The geological record and phylogeny of the Myriapoda

A new genus of scutigerid centipede from southern South America with the description of two new species and an updated molecular phylogeny of the myriapod order Scutigeromorpha (Myriapoda: Chilopoda)

Scutigeromorph centipedes are conspicuous, yet often ignored myriapods for which little work has been conducted in southern South America. After examining recent and museum collections from Chile and Argentina, two new species of generic uncertainty were identified. A new genus of scutigerid centipede, Edgethreua , is therefore described with two new species, E. chilensis from Central Chile (type species of the genus) and E. goloboffi from Argentinian Patagonia. The new genus is characterised by the presence of scattered setiform bristles with short paired spines and the absence of simple spinulae and spines on all stomatotergites, the presence of a single spine-bristle in the prefemur of the second maxilla, a patch of cuticular ridges and pores surrounding the sensilla of the proximal labral portion of the epipharynx, the morphology of the sensilla of the distal patch of the hypopharynx and the morphology of the female gonopods. A phylogenetic analysis of the new species using two nuclear ribosomal RNA genes (18S and 28S rRNA), two mitochondrial ribosomal RNA genes (12S and 16S rRNA) and the mitochondrial protein-encoding gene cytochrome c oxidase subunit I show that the new genus does not cluster with any other described genus of scutigeromorph represented in molecular phylogenies. The data indicate that the new genus is probably sister group to a clade including the genera Lassophora , Ballonema and the subfamily Thereuoneminae, although one analysis suggests a position as sister group to Scutigerinae. ZooBank: urn:lsid:zoobank.org:pub:A4D453F3-9031-4E21-84C7-87F16C07AD51.

From water to land: Evolution of photoreceptor circuits for vision in air

When vertebrates first conquered the land, they encountered a visual world that was radically distinct from that of their aquatic ancestors. Fish exploit the strong wavelength-dependent interactions of light with water by differentially feeding the signals from up to 5 spectral photoreceptor types into distinct behavioural programmes. However, above the water the same spectral rules do not apply, and this called for an update to visual circuit strategies. Early tetrapods soon evolved the double cone, a still poorly understood pair of new photoreceptors that brought the "ancestral terrestrial" complement from 5 to 7. Subsequent nonmammalian lineages differentially adapted this highly parallelised retinal input strategy for their diverse visual ecologies. By contrast, mammals shed most ancestral photoreceptors and converged on an input strategy that is exceptionally general. In eutherian mammals including in humans, parallelisation emerges gradually as the visual signal traverses the layers of the retina and into the brain.

Non-seed plants are emerging gene sources for agriculture and insect control proteins

The non-seed plants (e.g., charophyte algae, bryophytes, and ferns) have multiple human uses, but their contributions to agriculture and research have lagged behind seed plants. While sharing broadly conserved biology with seed plants and the major crops, non-seed plants sometimes possess alternative molecular and physiological adaptations. These adaptations may guide crop improvements. One such area is the presence of multiple classes of insecticidal proteins found in non-seed plant genomes which are either absent or widely diverged in seed plants. There are documented uses of non-seed plants, and ferns for example have been used in human diets. Among the occasional identifiable toxins or antinutritive components present in non-seed plants, none include these insecticidal proteins. Apart from these discrete risk factors which can be addressed in the safety assessment, there should be no general safety concern about sourcing genes from non-seed plant species.

Re-evaluating and dating myriapod diversification with phylotranscriptomics under a regime of dense taxon sampling

Recent transcriptomic studies of myriapod phylogeny have been based on relatively small datasets with <40 myriapod terminals and variably supported or contradicted the traditional morphological groupings of Progoneata and Dignatha. Here we amassed a large dataset of 104 myriapod terminals, including multiple species for each of the four myriapod classes. Across the tree, most nodes are stable and well supported. Most analyses across a range of gene occupancy levels provide moderate to strong support for a deep split of Myriapoda into Symphyla + Pauropoda (=Edafopoda) and an uncontradicted grouping of Chilopoda + Diplopoda (=Pectinopoda nov.), as in other recent transcriptome-based analyses; no analysis recovers Progoneata or Dignatha as clades. As in all recent multi-locus and phylogenomic studies, chilopod interrelationships resolve with Craterostigmus excluded from Amalpighiata rather than uniting with other centipedes with maternal brood care in Phylactometria. Diplopod ordinal interrelationships are largely congruent with morphology-based classifications. Chilognathan clades that are not invariably advocated by morphologists include Glomerida + Glomeridesmida, such that the volvation-related characters of pill millipedes may be convergent, and Stemmiulida + Polydesmida more closely allied to Juliformia than to Callipodida + Chordeumatida. The latter relationship implies homoplasy in spinnerets and contradicts Nematophora. A time-tree with nodes calibrated by 25 myriapod and six outgroup fossil terminals recovers Cambrian-Ordovician divergences for the deepest splits in Myriapoda, Edafopoda and Pectinopoda, predating the terrestrial fossil record of myriapods as in other published chronograms, whereas age estimates within Chilopoda and Diplopoda overlap with or do not appreciably predate the calibration fossils. The grouping of Chilopoda and Diplopoda is recovered in all our analyses and is formalized as Pectinopoda nov., named for the shared presence of mandibular comb lamellae. New taxonomic proposals for Chilopoda based on uncontradicted clades are Tykhepoda nov. for the three blind families of Scolopendromorpha that share a "sieve-type" gizzard, and Taktikospina nov. for Scolopendromorpha to the exclusion of Mimopidae.

Illusion of flight? Absence, evidence and the age of winged insects

The earliest fossils of winged insects (Pterygota) are mid-Carboniferous (latest Mississippian, 328–324 Mya), but estimates of their age based on fossil-calibrated molecular phylogenetic studies place their origin at 440–370 Mya during the Silurian or Devonian. This discrepancy would require that winged insects evaded fossilization for at least the first ~50 Myr of their history. Here, we examine the plausibility of such a gap in the fossil record, and possible explanations for it, based on comparisons with the fossil records of other arthropod groups, the distribution of first occurrence dates of pterygote families, phylogenetically informed simulations of the fossilization of Palaeozoic insects, and re-analysis of data presented by Misof and colleagues using updated fossil calibrations under a variety of prior probability settings. We do not find support for the mechanisms previously suggested to account for such an extended gap in the pterygote fossil record, including sampling bias, preservation bias, and body size. We suggest that inference of an early origin of Pterygota long prior to their first appearance in the fossil record is probably an analytical artefact of taxon sampling and choice of fossil calibration points, possibly compounded by heterogeneity in rates of sequence evolution or speciation, including radiations or ‘bursts’ during their early history.

Redescription of Ransomnema bravoae Sánchez-Velázquez, 1981 (Ransomnematoidea: Ransomnematidae) from Anadenobolus putealis Loomis, 1969 (Diplopoda: Rhinocricidae) from Mexico

Ransomnema bravoae from the intestine of the endemic rhinocricid, Anadenobolus putealis, from Mexico, is redescribed. This species was originally described from the spirobolid Hiltonius carpinus from Morelos State, Mexico. A redescription is presented and a neotype proposed due to the loss of the type material. Ransomnema bravoae can be distinguished from all other described species of Ransomnema by the presence of a cervical expansion extending from the base of the buccal cone to approximately the last one-third of the muscular pharynx and is offset by a slight constriction at the termination point, the presence of broad lateral alae beginning at the cervical expansion termination and terminating at the phasmids, the size and distribution of the male caudal papillae, and the size of the spicules, the right spicule being 3-4 times longer than the left. Ransomnema bravoae resembles R. alatum and R. bursatum in the presence of lateral alae but differs markedly in their distribution along the body. The right spicule is within the size range of that of R. longispiculum. Males of R. bravoae possess the usual papillal composition of the genus and are very similar to that of R. christiei in that the last pair of papillae is arranged in tandem. It is similar to species where males possess three precloacal papillae placed on lateral protuberances, showing morphological affinities to Caribbean species R. artigasi and R. habanense from Cuba.

Phylogenetic analyses suggest centipede venom arsenals were repeatedly stocked by horizontal gene transfer

Venoms have evolved over a hundred times in animals. Venom toxins are thought to evolve mostly by recruitment of endogenous proteins with physiological functions. Here we report phylogenetic analyses of venom proteome-annotated venom gland transcriptome data, assisted by genomic analyses, to show that centipede venoms have recruited at least five gene families from bacterial and fungal donors, involving at least eight horizontal gene transfer events. These results establish centipedes as currently the only known animals with venoms used in predation and defence that contain multiple gene families derived from horizontal gene transfer. The results also provide the first evidence for the implication of horizontal gene transfer in the evolutionary origin of venom in an animal lineage. Three of the bacterial gene families encode virulence factors, suggesting that horizontal gene transfer can provide a fast track channel for the evolution of novelty by the exaptation of bacterial weapons into animal venoms.

The tracheal system of scutigeromorph centipedes and the evolution of respiratory systems of myriapods

The tracheal system of scutigeromorph centipedes (Chilopoda) is special, as it consists of dorsally arranged unpaired spiracles. In this study, we investigate the tracheal systems of five different scutigeromorph species. They are strikingly similar to each other but depict unique characters compared to the tracheal systems of pleurostigmophoran centipedes, which has engendered an ongoing debate over a single versus independent origin of tracheal systems in Chilopoda. Up to now, only the respiratory system of Scutigera coleoptrata was investigated intensively using LM-, TEM-, and SEM-techniques. We supplement this with data for species from all three families of Scutigeromorpha. These reveal interspecific differences in atrial width and the shape and branching pattern of the tracheal tubules. Further, we investigated the tracheal system of Scutigera coleoptrata with three additional techniques: light sheet microscopy, microCT and synchrotron radiation based microCT analysis. This set of techniques allows a comparison between fresh versus fixed and dried material. The question of a unique vs. multiple origin of tracheal systems in centipedes and in Myriapoda as a whole is discussed with regard to their structural similarities and differences and the presence of hemocyanin as an oxygen carrier. We used morphological and molecular data and the fossil record to evaluate the alternative hypotheses.

Life after the mother's hug: Late post-embryonic development of Cryptops parisi (Chilopoda: Scolopendromorpha: Cryptopidae)

Here we describe in detail the late post-embryonic development of the common European scolopendromorph centipede Cryptops parisi. Canonical variate analyses of two groups of external morphological characters, viz., cephalic capsule characters (head length, length of the anterior and posterior paramedian cephalic sutures) and coxopleuron surface characters (number of pores in the coxal pore-field, number of setae on the posterior coxopleuron edge, their number on the coxal pore-field, and their number posterior to the coxal pore-field) were conducted on a large sample of specimens collected from two localities in Serbia. Ten free-living stages are recognized: three pre-adult stages (adolescens I, II, and III) and seven adult stages (one maturus junior stage, four maturus, and two maturus senior stages). The fourth late post-embryonic stage is the first mature stage in both sexes. Sexual dimorphism in the aforementioned characters was not observed. Morphological variation of coxopleuron characters was more informative for the discrimination of developmental stages in Cryptops than the morphological variation of cephalic capsule characters.

Evolution: An Irresistibly Clear View of Land

Visually guided predation is range-limited in water, but works over long distances on land. This may have driven our last aquatic ancestors to evolve large eyes on the top of their head for spotting crunchy meals on ancient riverbanks.

Dwarfs under dinosaur legs: a new millipede of the order Callipodida (Diplopoda) from Cretaceous amber of Burma

The entire Mesozoic Era is rather poor in millipede (class Diplopoda) fossils, with less than a dozen species being taxonomically described. Here, we describe the first fossil millipede of the order Callipodida, Burmanopetaluminexpectatum gen. nov. et sp. nov., found in early Cenomanian amber of Burma, 98.79±0.62 Mya. The species possesses a number of morphological traits that exclude it from all extant suborders, and Burmanopetalidea suborder nov. and Burmanopetalidae fam. nov. are here erected to accommodate it. The new suborder can be recognized by the following unique characters: pleurotergal setae absent; telson with a specific spatulate shape twice the size of the penultimate body ring; hypoproct devoid of setae; and eyes composed of five well-separated ommatidia. While the callipodidan habitus seems to have remained generally unchanged for at least 99 million years, pleurotergal and hypoproctal setation, as well as the complexity of eyes in ground-dwelling forms may have evolved recently in the order. As B.inexpectatum gen. nov. et sp. nov. is the first true callipodidan in the fossil record, the minimum age of Callipodida is thus at least 99 Mya.

Burgess Shale fossils shed light on the agnostid problem

Agnostids (agnostinids and eodiscinids) are a widespread and biostratigraphically important group of Cambro-Ordovician euarthropods whose evolutionary affinities have been highly controversial. Their dumbbell-shaped calcified tergum was traditionally suggested to unite them with trilobites, but agnostinids have alternatively been interpreted as stem-crustaceans, based on Orsten larval material from the Cambrian of Sweden. We describe exceptionally preserved soft tissues from mature individuals of the agnostinids Peronopsis and Ptychagnostus from the middle Cambrian (Wuliuan Stage) Burgess Shale (Walcott Quarry and Marble Canyon, British Columbia, Canada), facilitating the testing of alternative hypotheses. The digestive tract includes conspicuous ramifying cephalic diverticulae. The cephalon carries one pair of elongate spinous antennules projecting to the front, two pairs of appendages with distally setose, oar-like exopods, and three pairs of presumably biramous appendages with endopods sporting club-shaped exites. The trunk bears five appendage pairs, at least the first two of which are similar to the posteriormost cephalic pairs. The combined evidence supports a nektobenthic and detritivorous lifestyle for agnostinids. A head with six appendiferous segments contrasts strikingly with the four known in trilobites and five typical of mandibulates. Agnostinids are retrieved as the sister group to polymeroid trilobites in our phylogeny, implying that crustacean-like morphologies evolved homoplastically. This result highlights the variability in segmental composition of the artiopodan head. Finally, our study emphasizes the continued role of Burgess Shale-type fossils in resolving the affinities of problematic biomineralizing taxa.

Morphological study of the ovary in Hanseniella caldaria (Myriapoda; Symphyla): The position of oocyte-growth and evolution of ovarian structure in Arthropoda

In Arthropoda, the ovary is classified into Chelicerata-type and Mandibulata-type, based on the oocyte-growth position within the ovary. By contrast, oocytes of Diplopoda and Chilopoda grow within the hemocoelic space. However, as the position of oocyte-growth in Symphyla and Pauropoda has not been confirmed, whether the hemocoelic nature of oocyte-growth is common among myriapods remains ambiguous. This study described the ovarian structure of Hanseniella caldaria to reveal the oocyte-growth position in Symphyla. The oocyte is surrounded by the follicle epithelium, and the inner surface of the follicle epithelium, i.e., the space between follicle cells and oocytes, is lined with a basement membrane. The follicle epithelial layer continues to the ovarian epithelium via the follicle extension with a continuous layer of basement membrane. Data on the architecture of the follicle suggest that the follicle pouch opens to the hemocoel. Hence, the oocyte of H. caldaria grows within the hemocoelic space. Based on our findings in H. caldaria and previous studies in a millipede and in centipedes, the hemocoelic nature of oocyte-growth is considered as a common feature among myriapods and a synapomorphy of the Myriapoda for which morphological synapomorphies have been ambiguous.

An apparently non-swinging tentorium in the Diplopoda (Myriapoda): comparative morphology of the tentorial complex in giant pill-millipedes (Sphaerotheriida)

The presence of a swinging tentorium is a key apomorphy of Myriapoda, but this character has been studied in detail in only few species. Here the tentorium, i.e., the peristomatic skeleton of the preoral chamber, is comparatively studied in three species of the millipede order Sphaerotheriida Brandt, 1833. Since dissections of the fragile tentorial components proved to be difficult, despite the large head size, they were analysed mainly in situ via micro-computed tomography. Our results confirm previous observations of large differences in the tentorial construction in the giant pill-millipedes compared to chilognathan diplopods. The tentorium of Sphaerotheriida consists of a curved, plate-like epipharyngeal bar with distal projections, an elongate and thin hypopharyngeal bar, and a plate-like triangular posterior process; a transverse bar is absent. Only seven muscles attach at the tentorium in giant pill-millipedes, including two antennal muscles and two muscles of the gnathochilarium. Within the order Sphaerotheriida, the composition of the tentorium and its muscular equipment seems to be conserved, except for some variability in the shape of the epipharyngeal bar. As the transverse bar has been considered essential for the mobility of the tentorium in myriapods, its absence in Sphaerotheriida may indicate that their tentorium is not capable of performing a swing. Loss of tentorial mobility may also pertain to the order Glomerida Brandt, 1833, inferred here from the absence of a posterior process. An apparently immobile tentorium in Glomerida and Sphaerotheriida can straightforwardly be correlated with transformations of the head related to their ability of volvation. The different transformations of the tentorium, here hypothesised to cause immobility, may support current assumptions that the ability of volvation evolved convergently in Glomerida and Sphaerotheriida. This conclusion, however, still requires more detailed studies of the head anatomy in Glomerida and Glomeridesmida Cook, 1895.

Step-wise evolution of complex chemical defenses in millipedes: a phylogenomic approach

With fossil representatives from the Silurian capable of respiring atmospheric oxygen, millipedes are among the oldest terrestrial animals, and likely the first to acquire diverse and complex chemical defenses against predators. Exploring the origin of complex adaptive traits is critical for understanding the evolution of Earth's biological complexity, and chemical defense evolution serves as an ideal study system. The classic explanation for the evolution of complexity is by gradual increase from simple to complex, passing through intermediate "stepping stone" states. Here we present the first phylogenetic-based study of the evolution of complex chemical defenses in millipedes by generating the largest genomic-based phylogenetic dataset ever assembled for the group. Our phylogenomic results demonstrate that chemical complexity shows a clear pattern of escalation through time. New pathways are added in a stepwise pattern, leading to greater chemical complexity, independently in a number of derived lineages. This complexity gradually increased through time, leading to the advent of three distantly related chemically complex evolutionary lineages, each uniquely characteristic of each of the respective millipede groups.

Biotic and environmental dynamics through the Late Jurassic-Early Cretaceous transition: evidence for protracted faunal and ecological turnover

The Late Jurassic to Early Cretaceous interval represents a time of environmental upheaval and cataclysmic events, combined with disruptions to terrestrial and marine ecosystems. Historically, the Jurassic/Cretaceous (J/K) boundary was classified as one of eight mass extinctions. However, more recent research has largely overturned this view, revealing a much more complex pattern of biotic and abiotic dynamics than has previously been appreciated. Here, we present a synthesis of our current knowledge of Late Jurassic-Early Cretaceous events, focusing particularly on events closest to the J/K boundary. We find evidence for a combination of short-term catastrophic events, large-scale tectonic processes and environmental perturbations, and major clade interactions that led to a seemingly dramatic faunal and ecological turnover in both the marine and terrestrial realms. This is coupled with a great reduction in global biodiversity which might in part be explained by poor sampling. Very few groups appear to have been entirely resilient to this J/K boundary 'event', which hints at a 'cascade model' of ecosystem changes driving faunal dynamics. Within terrestrial ecosystems, larger, more-specialised organisms, such as saurischian dinosaurs, appear to have suffered the most. Medium-sized tetanuran theropods declined, and were replaced by larger-bodied groups, and basal eusauropods were replaced by neosauropod faunas. The ascent of paravian theropods is emphasised by escalated competition with contemporary pterosaur groups, culminating in the explosive radiation of birds, although the timing of this is obfuscated by biases in sampling. Smaller, more ecologically diverse terrestrial non-archosaurs, such as lissamphibians and mammaliaforms, were comparatively resilient to extinctions, instead documenting the origination of many extant groups around the J/K boundary. In the marine realm, extinctions were focused on low-latitude, shallow marine shelf-dwelling faunas, corresponding to a significant eustatic sea-level fall in the latest Jurassic. More mobile and ecologically plastic marine groups, such as ichthyosaurs, survived the boundary relatively unscathed. High rates of extinction and turnover in other macropredaceous marine groups, including plesiosaurs, are accompanied by the origin of most major lineages of extant sharks. Groups which occupied both marine and terrestrial ecosystems, including crocodylomorphs, document a selective extinction in shallow marine forms, whereas turtles appear to have diversified. These patterns suggest that different extinction selectivity and ecological processes were operating between marine and terrestrial ecosystems, which were ultimately important in determining the fates of many key groups, as well as the origins of many major extant lineages. We identify a series of potential abiotic candidates for driving these patterns, including multiple bolide impacts, several episodes of flood basalt eruptions, dramatic climate change, and major disruptions to oceanic systems. The J/K transition therefore, although not a mass extinction, represents an important transitional period in the co-evolutionary history of life on Earth.

The timetable of evolution

The integration of fossils, phylogeny, and geochronology has resulted in an increasingly well-resolved timetable of evolution. Life appears to have taken root before the earliest known minimally metamorphosed sedimentary rocks were deposited, but for a billion years or more, evolution played out beneath an essentially anoxic atmosphere. Oxygen concentrations in the atmosphere and surface oceans first rose in the Great Oxygenation Event (GOE) 2.4 billion years ago, and a second increase beginning in the later Neoproterozoic Era [Neoproterozoic Oxygenation Event (NOE)] established the redox profile of modern oceans. The GOE facilitated the emergence of eukaryotes, whereas the NOE is associated with large and complex multicellular organisms. Thus, the GOE and NOE are fundamental pacemakers for evolution. On the time scale of Earth's entire 4 billion-year history, the evolutionary dynamics of the planet's biosphere appears to be fast, and the pace of evolution is largely determined by physical changes of the planet. However, in Phanerozoic ecosystems, interactions between new functions enabled by the accumulation of characters in a complex regulatory environment and changing biological components of effective environments appear to have an important influence on the timing of evolutionary innovations. On the much shorter time scale of transient environmental perturbations, such as those associated with mass extinctions, rates of genetic accommodation may have been limiting for life.

Massive increase in visual range preceded the origin of terrestrial vertebrates

The evolution of terrestrial vertebrates, starting around 385 million years ago, is an iconic moment in evolution that brings to mind images of fish transforming into four-legged animals. Here, we show that this radical change in body shape was preceded by an equally dramatic change in sensory abilities akin to transitioning from seeing over short distances in a dense fog to seeing over long distances on a clear day. Measurements of eye sockets and simulations of their evolution show that eyes nearly tripled in size just before vertebrates began living on land. Computational simulations of these animal's visual ecology show that for viewing objects through water, the increase in eye size provided a negligible increase in performance. However, when viewing objects through air, the increase in eye size provided a large increase in performance. The jump in eye size was, therefore, unlikely to have arisen for seeing through water and instead points to an unexpected hybrid of seeing through air while still primarily inhabiting water. Our results and several anatomical innovations arising at the same time suggest lifestyle similarity to crocodiles. The consequent combination of the increase in eye size and vision through air would have conferred a 1 million-fold increase in the amount of space within which objects could be seen. The "buena vista" hypothesis that our data suggest is that seeing opportunities from afar played a role in the subsequent evolution of fully terrestrial limbs as well as the emergence of elaborated action sequences through planning circuits in the nervous system.

Exploring Phylogenetic Relationships within Myriapoda and the Effects of Matrix Composition and Occupancy on Phylogenomic Reconstruction

Myriapods, including the diverse and familiar centipedes and millipedes, are one of the dominant terrestrial arthropod groups. Although molecular evidence has shown that Myriapoda is monophyletic, its internal phylogeny remains contentious and understudied, especially when compared to those of Chelicerata and Hexapoda. Until now, efforts have focused on taxon sampling (e.g., by including a handful of genes from many species) or on maximizing matrix size (e.g., by including hundreds or thousands of genes in just a few species), but a phylogeny maximizing sampling at both levels remains elusive. In this study, we analyzed 40 Illumina transcriptomes representing 3 of the 4 myriapod classes (Diplopoda, Chilopoda, and Symphyla); 25 transcriptomes were newly sequenced to maximize representation at the ordinal level in Diplopoda and at the family level in Chilopoda. Ten supermatrices were constructed to explore the effect of several potential phylogenetic biases (e.g., rate of evolution, heterotachy) at 3 levels of gene occupancy per taxon (50%, 75%, and 90%). Analyses based on maximum likelihood and Bayesian mixture models retrieved monophyly of each myriapod class, and resulted in 2 alternative phylogenetic positions for Symphyla, as sister group to Diplopoda + Chilopoda, or closer to Diplopoda, the latter hypothesis having been traditionally supported by morphology. Within centipedes, all orders were well supported, but 2 deep nodes remained in conflict in the different analyses despite dense taxon sampling at the family level. Relationships among centipede orders in all analyses conducted with the most complete matrix (90% occupancy) are at odds not only with the sparser but more gene-rich supermatrices (75% and 50% supermatrices) and with the matrices optimizing phylogenetic informativeness or most conserved genes, but also with previous hypotheses based on morphology, development, or other molecular data sets. Our results indicate that a high percentage of ribosomal proteins in the most complete matrices, in conjunction with distance from the root, can act in concert to compromise the estimated relationships within the ingroup. We discuss the implications of these findings in the context of the ever more prevalent quest for completeness in phylogenomic studies.

Recognising moulting behaviour in trilobites by examining morphology, development and preservation: Comment on Błażejowski et al. 2015

A 365 million year-old trilobite moult-carcass assemblage was described by Błażejowski et al. (2015) as the oldest direct evidence of moulting in the arthropod fossil record. Unfortunately, their suppositions are insufficiently supported by the data provided. Instead, the morphology, configuration and preservational context of the highly fossiliferous locality (Kowala Quarry, Poland) suggest that the specimen consists of two overlapping, queued carcasses. The wider fossil record of moulting actually extends back 520 million years, providing an unparalleled opportunity to study behaviour, ecology and development in early animals. Taking cues from modern analogues, it is possible to quantify precise details about moulting behaviour to determine broad-scale evolutionary trends, ontogenetic sequences and morphological selection pressures. In this review, we argue that this rich source of data has been underused in evolutionary studies, though has great potential for investigating the life history and evolution of arthropods in deep time.

A molecular palaeobiological exploration of arthropod terrestrialization

Understanding animal terrestrialization, the process through which animals colonized the land, is crucial to clarify extant biodiversity and biological adaptation. Arthropoda (insects, spiders, centipedes and their allies) represent the largest majority of terrestrial biodiversity. Here we implemented a molecular palaeobiological approach, merging molecular and fossil evidence, to elucidate the deepest history of the terrestrial arthropods. We focused on the three independent, Palaeozoic arthropod terrestrialization events (those of Myriapoda, Hexapoda and Arachnida) and showed that a marine route to the colonization of land is the most likely scenario. Molecular clock analyses confirmed an origin for the three terrestrial lineages bracketed between the Cambrian and the Silurian. While molecular divergence times for Arachnida are consistent with the fossil record, Myriapoda are inferred to have colonized land earlier, substantially predating trace or body fossil evidence. An estimated origin of myriapods by the Early Cambrian precedes the appearance of embryophytes and perhaps even terrestrial fungi, raising the possibility that terrestrialization had independent origins in crown-group myriapod lineages, consistent with morphological arguments for convergence in tracheal systems.This article is part of the themed issue 'Dating species divergences using rocks and clocks'.

A general methodology for collecting and preserving xystodesmid and other large millipedes for biodiversity research

Background

With an estimated 80% of species remaining undescribed (but see Brewer et al. 2012), millipede taxonomy offers the opportunity to discover new species and explore biodiversity. The lack of basic alpha taxonomic information regarding millipedes belies their significant ecological role and potential as premier models in ecological and evolutionary studies. The group possesses many fascinating biological properties (e.g., bioluminescence, mimicry, and complex chemical secretions) that have been the focus of several recent studies and are emerging avenues of future investigation.

New information

Here we summarize a methodology for large-bodied millipede collection, curation, and preservation for genetic analyses with the hope that sharing these techniques will stimulate interest in these charismatic detritivores.

Tentorial mobility in centipedes (Chilopoda) revisited: 3D reconstruction of the mandibulo-tentorial musculature of Geophilomorpha

Mandibular mechanisms in Geophilomorpha are revised based on three-dimensional reconstructions of the mandibulo-tentorial complex and its muscular equipment in Dicellophiluscarniolensis (Placodesmata) and Hydroschendylasubmarina (Adesmata). Tentorial structure compares closely in the two species and homologies can be proposed for the 14/17 muscles that attach to the tentorium. Both species retain homologues of muscles that in other Pleurostigmophora are traditionally thought to cause swinging movements of the tentorium that complement the mobility of the mandibles. Although the original set of tentorial muscles is simplified in Geophilomorpha, the arrangement of the preserved homologues conforms to a system of six degrees of freedom of movement, as in non-geophilomorph Pleurostigmophora. A simplification of the mandibular muscles is confirmed for Geophilomorpha, but our results reject absence of muscles that in other Pleurostigmophora primarily support see-saw movements of the mandibles. In the construction of the tentorium, paralabial sclerites seem to be involved in neither Placodesmata nor Adesmata, and we propose their loss in Geophilomorpha as a whole. Current insights on the tentorial skeleton and its musculature permit two alternative conclusions on their transformation in Geophilomorpha: either tentorial mobility is primarily maintained in both Placodesmata and Adesmata (contrary to Manton's arguments for immobility), or the traditional assumption of the tentorium as being mobile is a misinterpretation for Pleurostigmophora as a whole.

Cave Diplopoda of southern China with reference to millipede diversity in Southeast Asia

The diversity of Diplopoda in caves of southern China is remarkably high, often 5–6 species per cave, consisting mostly of local endemics and presumed troglobionts. These are evidently biased to just a few lineages, mainly members of the orders Chordeumatida and Callipodida, the families Cambalopsidae (Spirostreptida) and Haplodesmidae (Polydesmida) or the genera Pacidesmus, Epanerchodus and Glenniea (all Polydesmida, Polydesmidae), Trichopeltis (Polydesmida, Cryptodesmidae), Dexmoxytes (Polydesmida, Paradoxosomatidae) and Hyleoglomeris (Glomerida, Glomeridae). All these taxa, especially the Paradoxosomatidae and Cambalopsidae (usually amounting to about 60% and 10% of the total species diversity in the Oriental fauna, respectively), are moderately to highly speciose across Southeast Asia, being largely epigean. However, the epigean Diplopoda of southern China are yet badly understudied, since much of the collecting and taxonomic exploration efforts still focus on cavernicoles. The Oriental Region is the only biogeographic realm globally that harbours all 16 orders of Diplopoda, of which 14 have already been encountered in China and/or the immediately adjacent parts of Indochina. Thus, China may actually prove to support no less than 1,000 millipede species of various origins, mainly Oriental and Palaearctic.

The complete mitochondrial genome of the geophilomorph centipede Strigamia maritima

Strigamia maritima (Myriapoda; Chilopoda) is a species from the soil-living order of geophilomorph centipedes. The Geophilomorpha is the most speciose order of centipedes with over a 1000 species described. They are notable for their large number of appendage bearing segments and are being used as a laboratory model to study the embryological process of segmentation within the myriapods. Using a scaffold derived from the recently published genome of Strigamia maritima that contained multiple mitochondrial protein-coding genes, here we report the complete mitochondrial genome of Strigamia, the first from any geophilomorph centipede. The mitochondrial genome of S. maritima is a circular molecule of 14,938 base pairs, within which we could identify the typical mitochondrial genome complement of 13 protein-coding genes and 2 ribosomal RNA genes. Sequences resembling 16 of the 22 transfer RNA genes typical of metazoan mitochondrial genomes could be identified, many of which have clear deviations from the standard ‘cloverleaf’ secondary structures of tRNA. Phylogenetic trees derived from the concatenated alignment of protein-coding genes of S. maritima and >50 other metazoans were unable to resolve the Myriapoda as monophyletic, but did support a monophyletic group of chilopods: Strigamia was resolved as the sister group of the scolopendromorph Scolopocryptos sp. and these two (Geophilomorpha and Scolopendromorpha), along with the Lithobiomorpha, formed a monophyletic group the Pleurostigmomorpha. Gene order within the S. maritima mitochondrial genome is unique compared to any other arthropod or metazoan mitochondrial genome to which it has been compared. The highly unusual organisation of the mitochondrial genome of Strigamia maritima is in striking contrast with the conservatively evolving nuclear genome: sampling of more members of this order of centipedes will be required to see whether this unusual organization is typical of the Geophilomorpha or results from a more recent reorganisation in the lineage leading to Strigamia.

Morphological and molecular study of Symphyla from Colombia

The symphylans are a poorly studied group. In Colombia the number of symphylan species is unknown with only Scutigerellaimmaculata (Symphyla: Scutigerellidae) being reported previously. The aim of this research was to collect and identify the symphylan pests of flower crops in Colombia. Morphological descriptions showed that our specimens shared more than one of the characters that define different genera within Scutigerellidae. The COI barcode haplotype showed interspecific level genetic divergence with Scutigerellacauseyae (at least 23%) and Hanseniella sp. (22%). Furthermore, our Colombian symphylans shared the same COI haplotype as some Symphyla found in Cameroon indicating a wide geographical distribution of this taxon. Our results suggest the presence of a new genus or subgenus in the class Symphyla.

Quo vadis venomics? A roadmap to neglected venomous invertebrates

Venomics research is being revolutionized by the increased use of sensitive -omics techniques to identify venom toxins and their transcripts in both well studied and neglected venomous taxa. The study of neglected venomous taxa is necessary both for understanding the full diversity of venom systems that have evolved in the animal kingdom, and to robustly answer fundamental questions about the biology and evolution of venoms without the distorting effect that can result from the current bias introduced by some heavily studied taxa. In this review we draw the outlines of a roadmap into the diversity of poorly studied and understood venomous and putatively venomous invertebrates, which together represent tens of thousands of unique venoms. The main groups we discuss are crustaceans, flies, centipedes, non-spider and non-scorpion arachnids, annelids, molluscs, platyhelminths, nemerteans, and echinoderms. We review what is known about the morphology of the venom systems in these groups, the composition of their venoms, and the bioactivities of the venoms to provide researchers with an entry into a large and scattered literature. We conclude with a short discussion of some important methodological aspects that have come to light with the recent use of new -omics techniques in the study of venoms.

The ultrastructure of the midgut epithelium in millipedes (Myriapoda, Diplopoda)

The midgut epithelia of the millipedes Polyxenus lagurus, Archispirostreptus gigas and Julus scandinavius were analyzed under light and transmission electron microscopies. In order to detect the proliferation of regenerative cells, labeling with BrdU and antibodies against phosphohistone H3 were employed. A tube-shaped midgut of three millipedes examined spreads along the entire length of the middle region of the body. The epithelium is composed of digestive, secretory and regenerative cells. The digestive cells are responsible for the accumulation of metals and the reserve material as well as the synthesis of substances, which are then secreted into the midgut lumen. The secretions are of three types - merocrine, apocrine and microapocrine. The oval or pear-like shaped secretory cells do not come into contact with the midgut lumen and represent the closed type of secretory cells. They possess many electron-dense granules (J. scandinavius) or electron-dense granules and electron-lucent vesicles (A. gigas, P. lagurus), which are accompanied by cisterns of the rough endoplasmic reticulum. The regenerative cells are distributed individually among the basal regions of the digestive cells. The proliferation and differentiation of regenerative cells into the digestive cells occurred in J. scandinavius and A. gigas, while these processes were not observed in P. lagurus. As a result of the mitotic division of regenerative cells, one of the newly formed cells fulfills the role of a regenerative cell, while the second one differentiates into a digestive cell. We concluded that regenerative cells play the role of unipotent midgut stem cells.

Two flat-backed polydesmidan millipedes from the Miocene Chiapas-amber Lagerstätte, Mexico

Two species of fossil polydesmidan millipedes (Diplopoda: Polydesmida) embedded in amber are described from Miocene strata near Simojovel, in the Chiapas Highlands, Mexico. Maatidesmus paachtungen. et sp. nov., placed into Chelodesmidae Cook, 1895, and Anbarrhacus adamantisgen. et sp. nov., assigned in the family Platyrhacidae Pocock, 1895. Morphological data from fossil specimens have been recovered using 3D X-ray micro-computed tomography and regular to infrared-reflected microscopy. Both fossil species are recognizable as new primarily but not exclusively, by collum margin modification and remarkable paranotal and metatergite dorsal sculpture.

Phylogenetic position of Myriapoda revealed by 454 transcriptome sequencing

Myriapods had been considered closely allied to hexapods (insects and relatives). However, analyses of molecular sequence data have consistently placed Myriapoda either as a sister group of Pancrustacea, comprising crustaceans and hexapods, and thereby supporting the monophyly of Mandibulata, or retrieved Myriapoda as a sister group of Chelicerata (spiders, ticks, mites and allies). In addition, the relationships among the four myriapod groups (Pauropoda, Symphyla, Diplopoda, Chilopoda) are unclear. To resolve the phylogeny of myriapods and their relationship to other main arthropod groups, we collected transcriptome data from the symphylan Symphylella vulgaris, the centipedes Lithobius forficatus and Scolopendra dehaani, and the millipedes Polyxenus lagurus, Glomeris pustulata and Polydesmus angustus by 454 sequencing. We concatenated a multiple sequence alignment that contained 1550 orthologous single copy genes (1,109,847 amino acid positions) from 55 euarthropod and 14 outgroup taxa. The final selected alignment included 181 genes and 37,425 amino acid positions from 55 taxa, with eight myriapods and 33 other euarthropods. Bayesian analyses robustly recovered monophyletic Mandibulata, Pancrustacea and Myriapoda. Most analyses support a sister group relationship of Symphyla in respect to a clade comprising Chilopoda and Diplopoda. Inclusion of additional sequence data from nine myriapod species resulted in an alignment with poor data density, but broader taxon average. With this dataset we inferred Diplopoda+Pauropoda as closest relatives (i.e., Dignatha) and recovered monophyletic Helminthomorpha. Molecular clock calculations suggest an early Cambrian emergence of Myriapoda ∼513 million years ago and a late Cambrian divergence of myriapod classes. This implies a marine origin of the myriapods and independent terrestrialization events during myriapod evolution.

Evaluating topological conflict in centipede phylogeny using transcriptomic data sets

Relationships between the five extant orders of centipedes have been considered solved based on morphology. Phylogenies based on samples of up to a few dozen genes have largely been congruent with the morphological tree apart from an alternative placement of one order, the relictual Craterostigmomorpha, consisting of two species in Tasmania and New Zealand. To address this incongruence, novel transcriptomic data were generated to sample all five orders of centipedes and also used as a test case for studying gene-tree incongruence. Maximum likelihood and Bayesian mixture model analyses of a data set composed of 1,934 orthologs with 45% missing data, as well as the 389 orthologs in the least saturated, stationary quartile, retrieve strong support for a sister-group relationship between Craterostigmomorpha and all other pleurostigmophoran centipedes, of which the latter group is newly named Amalpighiata. The Amalpighiata hypothesis, which shows little gene-tree incongruence and is robust to the influence of among-taxon compositional heterogeneity, implies convergent evolution in several morphological and behavioral characters traditionally used in centipede phylogenetics, such as maternal brood care, but accords with patterns of first appearances in the fossil record.

The evolution of centipede venom claws - open questions and possible answers

The maxilliped venom claw is an intriguing structure in centipedes. We address open questions concerning this structure. The maxillipeds of fossil centipedes from the Carboniferous (about 300 million years old) have been described, but not been depicted previously. Re-investigation demonstrates that they resemble their modern counterparts. A Jurassic geophilomorph centipede (about 150 million years old) was originally described as possessing a rather leg-like maxilliped. Our re-investigation shows that the maxilliped is, in fact, highly specialized as in modern Geophilomorpha. A scenario for the evolution of the centipede maxilliped is presented. It supports one of the two supposed hypotheses of centipede phylogeny, the Pleurostigmophora hypothesis. Although this hypothesis appears now well established, many aspects of character evolution resulting from this phylogeny remain to be told in detail. One such aspect is the special joint of the maxilliped in some species of Cryptops. Cryptops is an in-group of Scolopendromorpha, but its maxilliped joint can resemble that of Lithobiomorpha or even possess a mixture of characters between the both. Detailed investigation of fossils, larger sample sizes of extant species, and developmental data will be necessary to allow further improvements of the reconstruction of the evolutionary history of centipedes.

Ultrastructure, functional morphology and evolution of recto-canal epidermal glands in Myriapoda

In Chilopoda, solitary epidermal glands are composed of a couple of cells only. These glands are highly abundant on the entire body surface and are distributed throughout the single-layered epidermis. Some authors provided more or less comprehensive observations on the structure of epidermal glands of specific chilopod taxa. However, no information is hitherto available on the ultrastructural diversity of these glands. Furthermore, potential homologies of these chilopod epidermal glands and of their characteristic cellular components remain unknown. Based on our results, we are now able to distinguish two types of epidermal glands in Chilopoda that can be clearly distinguished by their structure and the course of their conducting canal: recto-canal epidermal glands (rceg) and flexo-canal epidermal glands (fceg). In the present paper, we focus on the rceg. We examined the ultrastructural organization of these glands in the head region and on the anterior trunk segments of various representatives of the five extant chilopod orders by light- and electron-microscopy. According to our terminology, rceg consist of up to five different cell types including: a) distal canal cells, b) proximal canal cells, c) intermediary cells, and d) two different types of secretory cells. Intermediary and canal cells form a common conducting canal. The rceg may taxon-specifically differ in relative size and subcellular architecture, but all have the following features in common: 1) a wide distribution on various body regions among all five chilopod subtaxa, 2) the straight, broad and locally dilated conducting canal surrounded by closely packed microvilli or microvilliform infoldings around the apex of the canal cell(s), and 3) the tendency to aggregate to form compound glandular organs of massive size and complexity. Tricellular glandular units established by three different cell types are observed in Scutigeromorpha and Geophilomorpha, whereas four cell types constitute rceg in Lithobiomorpha and Craterostigmomorpha. Five different cell types per glandular unit are found only in Scolopendromorpha. The partial cuticularization of the lower part of the conducting canal formed by the intermediary cell, as found in Chilopoda, differs from the pattern described for equivalent euarthropod epidermal glands, as for instance in Hexapoda. Their wide distribution in Chilopoda and Progoneata makes it likely that tricellular rceg were at least present in the last common ancestor of the Myriapoda. Concerning Chilopoda, the evolution of highly diverse rceg is well explained on the basis of the Pleurostigmophora concept. Glands of the recto-canal type are also found in other arthropods. The paper discusses cases where homology of rceg and also fceg may be assumed beyond Myriapoda and briefly evaluates the potentials and the still-to-be-solved issues prior to use them as an additional character system to reconstruct the phylogeny of the Euarthropoda.

Revival of forgotten characters and modern imaging techniques help to produce a robust phylogeny of the Diplopoda (Arthropoda, Myriapoda)

The external and internal anatomy of millipedes (Diplopoda) is poorly known compared to some of the other myriapod and arthropod groups. Due to both language barriers, which hindered the assessment of the character-rich older literature, and non-phylogenetic thinking, our knowledge of morphological characters useful for phylogenetic work diminished over the last decades. Here, a new character matrix with 64 characters, mainly derived from old literature data, is used to reconstruct a phylogeny of Diplopoda. As a tool to further our knowledge about the morphology of the different millipede orders, we show how micro-computer tomography (μCT) can be used to assess and illustrate specific parts of the Platydesmida. With the advent of μCT it is now possible to analyse many taxa and characters in a comparatively short time. A focus is put on potential phylogenetically useful characters. Our results support a Verhoeffian classification of the Diplopoda: Polyxenida + Chilognatha. Pentazonia are the sistergroup to the Helminthomorpha. Colobognatha form the sistergroup to Eugnatha, the latter split into monophyletic Juliformia and Polydesmida + Nematophora.

Ordinal-level phylogenomics of the arthropod class Diplopoda (millipedes) based on an analysis of 221 nuclear protein-coding loci generated using next-generation sequence analyses

BACKGROUND

The ancient and diverse, yet understudied arthropod class Diplopoda, the millipedes, has a muddled taxonomic history. Despite having a cosmopolitan distribution and a number of unique and interesting characteristics, the group has received relatively little attention; interest in millipede systematics is low compared to taxa of comparable diversity. The existing classification of the group comprises 16 orders. Past attempts to reconstruct millipede phylogenies have suffered from a paucity of characters and included too few taxa to confidently resolve relationships and make formal nomenclatural changes. Herein, we reconstruct an ordinal-level phylogeny for the class Diplopoda using the largest character set ever assembled for the group.

METHODS

Transcriptomic sequences were obtained from exemplar taxa representing much of the diversity of millipede orders using second-generation (i.e., next-generation or high-throughput) sequencing. These data were subject to rigorous orthology selection and phylogenetic dataset optimization and then used to reconstruct phylogenies employing Bayesian inference and maximum likelihood optimality criteria. Ancestral reconstructions of sperm transfer appendage development (gonopods), presence of lateral defense secretion pores (ozopores), and presence of spinnerets were considered. The timings of major millipede lineage divergence points were estimated.

RESULTS

The resulting phylogeny differed from the existing classifications in a number of fundamental ways. Our phylogeny includes a grouping that has never been described (Juliformia+Merocheta+Stemmiulida), and the ancestral reconstructions suggest caution with respect to using spinnerets as a unifying characteristic for the Nematophora. Our results are shown to have significantly stronger support than previous hypotheses given our data. Our efforts represent the first step toward obtaining a well-supported and robust phylogeny of the Diplopoda that can be used to answer many questions concerning the evolution of this ancient and diverse animal group.

Development of the venom ducts in the centipede Scolopendra: an example of recapitulation

In contrast to previous claims that (a) there is a law of recapitulation and, conversely, (b) recapitulation never happens, the evolutionary repatterning of development can take many forms, of which recapitulation is one. Here, we add another example to the list of case studies of recapitulation. This example involves the development of the venom claws (forcipules) in the centipede Scolopendra subspinipes mutilans, and in particular the development of the duct through which venom flows from the gland that produces it (proximal) to the opening called the meatus (distal) through which it is injected into prey. Most of the information we present is from early postembryonic stages--these have been neglected in previous work on centipede development. We show that the venom ducts arise from sutures that are invaginations of the cuticle. In S. s. mutilans, the invagination in each forcipule forms into a tubular structure that detaches itself from the exoskeleton and moves toward the center of the forcipule. This is in contrast to extant Scutigera, and also, probably, Scolopendra's extinct Scutigera-like ancestors, where the duct remains attached to the cuticle of throughout development. Thus, S. s. mutilans exhibits a recapitulatory repatterning of development.

Stable phylogenetic patterns in scutigeromorph centipedes (Myriapoda : Chilopoda : Scutigeromorpha): dating the diversification of an ancient lineage of terrestrial arthropods

Although stable and well-supported relationships are in place for the three main clades (families) of Scutigeromorpha, the interrelationships of particular taxa within the most diverse family, Scutigeridae, are less clearly resolved. Novel molecular data for taxa from Mesoamerica, the Caribbean, southern Africa, New Guinea and previously unsampled parts of the Pacific are incorporated into phylogenetic analyses. Relationships across the tree are stable under variable analytical conditions, whether these are homology-based (multiple sequence alignment versus implied alignment; untrimmed versus trimmed datasets) or method-based (parsimony versus maximum likelihood). Hypervariable regions, contrary to common belief, add phylogenetic structure to the data, as measured by the increased support for many nodes when compared with the same alignments trimmed with Gblocks. Our analyses show that a Yule-3-rate model best explained the diversification of Scutigeromorpha during their 400 million years of history. More complete molecular data for the New Guinea genus Ballonema stabilise its position as sister group to Thereuoneminae. To reconcile scutigeromorph systematics with the phylogeny, the monotypic genus Madagassophora Verhoeff, 1936, is placed in synonymy with Scutigerina Silvestri, 1901 (n. syn.), its type species M. hova becoming Scutigerina hova (de Saussure & Zehntner, 1902) new comb. (from Scutigera), and Lassophora Verhoeff, 1905, is re-established for an Afro-Malagasy clade containing Lassophora nossibei (de Saussure & Zehntner, 1902) new comb. (from Scutigera) and a newly sequenced species from Mozambique that diverged at the base of the lineage to Thereuoneminae. The dated phylogeny of Scutigeromorpha is more consistent with ancient vicariant splits between Madagascar–southern Africa and Australia–New Caledonia than with younger dispersal scenarios, though some geologically young Pacific islands that harbour lineages dating to the Cretaceous demonstrate the potential for trans-oceanic dispersal.

The complete mitochondrial genome of Pauropus longiramus (Myriapoda: Pauropoda): implications on early diversification of the myriapods revealed from comparative analysis

Myriapods are among the earliest arthropods and may have evolved to become part of the terrestrial biota more than 400 million years ago. A noticeable lack of mitochondrial genome data from Pauropoda hampers phylogenetic and evolutionary studies within the subphylum Myriapoda. We sequenced the first complete mitochondrial genome of a microscopic pauropod, Pauropus longiramus (Arthropoda: Myriapoda), and conducted comprehensive mitogenomic analyses across the Myriapoda. The pauropod mitochondrial genome is a circular molecule of 14,487 bp long and contains the entire set of thirty-seven genes. Frequent intergenic overlaps occurred between adjacent tRNAs, and between tRNA and protein-coding genes. This is the first example of a mitochondrial genome with multiple intergenic overlaps and reveals a strategy for arthropods to effectively compact the mitochondrial genome by overlapping and truncating tRNA genes with neighbor genes, instead of only truncating tRNAs. Phylogenetic analyses based on protein-coding genes provide strong evidence that the sister group of Pauropoda is Symphyla. Additionally, approximately unbiased (AU) tests strongly support the Progoneata and confirm the basal position of Chilopoda in Myriapoda. This study provides an estimation of myriapod origins around 555 Ma (95% CI: 444-704 Ma) and this date is comparable with that of the Cambrian explosion and candidate myriapod-like fossils. A new time-scale suggests that deep radiations during early myriapod diversification occurred at least three times, not once as previously proposed. A Carboniferous origin of pauropods is congruent with the idea that these taxa are derived, rather than basal, progoneatans.

Millipede taxonomy after 250 years: classification and taxonomic practices in a mega-diverse yet understudied arthropod group

BACKGROUND

The arthropod class Diplopoda is a mega-diverse group comprising >12,000 described millipede species. The history of taxonomic research within the group is tumultuous and, consequently, has yielded a questionable higher-level classification. Few higher-taxa are defined using synapomorphies, and the practice of single taxon descriptions lacking a revisionary framework has produced many monotypic taxa. Additionally, taxonomic and geographic biases render global species diversity estimations unreliable. We test whether the ordinal taxa of the Diplopoda are consistent with regards to underlying taxonomic diversity, attempt to provide estimates for global species diversity, and examine millipede taxonomic effort at a global geographic scale.

METHODOLOGY/PRINCIPAL FINDINGS

A taxonomic distinctness metric was employed to assess uniformity of millipede ordinal taxa. We found that ordinal-level taxa are not uniform and are likely overinflated with higher-taxa when compared to related groups. Several methods of estimating global species richness were employed (Bayesian, variation in taxonomic productivity, extrapolation from nearly fully described taxa). Two of the three methods provided estimates ranging from 13,413-16,760 species. Variations in geographic diversity show biases to North America and Europe and a paucity of works on tropical taxa.

CONCLUSIONS/SIGNIFICANCE

Before taxa can be used in an extensible way, they must be definable with respect to the diversity they contain and the diagnostic characters used to delineate them. The higher classification for millipedes is shown to be problematic from a number of perspectives. Namely, the ordinal taxa are not uniform in their underlying diversity, and millipedes appear to have a disproportionate number of higher-taxa. Species diversity estimates are unreliable due to inconsistent taxonomic effort at temporal, geographic, and phylogenetic scales. Lack of knowledge concerning many millipede groups compounds these issues. Diplopods are likely not unique in this regard as these issues may persist in many other diverse yet poorly studied groups.

Phylogenetics of the millipede genus Brachycybe Wood, 1864 (Diplopoda: Platydesmida: Andrognathidae): patterns of deep evolutionary history and recent speciation

The genus Brachycybe Wood is a little known group of millipedes comprising eight nominal species distributed throughout North America, Japan, South Korea, Taiwan, and China. The group's species are relatively morphologically homogenous and have been described primarily on the basis of differences in somatic morphology largely ignoring the often-used characters in millipede taxonomy and systematics - male genitalia (the gonopods). The objectives of this study were to survey male gonopods with the aim of evaluating inter-specific variation, assess existing species boundaries and phylogeny using molecular characters, examine the historical biogeography of the genus, and estimate the timing of lineage divergence using a molecular clock. We surveyed two mitochondrial genes (cytochrome c oxidase I and cytochrome b) and one nuclear protein-coding gene (glutamyl and prolyl-tRNA synthetase) each comprising 952, 746, and 555 aligned base pairs respectively. Phylogenetic inference coupled with an analysis of species delimitation using a generalized mixed Yule coalescent model recovered eight species, two of which were considered to be cryptic. Molecular dating analyses coupled with ancestral range reconstructions indicate that the group is quite ancient (age>50 million years) and its origins are likely traced back geographically to the mountains of California's Sierra Nevada and Coastal Ranges. The genus Brachycybe appears to have expanded its range at least twice out of present day California into eastern North America and at least once into Asia. This study highlights the need for integrative approaches to describe biodiversity and furthers the evidence for cryptic diversity even in groups where genitalia are generally thought to be rapidly evolving, diagnostic features.