Morphology of locomotor appendages in Arachnida: evolutionary trends and phylogenetic implications

The morphological diversity of locomotor appendages in Arachnida is surveyed lo reconstruct phylogenetic relationships and discover evolutionary trends in form and function. The appendicular skeleton and musculature of representatives from the ten living arachnid orders ate described, and a system of homology is proposed. Character polarities are established through comparison with an outgroup. Limulus polyphemus Xiphosura). Cladistic analysis suggests that Arachnida is monophyletic and that absence of extensor muscles is a primitive condition. Extensors are primitively absent in Araneae. Amblypygi, Uropygi, Palpigradi, Ricinulei and Acari. Most similarities in the appendages of these orders are symplesiomorphic so that phylogenetic relationships among the ‘extensorless’ groups cannot be resolved solely on the basis of appendicular characters. Extensor muscles appear to have evolved once, and their presence is considered a synapomorphic feature of Opiliones, Scorpiones, Pseudoscorpiones and Solifugae. Solifugae lack extensors, but a parsimonious interpretation of other characters indicates that this is a secondary, derived condition. The phylogenetic relationships among these four orders are clarified by modifications of the patellotibial joint. Cladistic analysis indicates that Opiliones may be the sister group of the other three orders and that Scorpiones is the sister group of Pseudoscorpiones and Solifugae. Conclusions concerning phylogenetic relationships and evolutionary morphology presented here differ substantially from those of earlier studies on the locomotor appendages of Arachnida.     

A phylogenetic analysis of the arachnid orders based on morphological characters

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.     

The Phylogeny of the Extant Chelicerate Orders

The phylogeny of the extant chelicerate orders is examined in the light of morphological and molecular evidence. Representatives from each of the chelicerate ‘orders’ and mandibulate and onychophoran outgroups are examined. Molecular (small and large ribosomal subunit DNA) and morphological information is combined in a total evidence regime to determine the most consistent picture of extant chelicerate relationships for these data. Multiple phylogenetic analyses are performed with variable analysis parameters yielding largely consistent results. A normalized incongruence length metric is used to assay the relative merit of the multiple analyses. The combined analysis with lowest character incongruence yields the scheme of relationships (Pycnogonida+ (Xiphosura+((Opiliones+((Solifugae+Pseudoscorpiones)+Scorpiones))+((Ricinulei+Acari)+(Palpigradi+ ((Thelyphonida+Schizomida=Uropygi)+(Amblypygi+ Araneae))))))). This result is fairly robust to variation in analysis parameters, with the placement of solifugids and the status of the pedipalps responsible for most disagreement.     

EVOLUTIONARY MORPHOLOGY AND PHYLOGENY OF ARAGHNIDA

Abstract— This paper reports results from a cladistic analysis of the 11 Recent arachnid orders. The polarities of 64 newly discovered and traditional characters were determined through outgroup comparisons that included Eurypterida, Xiphosura, Trilobita and Crustacea. A branch-and-bound algorithm was used to discover a single tree (consistency index 0–59). The relationships suggested by this analysis differ substantially from previous interpretations of arachnid phylogeny, and a new taxonomic system is introduced to accommodate these results. This analysis suggests that Arachnida is monophyletic and composed of two principal lineages, Micrura and Dromopoda. Possible synapomorphies of Micrura include a pygidmm, tntosternum, six principal lateral eyes, poorly sclerotized postgenital appendages, coxal gland orifices near leg 1, an array of micxotubules associated with the spermatozoan nucleus, and absence of coxal endites on the walking legs. The micruran orders appear to have the following relationships: (Palpigradi (Araneae (Amblypygi (I helyphonida, Schizomida)))) (Ricinulei, Acari). Possible synapomorphies of Dromopoda include transverse carapaeal furrows, greatly reduced prosomal sternum, prosomal endosternite with two segmental components, stomotheca, bicondylar femoropatellar and patellotibial joints and extensor muscles. The dromopodan orders appear to have the following relationships: Opiliones (Scorpioncs (Pscudo-scorpiones, Solifugae)).     

Nucleotide Composition of CO1 Sequences in Chelicerata (Arthropoda): Detecting New Mitogenomic Rearrangements

Here we study the evolution of nucleotide composition in third codon-positions of CO1 sequences of Chelicerata, using a phylogenetic framework, based on 180 taxa and three markers (CO1, 18S, and 28S rRNA; 5,218?nt). The analyses of nucleotide composition were also extended to all CO1 sequences of Chelicerata found in GenBank (1,701 taxa). The results show that most species of Chelicerata have a positive strand bias in CO1, i.e., in favor of C nucleotides, including all Amblypygi, Palpigradi, Ricinulei, Solifugae, Uropygi, and Xiphosura. However, several taxa show a negative strand bias, i.e., in favor of G nucleotides: all Scorpiones, Opisthothelae spiders and several taxa within Acari, Opiliones, Pseudoscorpiones, and Pycnogonida. Several reversals of strand-specific bias can be attributed to either a rearrangement of the control region or an inversion of a fragment containing the CO1 gene. Key taxa for which sequencing of complete mitochondrial genomes will be necessary to determine the origin and nature of mtDNA rearrangements involved in the reversals are identified. Acari, Opiliones, Pseudoscorpiones, and Pycnogonida were found to show a strong variability in nucleotide composition. In addition, both mitochondrial and nuclear genomes have been affected by higher substitution rates in Acari and Pseudoscorpiones. The results therefore indicate that these two orders are more liable to fix mutations of all types, including base substitutions, indels, and genomic rearrangements.     

Species richness and biogeography of non-acarine arachnids in Namibia

A total of 821 species, 296 genera, and 69 families of non-acarine arachnids (Araneae, Solifugae, Scorpiones, Pseudoscorpiones, Opiliones and Amblypygi) are presently known from Namibia. Patterns of spider, solifuge and scorpion species richness and endemism are summarized relative to the 14 major Namibian types of vegetation. Spiders are most speciose in the higher rainfall areas, solifuges in the dry areas, and scorpions in rocky areas. Namibia probably has the world's greatest diversity of solifuges. Many arachnids endemic to Namibia are associated with the Namib Desert sand dunes.     

Skeletomuscular anatomy of the harvestman Leiobunum aldrichi (Weed, 1893) (Arachnida: Opiliones: Palpatores) and its evolutionary significance

Skeletal muscles of the North American harvestman Leiobunum aldrichi are exhaustively surveyed and compared with other chelicerates to clarify the evolutionary morphology and phylogenetic relationships of arachnids. Representatives of 104 muscle groups are described and illustrated, and their possible functions are proposed. Comparisons of the feeding apparatus of L. aldrichi with that of other opilions, especially Sim (Cyphophthalmi) and Acromares (Laniatores), and two scorpion genera ( Centruroides, Pandinus ) indicate that the pharyngeal apparatus in L. aldrichi is derived and that its ability to accommodate large food particles is a secondary rather than primitive condition. Comparisons reveal several possible synapomorphies between Opiliones and Scorpiones suggesting that these orders may be sister groups. Apparently unique synapomorphies include an extrinsic cheliceral muscle that arises from the carapace and inserts on the second cheliceral article (deutomerite); an epistome divided into distal and proximal parts by a transverse sulcus; pharyngeal dilator muscles supported by a peripharyngeal skeleton formed by one dorsomedial and two ventrolateral epistomal processes, the latter also with muscular attachments to the endosternite; a specialized preoral chamber (stomodieca) derived from extensions (coxapophyses) of the coxae of the pedipalp and first two leg pairs; internal processes associated with the coxapophyses that serve, in part, as an attachment for muscles operating the coxa-trochanter joints, and lateral endosternal suspensor muscles that insert on the arthrodial membrane between the leg coxae. These are the first observations providing explicit support for an Opiliones-Scorpiones clade.     

钳蝎系统分类研究概况

传统观点认为蝎类是蛛形纲中最原始的类群,但是最近的形态分析和分子系统发生研究表明蝎类与单胫类(Haplocnemata=Solifugae Pseudoscorpiones)构成姐妹群,并与盲蛛类一起在蛛形纲形成一个独立的分支.钳蝎科是蝎目中种类最多分布最广的一个科,目前已知82属600余种.钳蝎一直被认为是蝎目中最原始的类群,但是随着Pseudochactidae的发现和建立,其原始的地位也被取代.钳蝎科中亚科的划分一直存在很大的争议,至今尚没有一个亚科划分意见被广泛接受.同样,钳蝎科中属种的界定也存在问题,一些建立较早的属亟待修订.本文对钳蝎的系统发生和分类进行了综述,并对国内外文献中已记载的中国钳蝎种类进行了汇总和记述,即东亚钳蝎Mesobuthus martensii(Karsch,1879),条斑钳蝎蒙古亚种M.eupeus mongolicus(Birula,1911)、凶恶亚种M.e.thersites(C.L.Koch,1839),高加索钳蝎间型亚种M.caucasicus intermedius(Birula,1897)、普氏亚种M.c.przewaklskii(Birula,1897),细尖狼蝎Lychas mucronatus(Fabricius,1798),斑等蝎Isometrus maculates(DeGeer,1778),凹直钳蝎Orthochirus scrobiculosus(Grube,1873).     

Walk it off: predictive power of appendicular characters toward inference of higher‐level relationships in Laniatores (Arachnida: Opiliones)

Morphological characters are essential for establishing phylogenetic relationships, delimiting higher‐level taxa, and testing phylogenetic relationships inferred from molecular sequence data. In cases where relationships between large clades remain unresolved, it becomes imperative to establish which character systems are sound predictors of phylogenetic signal. In the case of Laniatores, the largest suborder of Opiliones, some superfamilial relationships remain unresolved or unsupported, and traditionally employed phenotypic characters are typically of utility only at the family level. Here we investigated a promising set of morphological characters that can be discretized and scored in all Opiliones: cuticular structures of the distal podomeres (metatarsi and tarsi). We intensively sampled members of all known families of Laniatores, and define here three new, discrete appendicular characters toward refinement of Laniatores superfamilial systematics: metatarsal paired slits (MPS; occurring in all Laniatores except Sandokanidae), proximal tarsomeric gland (PTG; in Icaleptidae, Fissiphalliidae, and Zalmoxidae), and tarsal aggregate pores (TAP; found in Gonyleptoidea, Epedanoidea, and Pyramidopidae). We conducted statistical tests on each character to characterize the strength of phylogenetic signal and assess character independence, based on alternative tree topologies of Laniatores. All three characters had high retention indices and bore significantly strong phylogenetic signal. Excepting one pairwise comparison, morphological characters did not evolve in a correlated manner, indicating that appendicular morphology does not constitute a single character system. Our results demonstrate the predictive power and utility of appendicular characters in Opiliones phylogeny, and proffer a promising source of diagnostic synapomorphies for delimiting superfamilies.     

The affinities of mites and ticks: a review

Mites and ticks can be divided into two well-defined clades, Anactinotrichida and Actinotrichida, for which a recent work formalized a suite of putative autapomorphies and reciprocal differences. Whether they are sister-taxa – forming a monophyletic Acari – is more controversial. Earlier supporters of two independent origins for mites largely failed to demonstrate convincing synapomorphies between either of the two lineages and other arachnid orders; although recent work on reproductive biology revealed explicit characters of this nature. Furthermore, some of the characters proposed in support of a monophyletic Acari do not stand up to detailed scrutiny when compared with Arachnida in general. Effective morphological comparisons between mites and other arachnids are hindered by incompatible nomenclature and long-standing, mite-specific characters which are difficult to score for other arachnids. Furthermore, taxon-specific characters restricted to individual mite groups have sometimes been treated erroneously as 'typical' for all Acari. Here, previous hypotheses of mite affinities are reviewed. Historically, authors have debated whether mites are basal arachnids or highly derived. Excluding weakly supported early hypotheses, mites have been resolved – in whole or in part – as sister-group of all other Arachnida (based on tagmosis), closely related to Opiliones (based mostly on genital morphology), Palpigradi (based on controversial interpretations of limb morphology), Solifugae (based mostly on the mouthparts, but now perhaps also reproductive characters) and Ricinulei (based on hexapodal larvae and perhaps mouthparts). We cannot provide a final resolution here, but we aim to highlight important character sets which should be included in subsequent phylogenetic analyses, as well as useful areas for future investigations: particularly tagmosis and the nature of the gnathosoma.     

Evolutionary and functional substitution of extrinsic musculature in Solifugae (Arachnida)

The locomotory system of Solifugae is distinct from that of other Arachnida in several ways. Only three pairs of legs are involved in locomotion, while the first pair function as sensory appendages. Morphologically, the proximal region of the locomotory system in Solifugae is characterized by fused coxae. Within the prosoma of Solifugae, an endosternite is missing: in other Arachnida, this endosternite serves as the proximal attachment site for a portion of the extrinsic musculature. How then do these skeletal modifications influence the muscular anatomy in the proximal region of the locomotory system? To answer this question, we studied the skeletomuscular anatomy of Galeodes granti at the interface between the prosoma and legs, reinvestigating the complex muscular anatomy of this body region for the first time in over 80 years and—for the first time—using detailed micro-computed tomography scans to analyze the skeletomuscular morphology. Specimens of three further species were checked for comparison. The analysis revealed differences in the number and composition of coxa-trochanter muscles in each of the four pairs of legs. These are compared in the light of serial homology. The comparison between the proximal locomotory system of Solifugae and that of other Arachnida unveils a series of analogies. Primarily, the coxa-trochanter joint is the most proximal joint to move the leg relative to the prosoma. Therefore, we argue that from a morpho-functional point of view, the coxa-trochanter muscles in Solifugae should be considered secondary extrinsic musculature. Thus, the legs gain a stable, articulated joint in the most proximal region of the leg to the prosoma, which might be advantageous for agile locomotion.     

The mitochondrial genome sequence of the scorpion Centruroides limpidus (Karsch 1879) (Chelicerata; Arachnida)

The mitochondrial genome of the scorpion Centruroides limpidus (Chelicerata; Arachnida) has been completely sequenced and is 14519 bp long. The genome contains 13 protein-encoding genes, two ribosomal RNA genes, 21 transfer RNA genes and a large non-coding region related to the control region. The overall A + T composition is the lowest among the complete mitochondrial sequences published within the Chelicerata subphylum. Gene order and gene content differ slightly from that of Limulus polyphemus (Chelicerata: Xiphosura): i.e., the lack of the trnD gene, and the translocation–inversion of the trnI gene. Preliminary phylogenetic analysis of some Chelicerata shows that scorpions (C. limpidus and Mesobuthus gibbosus) make a tight cluster with the spiders (Arachnida; Araneae). Our analysis does not support that Scorpiones order is the sister group to all Arachnida Class, since it is closer to Araneae than to Acari orders.     

The evolution of pedipalps and glandular hairs as predatory devices in harvestmen (Arachnida,Opiliones)

Pedipalps are the most versatile appendages of arachnids. They can be equipped with spines (Amblypygi), chelae (Scorpiones), or adhesive pads (Solifugae), all of which are modifications to grasp and handle fast‐moving prey. Harvestmen (Opiliones) show a high diversity of pedipalpal morphologies. Some are obviously related to prey capture, like the enlargement and heavy spination of Laniatores pedipalps. Many Dyspnoi, by contrast, exhibit thin, thread‐like pedipalps that are covered with complex glandular setae (clavate setae). These extrude viscoelastic glue that is used to immobilize prey items. Comparable setae (plumose setae) have previously been found in representatives of both Eupnoi and Dyspnoi, yet comprehensive data on their distribution are lacking. This study examined the distribution and ultrastructure of glandular setae in harvestmen and related them to pedipalpal morphology. Pedipalpal and setal characters were analysed in a phylogenetic framework. We found that glandular setae are synapomorphic for and widespread in the Palpatores clade (Eupnoi plus Dyspnoi). Their occurrence correlates with pedipalp morphology and feeding habit. Remnants of arthropod cuticular structures or secretions, frequently found attached to glandular setae, and behavioural observations, underlined the importance of the setae for capturing and securing prey. We hypothesize that glandular setae evolved as an adaptation to capture small and agile prey, which are hard to catch with a capture basket. Details of ultrastructure indicate that the setae are derived sensilla chaetica, with both a secretory and sensory function. Derived ultrastructural characters of the glandular setae, such as slit‐like channel openings and a globular arrangement of the microtrichia, may increase their effectiveness. The functional role of further pedipalpal modifications, such as apophyses, stalked and hyperbendable joints, and curved segments, as well as sexual dimorphism and ontogenetic polymorphism, are discussed. Some implications of the results obtained for the taxonomic treatment of Phalangiidae are also discussed. These results shed new light on the biology and evolutionary history of this fascinating group of arthropods.     

Phylogeny of the arachnid order Opiliones (Arthropoda) inferred from a combined approach of complete 18S and partial 28S ribosomal DNA sequences and morphology

The phylogenetic relationships among the main evolutionary lines of the arachnid order Opiliones were investigated by means of molecular (complete 18S rDNA and the D3 region of the 28S rDNA genes) and morphological data sets. Equally and differentially weighted parsimony analyses of independent and combined data sets provide evidence for the monophyly of the Opiliones. In all the analyses, the internal relationships of the group coincide in the monophyly of the following main groups: Cyphophthalmi, Eupnoi Palpatores, Dyspnoi Palpatores, and Laniatores. The Cyphophthalmi are monophyletic and sister to a clade that includes all the remaining opilionid taxa (=Phalangida). Within the Phalangida the most supported hypothesis suggests that Palpatores are paraphyletic, as follows: (Eupnoi (Dyspnoi + Laniatores)), but the alternative hypothesis (Laniatores (Eupnoi + Dyspnoi)) is more parsimonious in some molecular data analyses. Relationships within the four main clades are also addressed. Evolution of some morphological characters is discussed, and plesiomorphic states of these characters are evaluated using molecular data outgroup polarization. Finally, Martens' hypothesis of opilionid evolution is assessed in relation to our results.     

Geological history and phylogeny of Chelicerata

Chelicerata probably appeared during the Cambrian period. Their precise origins remain unclear, but may lie among the so-called great appendage arthropods. By the late Cambrian there is evidence for both Pycnogonida and Euchelicerata. Relationships between the principal euchelicerate lineages are unresolved, but Xiphosura, Eurypterida and Chasmataspidida (the last two extinct), are all known as body fossils from the Ordovician. The fourth group, Arachnida, was found monophyletic in most recent studies. Arachnids are known unequivocally from the Silurian (a putative Ordovician mite remains controversial), and the balance of evidence favours a common, terrestrial ancestor. Recent work recognises four principal arachnid clades: Stethostomata, Haplocnemata, Acaromorpha and Pantetrapulmonata, of which the pantetrapulmonates (spiders and their relatives) are probably the most robust grouping. Stethostomata includes Scorpiones (Silurian–Recent) and Opiliones (Devonian–Recent), while Haplocnemata includes Pseudoscorpiones (Devonian–Recent) and Solifugae (Carboniferous–Recent). Recent works increasingly favour diphyletic mite origins, whereby Acaromorpha comprises Actinotrichida (Devonian–Recent), Anactinotrichida (Cretaceous–Recent) and Ricinulei (Carboniferous–Recent). The positions of the Phalangiotarbida (Devonian–Permian) and Palpigradi (Neogene–Recent) are poorly resolved. Finally, Pantetrapulmonata includes the following groups (listed here in their most widely recovered phylogenetic sequence): Trigonotarbida (Silurian–Permian), Uraraneida (Devonian–Permian), Araneae (Carboniferous–Recent), Haptopoda (Carboniferous), Amblypygi (?Devonian–Recent), Thelyphonida (Carboniferous–Recent) and Schizomida (Paleogene–Recent).     

A phylogenetic analysis of basal Anseriformes, the fossil Presbyornis, and the interordinal relationships of waterfowl

A phylogenetic analysis of 123 morphological characters of basal waterfowl (Aves: Anseriformes) and other selected avian orders confirmed that the screamers (Anhimae: Anhitn-idae) are the sister-group of other waterfowl (Anseres), and that the magpie goose (Anseranatidae: Anseranas semipalmata) is the sister group of other modern waterfowl exclusive of screamers (Anatidae sensu stricto). The analysis also supports the traditional hypothesis of the gallinaceous birds (Galliformes) as the sister group of the Anseriformes. Presbyornis, a fossil from the early Eocene of Wyoming and averred by Olson & Feduccia as showing that the Anseriformes were derived from shorebirds (Charadriiformes), was found to represent the sister group of the Anatidae. Associated hypotheses by Olson & Feduccia concerning the implications of Presbyornis for the phylogenetic relationships of flamingos (Phoenicopteriformes), the position of the Anhimidae within the waterfowl, relationships among modern Anatidae, and a plausible evolutionary scenario for waterfowl also are rejected. Analyses revealed that cranial characters were critical to the establishment of the Galliformes as the sister group of the Anseriformes; exclusion of the Anhimidae, especially in combination with Anseranas, also undermined the support for this inference. Placement of Presbyornis as the sister group of the Anatidae casts doubt on the role suggested by Feduccia of ‘transitional shorebirds' in the origin of modern avian orders, and calls into question the concept of ‘fossil mosaics’. The phylogenetic hypothesis is used to reconstruct an evolutionary scenario for selected ecomorphological characters in the galliform-anseriform transition, to predict the most parsimonious states of these characters for Presbyornis, and to propose a phylogenetic classification of the higher-order taxa of waterfowl. This re-examination of Presbyornis also is used to exemplify the fundamental methodological shortcomings of the intuitive approach to the reconstruction of phylogenetic relationships.     

Phylogeny and systematic position of Opiliones: a combined analysis of chelicerate relationships using morphological and molecular data

The ordinal level phylogeny of the Arachnida and the suprafamilial level phylogeny of the Opiliones were studied on the basis of a combined analysis of 253 morphological characters, the complete sequence of the 18S rRNA gene, and the D3 region of the 28S rRNA gene. Molecular data were collected for 63 terminal taxa. Morphological data were collected for 35 exemplar taxa of Opiliones, but groundplans were applied to some of the remaining chelicerate groups. Six extinct terminals, including Paleozoic scorpions, are scored for morphological characters. The data were analyzed using strict parsimony for the morphological data matrix and via direct optimization for the molecular and combined data matrices. A sensitivity analysis of 15 parameter sets was undertaken, and character congruence was used as the optimality criterion to choose among competing hypotheses. The results obtained are unstable for the high-level chelicerate relationships (except for Tetrapulmonata, Pedipalpi, and Camarostomata), and the sister group of the Opiliones is not clearly established, although the monophyly of Dromopoda is supported under many parameter sets. However, the internal phylogeny of the Opiliones is robust to parameter choice and allows the discarding of previous hypotheses of opilionid phylogeny such as the "Cyphopalpatores" or "Palpatores." The topology obtained is congruent with the previous hypothesis of "Palpatores" paraphyly as follows: (Cyphophthalmi (Eupnoi (Dyspnoi + Laniatores))). Resolution within the Eupnoi, Dyspnoi, and Laniatores (the latter two united as Dyspnolaniatores nov.) is also stable to the superfamily level, permitting a new classification system for the Opiliones.     

Muscular anatomy of the giant whipscorpion Mastigoproctus giganteus (Lucas) (Arachnida: Uropygi) and its evolutionary significance

Skeletal muscles in the whipscorpion Mastigoproctus giganteus are surveyed and compared with those of several other to clarify the evolutionary morphology and phylogenetic relationships of arachnids. Representatives from 90 muscle groups are described and illustrated, and their possible functions are proposed. Principal results of this analysis include new proposed homologies for the anterior opisthosomal appendages and sclerites in tetrapulmonate arachnids (that is, Trigonotarbida, Araneae, Amblypygi, Uropygi), the discovery that muscular attachments in arthropods can shift from the mesodermal endosternite to the ectodermal exoskeleton, a reconstruction of the evolutionary transformations associated with the apparent uncoupling of pharyngeal and locomotor complexes in the prosoma of Pedipalpi (that is, Amblypygi and Uropygi), and an expanded list of unique synapomorphies supporting the sister-group status of Amblypygi and Uropygi.     

REVIEW Evolution and systematics of the Chelicerata

After approximately 40 years of discussion about the question of whether the Arthropoda are a monophyletic or a paraphyletic group or even a polyphyletic assemblage of unrelated taxa, most morphologists, palaeontologists and molecular taxonomists agree that the Arthropoda are a monophylum. The Euarthropoda are composed of the Arachnomorpha and Mandibulata. Myriapods are usually considered to be mandibulates; however, new molecular data as well as some morphological characters show similarities which the Myriapoda share with the Chelicerata, suggesting that there is no taxon Antennata or Atelocerata. Chelicerata are usually considered to be the sister group of Trilobita or, more correctly, Trilobita branch off from the chelicerate stem line. The first adaptive radiation of the Chelicerata took place in the Cambrian. All extant and some extinct orders were present during the Carboniferous. Two systems are compared. It is suggested that the Chelicerata contain the Pantopoda and Euchelicerata. The Euchelicerata are divided into Xiphosura and terrestrial Arachnida. Scorpiones are considered to be the sister group of all other arachnids, the Lipoctena and these are further divided into the Megoperculata (Uropygi, Amblypygi, and Araneae) and Apulmonata (all other groups). The Acari are tentatively considered to be a monophylum and the sister group of the Ricinulei. However, the Actinotrichida and Anactinotrichida diverged early and therefore have had a long history of independent evolution.