The kinematics of amblypygid (Arachnida) pedipalps during predation: extreme elongation in raptorial appendages does not result in a proportionate increase in reach and closing speed.

The link between form and function is key to understanding the evolution of unique and/or extreme morphologies. Amblypygids, or whip spiders, are arachnids that often have highly elongated spined pedipalps. These limbs are used to strike at, and secure, prey before processing by the chelicerae. Amblypygi pedipalps are multifunctional, however, being used in courtship and contest, and vary greatly in form between species. Increased pedipalp length may improve performance during prey capture, but length could also be influenced by factors including territorial contest and sexual selection. Here, for the first time, we used high-speed videography and manual tracking to investigate kinematic differences in prey capture between amblypygid species. Across six morphologically diverse species, spanning four genera and two families, we created a total dataset of 86 trials (9-20 per species). Prey capture kinematics varied considerably between species, with differences being expressed in pedipalp joint angle ranges. In particular, maximum reach ratio did not remain constant with total pedipalp length, as geometric scaling would predict, but decreased with longer pedipalps. This suggests that taxa with the most elongated pedipalps do not deploy their potential length advantage to proportionally increase reach. Therefore, a simple mechanical explanation of increased reach does not sufficiently explain pedipalp elongation. We propose other factors to help explain this phenomenon, such as social interactions or sexual selection, which would produce an evolutionary trade-off in pedipalp length between prey capture performance and other behavioural and/or anatomical pressures.

Fossils improve phylogenetic analyses of morphological characters.

Fossils provide our only direct window into evolutionary events in the distant past. Incorporating them into phylogenetic hypotheses of living clades can help time-calibrate divergences, as well as elucidate macroevolutionary dynamics. However, the effect fossils have on phylogenetic reconstruction from morphology remains controversial. The consequences of explicitly incorporating the stratigraphic ages of fossils using tip-dated inference are also unclear. Here, we use simulations to evaluate the performance of inference methods across different levels of fossil sampling and missing data. Our results show that fossil taxa improve phylogenetic analysis of morphological datasets, even when highly fragmentary. Irrespective of inference method, fossils improve the accuracy of phylogenies and increase the number of resolved nodes. They also induce the collapse of ancient and highly uncertain relationships that tend to be incorrectly resolved when sampling only extant taxa. Furthermore, tip-dated analyses under the fossilized birth-death process outperform undated methods of inference, demonstrating that the stratigraphic ages of fossils contain vital phylogenetic information. Fossils help to extract true phylogenetic signals from morphology, an effect that is mediated by both their distinctive morphology and their temporal information, and their incorporation in total-evidence phylogenetics is necessary to faithfully reconstruct evolutionary history.

Evolutionary simulations clarify and reconcile biodiversity-disturbance models.

There is significant geographic variation in species richness. However, the nature of the underlying relationships, such as that between species richness and environmental stability, remains unclear. The stability-time hypothesis suggests that environmental instability reduces species richness by suppressing speciation and increasing extinction risk. By contrast, the patch-mosaic hypothesis suggests that small-scale environmental instability can increase species richness by providing a steady supply of non-equilibrium environments. Although these hypotheses are often applied to different time scales, their core mechanisms are in conflict. Reconciling these apparently competing hypotheses is key to understanding how environmental conditions shape the distribution of biodiversity. Here, we use REvoSim, an individual-based, eco-evolutionary system, to model the evolution of sessile organisms in environments with varying magnitudes and scales of environmental instability. We demonstrate that when environments have substantial permanent heterogeneity, a high level of localized environmental instability reduces biodiversity, whereas in environments lacking permanent heterogeneity, high levels of localized instability increase biodiversity. By contrast, broad-scale environmental instability, acting on the same time scale, invariably reduces biodiversity. Our results provide a new view of the biodiversity-disturbance relationship that reconciles contrasting hypotheses within a single model and implies constraints on the environmental conditions under which those hypotheses apply. These constraints can inform attempts to conserve adaptive potential in different environments during the current biodiversity crisis.

Morphological Phylogenetics Evaluated Using Novel Evolutionary Simulations.

Evolutionary inferences require reliable phylogenies. Morphological data have traditionally been analyzed using maximum parsimony, but recent simulation studies have suggested that Bayesian analyses yield more accurate trees. This debate is ongoing, in part, because of ambiguity over modes of morphological evolution and a lack of appropriate models. Here, we investigate phylogenetic methods using two novel simulation models-one in which morphological characters evolve stochastically along lineages and another in which individuals undergo selection. Both models generate character data and lineage splitting simultaneously: the resulting trees are an emergent property, rather than a fixed parameter. Standard consensus methods for Bayesian searches (Mki) yield fewer incorrect nodes and quartets than the standard consensus trees recovered using equal weighting and implied weighting parsimony searches. Distances between the pool of derived trees (most parsimonious or posterior distribution) and the true trees-measured using Robinson-Foulds (RF), subtree prune and regraft (SPR), and tree bisection reconnection (TBR) metrics-demonstrate that this is related to the search strategy and consensus method of each technique. The amount and structure of homoplasy in character data differ between models. Morphological coherence, which has previously not been considered in this context, proves to be a more important factor for phylogenetic accuracy than homoplasy. Selection-based models exhibit relatively lower homoplasy, lower morphological coherence, and higher inaccuracy in inferred trees. Selection is a dominant driver of morphological evolution, but we demonstrate that it has a confounding effect on numerous character properties which are fundamental to phylogenetic inference. We suggest that the current debate should move beyond considerations of parsimony versus Bayesian, toward identifying modes of morphological evolution and using these to build models for probabilistic search methods. [Bayesian; evolution; morphology; parsimony; phylogenetics; selection; simulation.].

Olfactory flow in the sea catfish, Ariopsis felis (L.): Origin, regulation, and resampling.

The olfactory epithelium of the sea catfish, Ariopsis felis, is found on a pinnate array of lamellae (the olfactory rosette) housed within a nasal chamber. The nasal anatomy of A. felis suggests an ability to capture external water currents. We prepared models from X-ray micro-computed tomography scans of two preserved specimens of A. felis. We then used dye visualisation and computational fluid dynamics to show that an external current induced a flow of water through a) the nasal chamber and b) the sensory channels of the olfactory rosette. The factors responsible for inducing flow through the nasal chamber are common to fishes from two other orders. The dye visualisation experiments, together with observations of sea catfishes in vivo, indicate that flow through the nasal chamber is regulated by a mobile nasal flap. The position of the nasal flap - elevated (significant flow) or depressed (reduced flow) - is controlled by the sea catfish's movements. Flow in the sensory channels of the olfactory rosette can pass through either a single channel or, via multiple pathways, up to four consecutive channels. Flow through consecutive sensory channels (olfactory resampling) is more extensive at lower Reynolds numbers (200 and 300, equivalent to swimming speeds of 0.5-1.0 total lengths s-1), coinciding with the mean swimming speed of the sea catfishes observed in vivo (0.6 total lengths s-1). Olfactory resampling may also occur, via a vortex, within single sensory channels. In conclusion, olfactory flow in the sea catfish is regulated and thoroughly sampled by novel mechanisms.

The functional nasal anatomy of the pike, Esox lucius L.

Olfactory flow in fishes is a little-explored area of fundamental and applied importance. We investigated olfactory flow in the pike, Esox lucius, because it has an apparently simple and rigid nasal region. We characterised olfactory flow by dye visualisation and computational fluid dynamics, using models derived from X-ray micro-computed tomography scans of two preserved specimens. An external current induced a flow of water through the nasal chamber at physiologically relevant Reynolds numbers (200-300). We attribute this externally-induced flow to: the location of the incurrent nostril in a region of high static pressure; the nasal bridge deflecting external flow into the nasal chamber; an excurrent nostril normal to external flow; and viscous entrainment. A vortex in the incurrent nostril may be instrumental in viscous entrainment. Flow was dispersed over the olfactory sensory surface when it impacted on the floor of the nasal chamber. Dispersal may be assisted by: the radial array of nasal folds; a complementary interaction between a posterior nasal fold and the ventral surface of the nasal bridge; and the incurrent vortex. The boundary layer could delay considerably (up to ~ 3 s) odorant transport from the external environment to the nasal region. The drag incurred by olfactory flow was almost the same as the drag incurred by models in which the nasal region had been replaced by a smooth surface. The boundary layer does not detach from the nasal region. We conclude that the nasal bridge and the incurrent vortex are pivotal to olfaction in the pike.

Olfactory flow in the sturgeon is externally driven.

Fluid dynamics plays an important part in olfaction. Using the complementary techniques of dye visualisation and computational fluid dynamics (CFD), we investigated the hydrodynamics of the nasal region of the sturgeon Huso dauricus. H. dauricus offers several experimental advantages, including a well-developed, well-supported, radial array (rosette) of visible-by-eye olfactory sensory channels. We represented these features in an anatomically accurate rigid model derived from an X-ray scan of the head of a preserved museum specimen. We validated the results from the CFD simulation by comparing them with data from the dye visualisation experiments. We found that flow through both the nasal chamber and, crucially, the sensory channels could be induced by an external flow (caused by swimming in vivo) at a physiologically relevant Reynolds number. Flow through the nasal chamber arises from the anatomical arrangement of the incurrent and excurrent nostrils, and is assisted by the broad, cartilage-supported, inner wall of the incurrent nostril. Flow through the sensory channels arises when relatively high speed flow passing through the incurrent nostril encounters the circular central support of the olfactory rosette, decelerates, and is dispersed amongst the sensory channels. Vortices within the olfactory flow may assist odorant transport to the sensory surfaces. We conclude that swimming alone is sufficient to drive olfactory flow in H. dauricus, and consider the implications of our results for the three other extant genera of sturgeons (Acipenser, Pseudoscaphirhynchus and Scaphirhynchus), and for other fishes with olfactory rosettes.

Amber fossils demonstrate deep-time stability of Caribbean lizard communities.

Whether the structure of ecological communities can exhibit stability over macroevolutionary timescales has long been debated. The similarity of independently evolved Anolis lizard communities on environmentally similar Greater Antillean islands supports the notion that community evolution is deterministic. However, a dearth of Caribbean Anolis fossils--only three have been described to date--has precluded direct investigation of the stability of anole communities through time. Here we report on an additional 17 fossil anoles in Dominican amber dating to 15-20 My before the present. Using data collected primarily by X-ray microcomputed tomography (X-ray micro-CT), we demonstrate that the main elements of Hispaniolan anole ecomorphological diversity were in place in the Miocene. Phylogenetic analysis yields results consistent with the hypothesis that the ecomorphs that evolved in the Miocene are members of the same ecomorph clades extant today. The primary axes of ecomorphological diversity in the Hispaniolan anole fauna appear to have changed little between the Miocene and the present, providing evidence for the stability of ecological communities over macroevolutionary timescales.

The phylogeny of fossil whip spiders.

BACKGROUND: Arachnids are a highly successful group of land-dwelling arthropods. They are major contributors to modern terrestrial ecosystems, and have a deep evolutionary history. Whip spiders (Arachnida, Amblypygi), are one of the smaller arachnid orders with ca. 190 living species. Here we restudy one of the oldest fossil representatives of the group, Graeophonus anglicus Pocock, 1911 from the Late Carboniferous (Duckmantian, ca. 315 Ma) British Middle Coal Measures of the West Midlands, UK. Using X-ray microtomography, our principal aim was to resolve details of the limbs and mouthparts which would allow us to test whether this fossil belongs in the extant, relict family Paracharontidae; represented today by a single, blind species Paracharon caecus Hansen, 1921. RESULTS: Tomography reveals several novel and significant character states for G. anglicus; most notably in the chelicerae, pedipalps and walking legs. These allowed it to be scored into a phylogenetic analysis together with the recently described Paracharonopsis cambayensis Engel & Grimaldi, 2014 from the Eocene (ca. 52 Ma) Cambay amber, and Kronocharon prendinii Engel & Grimaldi, 2014 from Cretaceous (ca. 99 Ma) Burmese amber. We recovered relationships of the form ((Graeophonus (Paracharonopsis + Paracharon)) + (Charinus (Stygophrynus (Kronocharon (Charon (Musicodamon + Paraphrynus)))))). This tree largely reflects Peter Weygoldt's 1996 classification with its basic split into Paleoamblypygi and Euamblypygi lineages; we were able to score several of his characters for the first time in fossils. Our analysis draws into question the monophyly of the family Charontidae. CONCLUSIONS: Our data suggest that Graeophonus is a crown group amblypygid, and falls within a monophyletic Paleoamblypgi clade, but outside the family Paracharontidae (= Paracharonopsis + Paracharon). Our results also suggest a new placement for the Burmese amber genus Kronocharon, a node further down from its original position. Overall, we offer a broad phylogenetic framework for both the fossil and Recent whip spiders against which future discoveries can be tested.

Open data and digital morphology.

Over the past two decades, the development of methods for visualizing and analysing specimens digitally, in three and even four dimensions, has transformed the study of living and fossil organisms. However, the initial promise that the widespread application of such methods would facilitate access to the underlying digital data has not been fully achieved. The underlying datasets for many published studies are not readily or freely available, introducing a barrier to verification and reproducibility, and the reuse of data. There is no current agreement or policy on the amount and type of data that should be made available alongside studies that use, and in some cases are wholly reliant on, digital morphology. Here, we propose a set of recommendations for minimum standards and additional best practice for three-dimensional digital data publication, and review the issues around data storage, management and accessibility.

Almost a spider: a 305-million-year-old fossil arachnid and spider origins.

Spiders are an important animal group, with a long history. Details of their origins remain limited, with little knowledge of their stem group, and no insights into the sequence of character acquisition during spider evolution. We describe a new fossil arachnid, Idmonarachne brasierigen. et sp. nov. from the Late Carboniferous (Stephanian,ca 305-299 Ma) of Montceau-les-Mines, France. It is three-dimensionally preserved within a siderite concretion, allowing both laboratory- and synchrotron-based phase-contrast computed tomography reconstruction. The latter is a first for siderite-hosted fossils and has allowed us to investigate fine anatomical details. Although distinctly spider-like in habitus, this remarkable fossil lacks a key diagnostic character of Araneae: spinnerets on the underside of the opisthosoma. It also lacks a flagelliform telson found in the recently recognized, spider-related, Devonian-Permian Uraraneida. Cladistic analysis resolves our new fossil as sister group to the spiders: the spider stem-group comprises the uraraneids and I. brasieri While we are unable to demonstrate the presence of spigots in this fossil, the recovered phylogeny suggests the earliest character to evolve on the spider stem-group is the secretion of silk. This would have been followed by the loss of a flagelliform telson, and then the ability to spin silk using spinnerets. This last innovation defines the true spiders, significantly post-dates the origins of silk, and may be a key to the group's success. The Montceau-les-Mines locality has previously yielded a mesothele spider (with spinnerets). Evidently, Late Palaeozoic spiders lived alongside Palaeozoic arachnid grades which approached the spider condition, but did not express the full suite of crown-group autapomorphies.

Carboniferous Onychophora from Montceau-les-Mines, France, and onychophoran terrestrialization.

The geological age of the onychophoran crown-group, and when the group came onto land, have been sources of debate. Although stem-group Onychophora have been identified from as early as the Cambrian, the sparse record of terrestrial taxa from before the Cretaceous is subject to contradictory interpretations. A Late Carboniferous species from the Mazon Creek biota of the USA, Helenodora inopinata, originally interpreted as a crown-group onychophoran, has recently been allied to early Cambrian stem-group taxa. Here we describe a fossil species from the Late Carboniferous Montceau-les-Mines Lagerstätte, France, informally referred to as an onychophoran for more than 30 years. The onychophoran affinities of Antennipatus montceauensis gen. nov., sp. nov. are indicated by the form of the trunk plicae and the shape and spacing of their papillae, details of antennal annuli, and the presence of putative slime papillae. The poor preservation of several key systematic characters for extant Onychophora, however, prohibits the precise placement of the Carboniferous fossil in the stem or crown of the two extant families, or the onychophoran stem-group as a whole. Nevertheless, A. montceauensis is the most compelling candidate to date for a terrestrial Paleozoic onychophoran.

Silurian horseshoe crab illuminates the evolution of arthropod limbs.

The basic arrangement of limbs in euarthropods consists of a uniramous head appendage followed by a series of biramous appendages. The body is divided into functional units or tagmata which are usually distinguished by further differentiation of the limbs. The living horseshoe crabs are remnants of a much larger diversity of aquatic chelicerates. The limbs of the anterior and posterior divisions of the body of living horseshoe crabs differ in the loss of the outer and inner ramus, respectively, of an ancestral biramous limb. Here we report a new fossil horseshoe crab from the mid-Silurian Lagerstätte in Herefordshire, United Kingdom (approximately 425 Myr B.P.), a site that has yielded a remarkably preserved assemblage of soft-bodied fossils. The limbs of the new form can be homologized with those of living Limulus, but retain an ancestral biramous morphology. Remarkably, however, the two limb branches originate separately, providing fossil evidence to suggest that repression or loss of gene expression might have given rise to the appendage morphology of Limulus. Both branches of the prosomal limbs of this new fossil are robust and segmented in contrast to their morphology in Cambrian arthropods, revealing that a true biramous limb was once present in chelicerates as well as in the mandibulates.

Phylogenetic congruence, conflict and consilience between molecular and morphological data.

Morphology and molecules are important data sources for estimating evolutionary relationships. Modern studies often utilise morphological and molecular partitions alongside each other in combined analyses. However, the effect of combining phenomic and genomic partitions is unclear. This is exacerbated by their size imbalance, and conflict over the efficacy of different inference methods when using morphological characters. To systematically address the effect of topological incongruence, size imbalance, and tree inference methods, we conduct a meta-analysis of 32 combined (molecular + morphology) datasets across metazoa. Our results reveal that morphological-molecular topological incongruence is pervasive: these data partitions yield very different trees, irrespective of which method is used for morphology inference. Analysis of the combined data often yields unique trees that are not sampled by either partition individually, even with the inclusion of relatively small quantities of morphological characters. Differences between morphology inference methods in terms of resolution and congruence largely relate to consensus methods. Furthermore, stepping stone Bayes factor analyses reveal that morphological and molecular partitions are not consistently combinable, i.e. data partitions are not always best explained under a single evolutionary process. In light of these results, we advise that the congruence between morphological and molecular data partitions needs to be considered in combined analyses. Nonetheless, our results reveal that, for most datasets, morphology and molecules can, and should, be combined in order to best estimate evolutionary history and reveal hidden support for novel relationships. Studies that analyse only phenomic or genomic data in isolation are unlikely to provide the full evolutionary picture.

A Review of the "Black Beauty" Martian Regolith Breccia and Its Martian Habitability Record.

The regolith breccia Northwest Africa (NWA) 7034 and paired samples are unique meteorite representatives of the martian crust. They are water rich, lithologically varied, and preserve the oldest martian zircon grains yet discovered that formed ca. 4500-4300 Ma. The meteorite thus provides us with an invaluable record of the crustal and environmental conditions on early Mars. Resetting of some radioisotopic chronometers occurred in response to a major thermal disturbance event ca. 1500-1400 Ma, likely caused by an impactor that brecciated and redeposited NWA 7034 near the surface in an ejecta blanket. Lithologies comprising NWA 7034 were then aqueously altered by a long-lasting impact-induced hydrothermal system, before being excavated and ejected by a subsequent impact at ca. 5-15 Ma. This review compiles chronological and petrological information into an overarching geochronological summary for NWA 7034 and paired samples. We then provide a synopsis for the volatile (H2O, C) inventory and hydrothermal alteration history of NWA 7034. From this geochronological history and volatile inventory, we interpret and assess two potential periods of martian habitability: (1) an early window of pre-Noachian planetary habitability, and (2) impact-derived hydrothermal systems that allowed intermittent habitable crater environments well into the Amazonian.

Productivity, niche availability, species richness, and extinction risk: Untangling relationships using individual-based simulations.

It has often been suggested that the productivity of an ecosystem affects the number of species that it can support. Despite decades of study, the nature, extent, and underlying mechanisms of this relationship are unclear. One suggested mechanism is the "more individuals" hypothesis (MIH). This proposes that productivity controls the number of individuals in the ecosystem, and that more individuals can be divided into a greater number of species before their population size is sufficiently small for each to be at substantial risk of extinction. Here, we test this hypothesis using REvoSim: an individual-based eco-evolutionary system that simulates the evolution and speciation of populations over geological time, allowing phenomena occurring over timescales that cannot be easily observed in the real world to be evaluated. The individual-based nature of this system allows us to remove assumptions about the nature of speciation and extinction that previous models have had to make. Many of the predictions of the MIH are supported in our simulations: Rare species are more likely to undergo extinction than common species, and species richness scales with productivity. However, we also find support for relationships that contradict the predictions of the strict MIH: species population size scales with productivity, and species extinction risk is better predicted by relative than absolute species population size, apparently due to increased competition when total community abundance is higher. Furthermore, we show that the scaling of species richness with productivity depends upon the ability of species to partition niche space. Consequently, we suggest that the MIH is applicable only to ecosystems in which niche partitioning has not been halted by species saturation. Some hypotheses regarding patterns of biodiversity implicitly or explicitly overlook niche theory in favor of neutral explanations, as has historically been the case with the MIH. Our simulations demonstrate that niche theory exerts a control on the applicability of the MIH and thus needs to be accounted for in macroecology.

Time-lapse three-dimensional imaging of crack propagation in beetle cuticle.

Arthropod cuticle has extraordinary properties. It is very stiff and tough whilst being lightweight, yet it is made of rather ordinary constituents. This desirable combination of properties results from a hierarchical structure, but we currently have a poor understanding of how this impedes damage propagation. Here we use non-destructive, time-lapse in situ tensile testing within an X-ray nanotomography (nCT) system to visualise crack progression through dry beetle elytron (wing case) cuticle in 3D. We find that its hierarchical pseudo-orthogonal laminated microstructure exploits many extrinsic toughening mechanisms, including crack deflection, fibre and laminate pull-out and crack bridging. We highlight lessons to be learned in the design of engineering structures from the toughening methods employed. STATEMENT OF SIGNIFICANCE: We present the first comprehensive study of the damage and toughening mechanisms within arthropod cuticle in a 3D time-lapse manner, using X-ray nanotomography during crack growth. This technique allows lamina to be isolated despite being convex, which limits 2D analysis of microstructure. We report toughening mechanisms previously unobserved in unmineralised cuticle such as crack deflection, fibre and laminate pull-out and crack bridging; and provide insights into the effects of hierarchical microstructure on crack propagation. Ultimately the benefits of the hierarchical microstructure found here can not only be used to improve biomimetic design, but also helps us to understand the remarkable success of arthropods on Earth.

Terrestrial invertebrates in the Rhynie chert ecosystem.

The Early Devonian Rhynie and Windyfield cherts remain a key locality for understanding early life and ecology on land. They host the oldest unequivocal nematode worm (Nematoda), which may also offer the earliest evidence for herbivory via plant parasitism. The trigonotarbids (Arachnida: Trigonotarbida) preserve the oldest book lungs and were probably predators that practiced liquid feeding. The oldest mites (Arachnida: Acariformes) are represented by taxa which include mycophages and predators on nematodes today. The earliest harvestman (Arachnida: Opiliones) includes the first preserved tracheae, and male and female genitalia. Myriapods are represented by a scutigeromorph centipede (Chilopoda: Scutigeromorpha), probably a cursorial predator on the substrate, and a putative millipede (Diplopoda). The oldest springtails (Hexapoda: Collembola) were probably mycophages, and another hexapod of uncertain affinities preserves a gut infill of phytodebris. The first true insects (Hexapoda: Insecta) are represented by a species known from chewing (non-carnivorous?) mandibles. Coprolites also provide insights into diet, and we challenge previous assumptions that several taxa were spore-feeders. Rhynie appears to preserve a largely intact community of terrestrial animals, although some expected groups are absent. The known fossils are (ecologically) consistent with at least part of the fauna found around modern Icelandic hot springs.This article is part of a discussion meeting issue 'The Rhynie cherts: our earliest terrestrial ecosystem revisited'.

Sexual dimorphism in the Arachnid orders.

Sexual differences in size and shape are common across the animal kingdom. The study of sexual dimorphism (SD) can provide insight into the sexual- and natural-selection pressures experienced by males and females in different species. Arachnids are diverse, comprising over 100,000 species, and exhibit some of the more extreme forms of SD in the animal kingdom, with the males and females of some species differing dramatically in body shape and/or size. Despite this, research on arachnid SD has primarily focused on specific clades as opposed to observing traits across arachnid orders, the smallest of which have received comparatively little attention. This review provides an overview of the research to date on the trends and potential evolutionary drivers for SD and sexual size dimorphism (SSD) in individual arachnid orders, and across arachnids as a whole. The most common trends across Arachnida are female-biased SSD in total body size, male-biased SSD in relative leg length and SD in pedipalp length and shape. However, the evolution of sexually dimorphic traits within the group is difficult to elucidate due to uncertainty in arachnid phylogenetic relationships. Based on the dataset we have gathered here, we highlight gaps in our current understanding and suggest areas for future research.

Cretaceous arachnid Chimerarachne yingi gen. et sp. nov. illuminates spider origins.

Spiders (Araneae) are a hugely successful lineage with a long history. Details of their origins remain obscure, with little knowledge of their stem group and few insights into the sequence of character acquisition during spider evolution. Here, we describe Chimerarachne yingi gen. et sp. nov., a remarkable arachnid from the mid-Cretaceous (approximately 100 million years ago) Burmese amber of Myanmar, which documents a key transition stage in spider evolution. Like uraraneids, the two fossils available retain a segmented opisthosoma bearing a whip-like telson, but also preserve two traditional synapomorphies for Araneae: a male pedipalp modified for sperm transfer and well-defined spinnerets resembling those of modern mesothele spiders. This unique character combination resolves C. yingi within a clade including both Araneae and Uraraneida; however, its exact position relative to these orders is sensitive to different parameters of our phylogenetic analysis. Our new fossil most likely represents the earliest branch of the Araneae, and implies that there was a lineage of tailed spiders that presumably originated in the Palaeozoic and survived at least into the Cretaceous of Southeast Asia.