stoermeropterus to Moselopteridae, the most basal eurypterine clade, including its possession of a pediform appendage VI with a modified ‘podomere’ 7a and the possession of a coxal ‘ear’, which may represent the remains of a much-reduced exopod. Stoermeropterus conicus resolves phylogenetically as the most basal known eurypterine, and can aid in reconstructing the eurypterid ground plan. As such an attempt is made to reconstruct the characteristics that are plesiomorphic for Eurypterida through comparison with basal Eurypterina, Stylonurina, chasmataspidids and synziphosurines. Several characteristics previously thought to be autapomorphies of Stylonurina, such as a three-segmented genital operculum, are now shown to actually be plesiomorphic conditions in respect to Eurypterida as a whole, while other apparently derived characteristics such as an epistoma and genital spatulae may be characters that are present in all eurypterids during the juvenile and are respectively either paedomorphically retained into adulthood or become hypertrophied in various species. Following the identification of a metastoma and genital appendage in some chasmataspidid species, the sole currently known eurypterid autapomorphy is identified as the fusion of the opercular plates of somites VIII and IX. Evolutionary relationships among the traditional ‘merostome’ groups are reviewed, primarily in light of segment articulations and the development of the appendage of somite VII. The concept that synziphosurines may represent a paraphyletic stem grade to a group inclusive of xiphosurids, chasmataspidids, eurypterids, and arachnids is proposed.
The monophyly of the class Xiphosura is critically re-examined. For the first time a phylogenetic analysis of a number of synziphosurine and xiphosurid taxa is performed together with representatives of the other chelicerate orders also included as ingroup taxa. Xiphosura as currently defined is shown to be paraphyletic, and a revised classification is presented. Previous characteristics used to unite the xiphosurids (possessing a fused thoracetron) and a paraphyletic grade of synziphosurines (retaining freely articulating opisthosomal tergites) include the presence of a cardiac lobe, ophthalmic ridges, an axial region of the opisthosoma, and a reduced first opisthosomal segment. All of these characteristics are, however, here shown to be present in other chelicerate groups, leaving Xiphosura without any defining synapomorphies. A number of other characters, including the form of the chelicerae and appendage VII, indicate that xiphosurans may be paraphyletic with respect to a clade consisting of chasmataspidids, eurypterids, and arachnids. What ramifications this has for the evolution of basal chelicerates is briefly discussed, and it is recognized that most of the currently known ‘synziphosurine’ taxa represent offshoots from the main chelicerate lineage with ghost ranges extending into at least the Middle Ordovician.
Drepanopterus pentlandicus Laurie, 1892 is redescribed from the original type material along with previously unfigured specimens. A cleft metastoma is confirmed as a characteristic of the genus, along with the armature of the second and third prosomal appendages being modified into flattened blades, while the species is shown to possess a somewhat enlarged second tergite and lateral prosomal margins that overlap the first opisthosomal tergite. Different ontogenetic stages of D. pentlandicus are described, and reveal that these latter characters develop only later in ontogeny, suggesting that described specimens of Drepanopterus abonensis Simpson, 1951 may represent juveniles. Cladistic analysis of Stylonurina shows the genus Drepanopterus to be monophyletic consisting of D. pentlandicus, D. abonensis and D. odontospathus sp. nov.: it forms a basal clade of mycteropoids. Hibbertopteroidea Kjellesvig-Waering, 1959 is shown to be a junior subjective synonym of Mycteropoidea Cope, 1886, with the synonymy of many of the hibbertopterid genera hypothesised and Hibbertopterus Kjellesvig-Waering, 1959 suggested to represent juvenile specimens of Cyrtoctenus Størmer & Waterston, 1968. Hibbertopterus permianus Ponomarenko, 1985 is transferred to Campylocephalus Eichwald, 1860. The role of heterochrony in the morphological development of the mycteropoid lineage is discussed, with both hibbertopterids and mycteropids suggested to be hypertrophic and pre-displacement peramorphs respectively.
A single specimen of a new species of the synziphosurine Kasibelinurus Pickett, 1993 is described from the Lower Devonian (Lochkovian) Xiaxishancun Formation of Yunnan Province, China. The new species, K. yueya sp. nov., extends the geographic extent of the family Kasibelinuridae from the Australian palaeocontinent to the South China palaeocontinent, and the stratigraphic range back some 50 Ma from Late to Early Devonian.
A megafossil plant assemblage containing three zosterophyll plants (Gen. nov. A, aff. Huia sp. and a unnamed spike) is described from the Lower Devonian Xitun Formation (Lochkovian) of Qujing, Yunnan Province, China, providing new data on the diversity of plant types during the Lochkovian, a time poorly represented by fossil vascular plants. Gen. nov. A has a character combination of naked axes, a diagnostic branching pattern (i.e. K-type branching as well as small lateral branches scattered along the axes) and lateral stalked sporangia. Aff. Huia sp. has ovate and stalked sporangia which are arranged in loose spirals and are reflexed adaxially. Combining the plants previously reported from the Xitun Formation (i.e. Xitunia spinitheca, Zosterophyllum shengfengense and Z. minorstachyum), the Xitun flora is dominated by zosterophylls and exhibits varied vegetative and fertile morphologies. Unlike the coeval plant assemblages in many localities of Laurussia and western Gondwana, which are mainly composed of plants with isotomous branching and terminal sporangia, the Lochkovian flora of South China is dominated by zosterophylls, documenting the existence of a distinct Northwest Gondwanan phytogeographic unit in the Lochkovian and a strong effect of phytogeographical isolation.
The prosomal morphology of Baltoeurypterus tetragonophthalmus (Fischer) from the Baltic Silurian is redescribed and reconstructed. The first eurypterid labrum and new secondary sexual characters of Baltoeurypterus are described. The radially-arranged coxae of Baltoeurypterus were capable of adduction and abduction for food mastication, but not promotor-remotor movements for locomotion. Joint diagrams are presented for the first time for an extinct arthropod. Promotion and remotion of the limbs occurred about subvertical trochanteral pivots, as in all other chelicerates except xiphosurans. Baltoeurypterus probably walked in a “slow” gait; a method of choosing possible gaits for extinct arthropods is outlined. Swimming in Baltoeurypterus was effected by means of a rowing action of the posterior limb pair, which is provided with complex joints for collapsing the paddle during the recovery stroke. The limb arrangement and joint mechanisms of Baltoeurypterus are intermediate between those of the xiphosurans and the arachnids. It is possible that a sister relationship exists between the eurypterids and some arachnid groups, which would render Merostomata and Arachnida unnatural assemblages.
A new species of eurypterid, Paraeurypterus anatoliensis gen. et sp. nov., is described from the Upper Ordovician (Katian) Şort Tepe Formation of southeast Turkey. The single specimen, preserving the carapace, mesosoma and fragments of appendages, appears morphologically intermediate between the eurypteroid families Dolichopteridae and Eurypteridae. P. anatoliensis retains the plesiomorphic conditions of crescentic eyes with enlarged palpebral lobes and a quadrate carapace with ornamentation consisting of small pustules but also displaying the derived characteristics of genal facets and a row of large acicular scales across the posterior of each tergite. Phylogenetic analysis incorporating each of the major eurypterine clades and all Eurypterina having a three-segmented genital operculum (the triploperculate condition) resolves eurypteroids to be an unnatural group, with Dolichopteridae and Eurypteridae forming part of a grade leading to diploperculate Eurypterina. P. anatoliensis is intermediate between the two eurypteroid families, as is ‘Eurypterus’ minor from the Pentland Hills of Scotland, which is shown to be a distinct genus and assigned to Pentlandopterus gen. nov. Using the phylogenetic topology to infer ghost ranges for each of the major eurypterid clades reveals that the majority of eurypterid superfamilies must have originated by the Katian, indicating a largely unsampled record of Ordovician eurypterids. The occurrence of poor dispersers such as Paraeurypterus in the Ordovician of Gondwana is puzzling, and it has been suggested that they dispersed to the continent during periods of sea level lowstand in the Sandbian and Hirnantian, however this does not explain the lack of Ordovician species in North America and Europe, given the well-sampled nature of these continents, and an alternative is proposed whereby eurypterids originated in Gondwana and radiated out to Laurentia and Baltica in the late Ordovician and early Silurian, thus explaining their sudden appearance in the European and North American rock record.
Stylonurid eurypterids (Arthropoda: Chelicerata) include some of the largest known arthropods – bizarre sweep-feeding hibbertopterids from the Carboniferous to end-Permian. New material of Drepanopterus abonensis, a stylonurid from the Late Devonian (Famennian) of Portishead, south-west England, offers key insights into this genus and its affinities. A redescription utilising the new material enables D. abonensis to be assigned as basal member of the Superfamily Hibbertopteroidea, the large-sweep-feeding forms, possessing a cleft metastoma and blades (modified blunt spines) on their anterior prosomal appendages. D. abonensis also shares characters such as a clavate telson and median ridge on the carapace with the proposed hibbertopteroid sister group the Kokomopteroidea. Hibbertopteroid eurypterids are the most long-ranging stylonurids, surviving the decline and extinction of the other eurypterid families in the Late Devonian, their survival probably because of their sweep-feeding mode of life, which was not in direct competition with their eurypterine relatives and other predators.
Background
Few studies on eurypterids have taken into account morphological changes that occur throughout postembryonic development. Here two species of eurypterid are described from the Pragian Beartooth Butte Formation of Cottonwood Canyon in Wyoming and included in a phylogenetic analysis. Both species comprise individuals from a number of instars, and this allows for changes that occur throughout their ontogeny to be documented, and how ontogenetically variable characters can influence phylogenetic analysis to be tested.
Results
The two species of eurypterid are described as Jaekelopterus howelli (Kjellesvig-Waering and Størmer, 1952) and Strobilopterus proteus sp. nov. Phylogenetic analysis places them within the Pterygotidae and Strobilopteridae respectively, both families within the Eurypterina. Jaekelopterus howelli shows positive allometry of the cheliceral denticles throughout ontogeny, while a number of characteristics including prosomal appendage length, carapace shape, lateral eye position, and relative breadth all vary during the growth of Strobilopterus proteus.
Conclusions
The ontogeny of Strobilopterus proteus shares much in common with that of modern xiphosurans, however certain characteristics including apparent true direct development suggest a closer affinity to arachnids. The ontogenetic development of the genital appendage also supports the hypothesis that the structure is homologous to the endopods of the trunk limbs of other arthropods. Including earlier instars in the phylogenetic analysis is shown to destabilise the retrieved topology. Therefore, coding juveniles as individual taxa in an analysis is shown to be actively detrimental and alternative ways of coding ontogenetic data into phylogenetic analyses should be explored.
The first well-resolved phylogeny of stylonurine eurypterids (30 taxa, 58 characters) is presented, prompting a taxonomic revision at the familial and superfamilial levels. The monophyletic suborder Stylonurina consists of four superfamilies: Rhenopteroidea, Stylonuroidea, Kokomopteroidea and Hibbertopteroidea. The enigmatic hibbertopterids – large sweep-feeding forms from the Carboniferous to end-Permian – are therefore demonstrated to be an in-group Stylonurina clade, within Eurypterida, in contrast to some earlier hypotheses. Furthermore, the genus Drepanopterus is shown to be polyphyletic: ‘Drepanopterus’ bembycoides is transferred to Moselopteridae fam. nov. along with Moselopterus and Vinetopterus at the base of the Eurypterina, defined by their possession of a pediform appendage VI bearing a modified podomere 7a. Evolution towards a sweep-feeding mode of life occurred independently in stylonuroids and hibbertopteroids, involving either multiple rows of fixed spines on the prosomal appendages (in stylonuroids) or paired movable flattened spines (‘blades’) on the prosomal appendages alongside a posteriorly cleft metastoma and coxal laden (in hibbertopteroids). The Stylonurina have a relatively poor fossil record (RCI 15%), when compared to more derived Eurypterina clades (e.g. Adelophthalmoidea RCI 66%; Pterygotoidea RCI 53%), but is relatively more complete than basal Eurypterina clades (RCI -21%). The fit between phylogeny and stratigraphical occurrences of stylonurid taxa is good (SCI 0.65 and GER 0.77, with only 0.3% of 1000 randomisation tests yielding greater congruence; GER* 0.995), and generic-level collector curves of the Stylonurina and Eurypterina show no major discrepancies in their sampling histories. These differences could be explained by geographic collection bias, taxa having different habitat preferences (and hence fossilisation potential), and ontogenetic factors: these results support previous suggestions that stylonurine eurypterids are oversplit.
Zusammenfassung und Literatur folgen am Schluß des 2. Teiles der Untersuchung.
1997 04 15: Opisthosomal fusion and phylogeny of Palaeozoic Xiphosura. Lethaia, Vol. 30, pp. 19-31. Oslo. ISSN 0024-1 164. Fusion of opisthosomal tergites to form a thoracetron has previously been considered a char-acteristic of the xiphosuran superfamilies Euproopoidea Eller, 1938, and Limuloidea Zittel, 1885. Evidence is presented here that fusion also occurs in Bellinuroidea Zittel & Eastman, 1913. Results of a cladistic analysis of Palaeozoic xiphosuran genera indicate that Synziphosu-rina Packard, 1886, is a paraphyletic assemblage of stem-group Xiphosura. Superfamily Pale-olimulidae superfam. nov. is erected for families Paleolimulidae Raymond, 1944, and Moravu-ridae Pfibyl, 1967. OChelicerata, cladistics, evolution. Horseshoe crabs (Xiphosura) have been held up as the prime example of a 'living fossil' group -one that has remained conservative in morphology and at low diver-sity for much of its geological history (Fisher 1984). The distinctive features of a large carapace concealing the pro-soma1 appendages, fused opisthosomal tergites (a thora-cetron), and a styliform tail spine, can be traced back to Carboniferous times, when the group was apparently much more diverse than today. Three distinct groups of Carboniferous Xiphosura are generally recognized, the superfamilies Bellinuroidea Zittel & Eastman, 19 13, Euproopoidea Eller, 1938, and Limuloidea Zittel, 1885. The latter two groups share the possession of a thoracet-ron, whilst bellinuroids have hitherto been distinguished by their free opisthosomal tergites. During a restudy of upper Palaeozoic Xiphosura by one of us (LIA), it was dis-covered that all specimens that could be referred to Bell-inurus Pictet, 1846 (Fig. 2J), and also Bellinuroopsis Chernyshev, 1933 (Fig. 2M), have all post-opercular tergites fused into a thoracetron (Anderson 1995). This observation has important consequences for xiphosuran taxonomy and phylogeny. We present here the evidence for fusion in the opisthosoma of bellinuroids, followed by a phylogenetic analysis of late Palaeozoic Xiphosura to generic level. The character matrix relies to a large extent on new information resulting from restudy of large num-bers of late Palaeozoic xiphosurans, including all available holotypes. Further details are given by Anderson (1996), and taxonomic revisions will be published elsewhere. A major conclusion of this study is that Synziphosurina Packard, 1886, is a paraphyletic assemblage and the name must be abandoned; xiphosurans with a thoracetron (and other synapomorphies) are united herein in the order Xiphosurida Latreille, 1802.
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.
The renowned soft-bodied faunas of the Cambrian period, which include the Burgess Shale, disappear from the fossil record in the late Middle Cambrian, after which the Palaeozoic fauna dominates. The disappearance of faunas of Burgess Shale type curtails the stratigraphic record of a number of iconic Cambrian taxa. One possible explanation for this loss is a major extinction, but more probably it reflects the absence of preservation of similar soft-bodied faunas in later periods. Here we report the discovery of numerous diverse soft-bodied assemblages in the Lower and Upper Fezouata Formations (Lower Ordovician) of Morocco, which include a range of remarkable stem-group morphologies normally considered characteristic of the Cambrian. It is clear that biotas of Burgess Shale type persisted after the Cambrian and are preserved where suitable facies occur. The Fezouata biota provides a link between the Burgess Shale communities and the early stages of the Great Ordovician Biodiversification Event.
Abstract- Because they are designed to produced just one tree, neighbor-joining programs can obscure ambiguities in data. Ambiguities can be uncovered by resampling, but existing neighbor-joining programs may give misleading bootstrap frequencies because they do not suppress zero-length branches and/or are sensitive to the order of terminals in the data. A new procedure, parsimony jackknifing, overcomes these problems while running hundreds of times faster than existing programs for neighbor-joining bootstrapping. For analysis of large matrices, parsimony jackknifing is hundreds of thousands of times faster than extensive branch-swapping, yet is better able to screen out poorly-supported groups.
The recently-developed statistical method known as the "bootstrap" can be used to place confidence intervals on phylogenies. It involves resampling points from one's own data, with replacement, to create a series of bootstrap samples of the same size as the original data. Each of these is analyzed, and the variation among the resulting estimates taken to indicate the size of the error involved in making estimates from the original data. In the case of phylogenies, it is argued that the proper method of resampling is to keep all of the original species while sampling characters with replacement, under the assumption that the characters have been independently drawn by the systematist and have evolved independently. Majority-rule consensus trees can be used to construct a phylogeny showing all of the inferred monophyletic groups that occurred in a majority of the bootstrap samples. If a group shows up 95% of the time or more, the evidence for it is taken to be statistically significant. Existing computer programs can be used to analyze different bootstrap samples by using weights on the characters, the weight of a character being how many times it was drawn in bootstrap sampling. When all characters are perfectly compatible, as envisioned by Hennig, bootstrap sampling becomes unnecessary; the bootstrap method would show significant evidence for a group if it is defined by three or more characters.
The ecdysed carapace of a xiphosuran from the Late Devonian Mandagery Sst. of central western New South Wales is described as Kasibelinurus amicorum gen. et sp. nov., and the Kasibelinuridae established to receive it and three other Late Devonian species from Europe and North America. Interrelationships of later Palaeozoic suprafamilial groups are discussed. -Author
The main features of the phylogeny program TNT are discussed. Windows versions have a menu interface, while Macintosh and Linux versions are command-driven. The program can analyze data sets with discrete (additive, non-additive, step-matrix) as well as continuous ...
Synopsis
Lower Carboniferous eurypterids are rare, and consequently the Scottish eurypterid fauna is of importance, in a world setting, because of the number of species of that age which it contains. Since the Scottish forms were originally described, great advances have been made in our knowledge of the Eurypterida which have made necessary the present critical re-examination of all available Scottish Carboniferous eurypterid specimens. Five species, belonging to four genera, are described and illustrated. Fresh evidence of the anatomy of Campylocephalus scouleri (Hibbert) is presented and the wider issues of eurypterid morphology raised by homology with this specialised species are discussed.
Because they are designed to produced just one tree, neighbor-joining programs can obscure ambiguities in data. Ambiguities can be uncovered by resampling, but existing neighbor-joining programs may give misleading bootstrap frequencies because they do not suppress zero-length branches and/or are sensitive to the order of terminals in the data. A new procedure, parsimony jackknifing, overcomes these problems while running hundreds of times faster than existing programs for neighbor-joining bootstrapping. For analysis of large matrices, parsimony jackknifing is hundreds of thousands of times faster than extensive branch-swapping, yet is better able to screen out poorly-supported groups.
The recently-developed statistical method known as the "bootstrap" can be used to place confidence intervals on phylogenies. It involves resampling points from one's own data, with replacement, to create a series of bootstrap samples of the same size as the original data. Each of these is analyzed, and the variation among the resulting estimates taken to indicate the size of the error involved in making estimates from the original data, In the case of phylogenies, it is argued that the proper method of resampling is to keep all of the original species while sampling characters with replacement, under the assumption that the characters have been independently drawn by the systematist and have evolved independently. Majority-rule consensus trees can be used to construct a phylogeny showing all of the inferred monophyletic groups that occurred in a majority of the bootstrap samples. If a group shows up 95% of the time or more, the evidence for it is taken to be statistically significant. Existing computer programs can be used to analyze different bootstrap samples by using weights on the characters, the weight of a character being how many times it was drawn in bootstrap sampling. When all characters are perfectly compatible, as envisioned by Hennig, bootstrap sampling becomes unnecessary; the bootstrap method would show significant evidence for a group if it is defined by three or more characters.
The main features of the phylogeny program TNT are discussed. Windows versions have a menu interface, while Macintosh and Linux versions are command-driven. The program can analyze data sets with discrete (additive, non-additive, step-matrix) as well as continuous characters (evaluated with Farris optimization). Effective analysis of large data sets can be carried out in reasonable times, and a number of methods to help identifying wildcard taxa in the case of ambiguous data sets are implemented. A variety of methods for diagnosing trees and exploring character evolution is available in TNT, and publication-quality tree-diagrams can be saved as metafiles. Through the use of a number of native commands and a simple but powerful scripting language, TNT allows the user an enormous flexibility in phylogenetic analyses or simulations.
© The Willi Hennig Society 2008.
Abstract— Branch support is quantified as the extra length needed to lose a branch in the consensus of near-most-parsimonious trees. This approach is based solely on the original data, as opposed to the data perturbation used in the bootstrap procedure. If trees have been generated by Farris's successive approximations approach to character weighting, branch support should be examined in terms of weighted extra length needed to lose a branch. The sum of all branch support values over the tree divided by the length of the most parsimonious tree[s] provides a new index, the total support index. This index is a measure of tree stability in terms of supported resolutions, which is of prime importance in cladistic analysis.
Fusion of opisthosomal tergites to form a thoracetron has previously been considered a characteristic of the xiphosuran superfamilies Euproopoidea Eller, 1938, and Limuloidea Zittel, 1885. Evidence is presented here that fusion also occurs in Bellinuroidea Zittel & Eastman, 1913. Results of a cladistic analysis of Palaeozoic xiphosuran genera indicate that Synziphosurina Packard, 1886, is a paraphyletic assemblage of stem-group Xiphosura. Superfamily Paleolimulidae superfam. nov. is erected for families Paleolimulidae Raymond, 1944, and Moravuridae P&íbyl, 1967.
Morphological evidence for resolving relationships among arachnid orders was surveyed and assembled in a matrix comprising 59 euchelicerate genera (41 extant, 18 fossil) and 202 binary and unordered multistate characters. Parsimony analysis of extant genera recovered a monophyletic Arachnida with the topology (Palpigradi (Acaromorpha (Tetrapulmonata (Haplocnemata, Stomothecata nom. nov.)))), with Acaromorpha containing Ricinulei and Acari, Tetrapulmonata containing Araneae and Pedipalpi (Amblypygi, Uropygi), Haplocnemata (Pseudoscorpiones, Solifugae) and Stomothecata (Scorpiones, Opiliones). However, nodal support and results from exploratory implied weights analysis indicated that relationships among the five clades were effectively unresolved. Analysis of extant and fossil genera recovered a clade, Pantetrapulmonata nom nov., with the topology (Trigonotarbida (Araneae (Haptopoda (Pedipalpi)))). Arachnida was recovered as monophyletic with the internal relationships (Stomothecata (Palpigradi, Acaromorpha (Haplocnemata, Pantetrapulmonata))). Nodal support and exploratory implied weights indicated that relationships among these five clades were effectively unresolved. Thus, some interordinal relationships were strongly and/or consistently supported by morphology, but arachnid phylogeny is unresolved at its deepest levels. Alternative hypotheses proposed in the recent literature were evaluated by constraining analyses to recover hypothesized clades, an exercise that often resulted in the collapse of otherwise well-supported clades. These results suggest that attempts to resolve specific nodes based on individual characters, lists of similarities, evolutionary scenarios, etc., are problematic, as they ignore broader impacts on homoplasy and analytical effects on non-target nodes. © 2007 The Linnean Society of London, Zoological Journal of the Linnean Society, 2007, 150, 221–265.
A new synziphosurine, Anderella parva gen. et sp. nov., extends the known range of this group from the Silurian to the Carboniferous and is the youngest known so far from the fossil record. Previously the youngest synziphosurine, Kasibelinurus, was from the Devonian of North America. Anderella parva has a semi-oval carapace with pointed genal regions, nine freely articulating opisthosomal segments and a long styliform tail spine. It is the third xiphosuran genus to be described from the Bear Gulch Limestone and its discovery highlights this deposit as containing an unusually diverse xiphosuran biota. It is also the only known locality where synziphosurines occur alongside the more derived xiphosurids. Xiphosurans reached their greatest diversity in the Carboniferous when the xiphosurids began to occupy brackish and freshwater habitats and became dominant over the synziphosurines. The discovery of the only known Carboniferous synziphosurine in marine sediments may indicate their inability to exploit these same environments.
A new specimen of the synziphosurine arthropodWeinbergina opitzi is described from the Lower Devonian (Lower Emsian) Hunsrück Slate of Germany (Rhenish Slate Mountains). It is the smallest and only the fifth specimen of this taxon to be described and is preserved in ventral aspect with exceptional preservation of prosomal and opisthosomal appendages. This specimen confirms the presence of a seventh appendage, similar in morphology to the preceding prosomal appendages, associated with opisthosomal segment one. In addition, at least three opisthosomal plates fringed with teeth are confirmed. Correlation of prosomal appendage podomeres betweenWeinbergina and selected chelicerate taxa shows that appendage structure is most similar to eurypterid appendages III–IV and Araneae appendages III–VI. This is in contrast to modern horseshoe crabs which have fewer podomeres in appendages II–V due to an undifferentiated tibiotarsus.
Weinbergina ist der erste Synziphosuride, bei dem die ventralen Gliedmaßen gefunden werden konnten. Unter den Cheliceraten istWeinbergina mit 6 prosomalen Beinpaaren zusätzlich zu den Cheliceren einmalig. Daraus folgt, daß das prägenitale Segment mit dem letzten
Paar Laufbeinen, entsprechend den Chilarien beiLimulus, zum Prosoma gerechnet werden muß. Eine erste Deutung der opisthosomalen Gliedmaßäen wird versucht. Sie erscheinen abgeflacht
und eine Kombination der lamellaren Kiemen desLimulus-Typs mit den abgeflachten Dornen (»Kiemen-Fäden«) des trilobitomorphen Typs zu sein.Weinbergina war offensichtlich an ein Leben auf einem weichen Substrat, nicht aber an Wühlen und Graben angepaßt.
Weinbergina is the first synziphosurid in which ventral appendages have been found. It is so far unique among chelicerates in having
six pairs of prosomal legs in addition to the chelicerae. This indicates that the pregeni tal segment with the last pair of
walking legs, corresponding to the chilaria ofLimulus, must be counted with the prosoma. The first interpretation of the opisthosomal appendages is given. They appear to be plate-like
and to combine the presence of lamellar gills of limulid type with flattened spines (so called gill filaments) of trilobitomorph
type.Weinbergina apparently was adapted for a life on soft substrate but not for burrowing.
The early history and phylogeny of the chelicerates
- J A Dunlop
- P A Selden
Dunlop, J. A. & Selden, P. A. (1997). The early history and phylogeny of the chelicerates. In R. A. Fortey & R. H. Thomas (Eds)
Arthropod Relationships (pp. 221-235). London: Chapman & Hall.
Unterschungen zur Morphologie
- P Weygoldt
- H F Paulus
Weygoldt, P. & Paulus, H. F. (1979). Unterschungen zur Morphologie, Taxonomie und Phylogenie der Chelicerata. Zeitschrift
f€ ur Zoologische Systematik und Evolutionsforschung, 17, 85-116,
177-120.
[New occurrence of Xiphosura in China]
- Zhang