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

An updated world checklist of velvet worms (Onychophora) with notes on nomenclature and status of names

More than a decade has passed since the publication of the only world checklist available for Onychophora. During this period, numerous nomenclatural acts and taxonomic changes have been suggested within the group and a wealth of novel data has been published on many taxa. Herein, the up-to-date taxonomic scenario within Onychophora is presented, with appraisal of name status. This checklist covers both extant (Peripatidae and Peripatopsidae) and fossil taxa, and each species is accompanied by information on synonyms, type designation, holotype location, type locality, and language of original description. Additional remarks include nomenclatural inconsistencies, synonymizations, name misspellings, conflicting collecting event data, availability of taxonomically informative molecular data, etc. According to the data, 237 species are currently assigned to Onychophora: 140 of Peripatopsidae, 92 of Peripatidae, and five fossil species with unclear relationship to extant taxa. Since the previous checklist, 37 species have been added to Onychophora, representing an increase of 18.5% in the diversity described for the group. Yet, taxonomic descriptions seem slow-paced, with an average of 3.6 onychophoran species being described annually. From the taxonomic standpoint, 216 species are valid, although many of them require morphological revision and molecular characterization; 21 species exhibit major taxonomic ambiguities and have been regarded as nomina dubia. Recurrent taxonomic issues identified in the literature include inaccurate collecting event data, doubtful taxonomic assignment of molecular sequences, and non-observance of nomenclatural rules. These and other taxonomic aspects are addressed herein in the light of the directives established by the International Code of Zoological Nomenclature.

Was There a Cambrian Explosion on Land? The Case of Arthropod Terrestrialization

Arthropods, the most diverse form of macroscopic life in the history of the Earth, originated in the sea. Since the early Cambrian, at least ~518 million years ago, these animals have dominated the oceans of the world. By the Silurian-Devonian, the fossil record attests to arthropods becoming the first animals to colonize land, However, a growing body of molecular dating and palaeontological evidence suggests that the three major terrestrial arthropod groups (myriapods, hexapods, and arachnids), as well as vascular plants, may have invaded land as early as the Cambrian-Ordovician. These dates precede the oldest fossil evidence of those groups and suggest an unrecorded continental "Cambrian explosion" a hundred million years prior to the formation of early complex terrestrial ecosystems in the Silurian-Devonian. We review the palaeontological, phylogenomic, and molecular clock evidence pertaining to the proposed Cambrian terrestrialization of the arthropods. We argue that despite the challenges posed by incomplete preservation and the scarcity of early Palaeozoic terrestrial deposits, the discrepancy between molecular clock estimates and the fossil record is narrower than is often claimed. We discuss strategies for closing the gap between molecular clock estimates and fossil data in the evolution of early ecosystems on land.

The velvet worm brain unveils homologies and evolutionary novelties across panarthropods

Background

The evolution of the brain and its major neuropils in Panarthropoda (comprising Arthropoda, Tardigrada and Onychophora) remains enigmatic. As one of the closest relatives of arthropods, onychophorans are regarded as indispensable for a broad understanding of the evolution of panarthropod organ systems, including the brain, whose anatomical and functional organisation is often used to gain insights into evolutionary relations. However, while numerous recent studies have clarified the organisation of many arthropod nervous systems, a detailed investigation of the onychophoran brain with current state-of-the-art approaches is lacking, and further inconsistencies in nomenclature and interpretation hamper its understanding. To clarify the origins and homology of cerebral structures across panarthropods, we analysed the brain architecture in the onychophoran Euperipatoides rowelli by combining X-ray micro-computed tomography, histology, immunohistochemistry, confocal microscopy, and three-dimensional reconstruction.

Results

Here, we use this detailed information to generate a consistent glossary for neuroanatomical studies of Onychophora. In addition, we report novel cerebral structures, provide novel details on previously known brain areas, and characterise further structures and neuropils in order to improve the reproducibility of neuroanatomical observations. Our findings support homology of mushroom bodies and central bodies in onychophorans and arthropods. Their antennal nerve cords and olfactory lobes most likely evolved independently. In contrast to previous reports, we found no evidence for second-order visual neuropils, or a frontal ganglion in the velvet worm brain.

Conclusion

We imaged the velvet worm nervous system at an unprecedented level of detail and compiled a comprehensive glossary of known and previously uncharacterised neuroanatomical structures to provide an in-depth characterisation of the onychophoran brain architecture. We expect that our data will improve the reproducibility and comparability of future neuroanatomical studies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01196-w.

Phylogenomic Analysis of Velvet Worms (Onychophora) Uncovers an Evolutionary Radiation in the Neotropics

Onychophora ("velvet worms") are charismatic soil invertebrates known for their status as a "living fossil," their phylogenetic affiliation to arthropods, and their distinctive biogeographic patterns. However, several aspects of their internal phylogenetic relationships remain unresolved, limiting our understanding of the group's evolutionary history, particularly with regard to changes in reproductive mode and dispersal ability. To address these gaps, we used RNA sequencing and phylogenomic analysis of transcriptomes to reconstruct the evolutionary relationships and infer divergence times within the phylum. We recovered a fully resolved and well-supported phylogeny for the circum-Antarctic family Peripatopsidae, which retains signals of Gondwanan vicariance and showcases the evolutionary lability of reproductive mode in the family. Within the Neotropical clade of Peripatidae, though, we found that amino acid-translated sequence data masked nearly all phylogenetic signal, resulting in highly unstable and poorly supported relationships. Analyses using nucleotide sequence data were able to resolve many more relationships, though we still saw discordant phylogenetic signal between genes, probably indicative of a rapid, mid-Cretaceous radiation in the group. Finally, we hypothesize that the unique reproductive mode of placentotrophic viviparity found in all Neotropical peripatids may have facilitated the multiple inferred instances of over-water dispersal and establishment on oceanic islands.

Panarthropod tiptop/teashirt and spalt orthologs and their potential role as "trunk"-selector genes

BACKGROUND

In the vinegar fly Drosophila melanogaster, the homeodomain containing transcription factor Teashirt (Tsh) appears to specify trunk identity in concert with the function of the Hox genes. While in Drosophila there is a second gene closely related to tsh, called tiptop (tio), in other arthropods species only one copy exists (called tio/tsh). The expression of tsh and tio/tsh, respectively, is surprisingly similar among arthropods suggesting that its function as trunk selector gene may be conserved. Other research, for example on the beetle Tribolium castaneum, questions even conservation of Tsh function among insects. The zinc-finger transcription factor Spalt (Sal) is involved in the regulation of Drosophila tsh, but this regulatory interaction does not appear to be conserved in Tribolium either. Whether the function and interaction of tsh and sal as potential trunk-specifiers, however, is conserved is still unclear because comparative studies on sal expression (except for Tribolium) are lacking, and functional data are (if at all existing) restricted to Insecta.

RESULTS

Here, we provide additional data on arthropod tsh expression, show the first data on onychophoran tio/tsh expression, and provide a comprehensive investigation on sal expression patterns in arthropods and an onychophoran.

CONCLUSIONS

Our data support the idea that tio/tsh genes are involved in the development of "trunk" segments by regulating limb development. Our data suggest further that the function of Sal is indeed unlikely to be conserved in trunk vs head development like in Drosophila, but early expression of sal is in line with a potential homeotic function, at least in Arthropoda.

Cuticular reticulation replicates the pattern of epidermal cells in lowermost Cambrian scalidophoran worms

The cuticle of ecdysozoans (Panarthropoda, Scalidophora, Nematoida) is secreted by underlying epidermal cells and renewed via ecdysis. We explore here the relationship between epidermis and external cuticular ornament in stem-group scalidophorans from the early Cambrian of China (Kuanchuanpu Formation; ca 535 Ma) that had two types of microscopic polygonal cuticular networks with either straight or microfolded boundaries. Detailed comparisons with modern scalidophorans (priapulids) indicate that these networks faithfully replicate the cell boundaries of the epidermis. This suggests that the cuticle of early scalidophorans formed through the fusion between patches of extracellular material secreted by epidermal cells, as observed in various groups of present-day ecdysozoans, including arthropods. Key genetic, biochemical and mechanical processes associated with ecdysis and cuticle formation seem to have appeared very early (at least not later than 535 Ma) in the evolution of ecdysozoans. Microfolded reticulation is likely to be a mechanical response to absorbing contraction exerted by underlying muscles. The polygonal reticulation in early and extant ecdysozoans is clearly a by-product of the epidermal cell pavement and interacted with the sedimentary environment.

A three-dimensionally preserved lobopodian from the Herefordshire (Silurian) Lagerstätte, UK

The Herefordshire (Silurian) Lagerstätte (approx. 430 Myr BP) has yielded, among many exceptionally preserved invertebrates, a wide range of new genera belonging to crown-group Panarthropoda. Here, we increase this panarthropod diversity with the lobopodian Thanahita distos, a new total-group panarthropod genus and species. This new lobopodian preserves at least nine paired, long, slender appendages, the anterior two in the head region and the posterior seven representing trunk lobopods. The body ends in a short post-appendicular extension. Some of the trunk lobopods bear two claws, others a single claw. The body is covered by paired, tuft-like papillae. Thanahita distos joins only seven other known three-dimensionally preserved lobopodian or onychophoran (velvet worm) fossil specimens and is the first lobopodian to be formally described from the Silurian. Phylogenetic analysis recovered it, together with all described Hallucigenia species, in a sister-clade to crown-group panarthropods. Its placement in a redefined Hallucigeniidae, an iconic Cambrian clade, indicates the survival of this clade to Silurian times.

The ‘Peripatos' in Eurogondwana? — Lack of evidence that southeast Asian onychophorans walked through Europe

Onychophorans, or velvet worms, are cryptic but extremely charismatic terrestrial invertebrates that have often been the subject of interesting biogeographic debate. Despite great interest, a well resolved and complete phylogeny of the group and a reliable chronogram have been elusive due to their broad geographic distribution, paucity of samples, and challenging molecular composition. Here we present a molecular phylogenetic analysis of Onychophora that includes previously unsampled and undersampled lineages and we analyse the expanded dataset using a series of nested taxon sets designed to increase the amount of information available for particular subclades. These include a dataset with outgroups, one restricted to the ingroup taxa, and three others for Peripatopsidae, Peripatidae and Neopatida (= the Neotropical Peripatidae). To explore competing biogeographic scenarios we generate a new time tree for Onychophora using the few available reliable fossils as calibration points. Comparing our results to those of Cyphophthalmi, we reconsider the hypothesis that velvet worms reached Southeast Asia via Eurogondwana, and conclude that a more likely scenario is that they reached Southeast Asia by rafting on the Sibumasu terrane. Our phylogenetic results support the reciprocal monophyly of both families as well as an early division between East and West Gondwana, also in both families, each beginning to diversify between the Permian and the Jurassic. Peripatopsidae clearly supports paraphyly of South Africa with respect to southern South America (Chile) and a sister group relationship of the Southeast Asian/New Guinean Paraperipatus to the Australian/New Zealand taxa. The latter includes a clade that divides between Western Australia and Eastern Australia and two sister clades of trans-Tasman species (one oviparous and one viviparous). This pattern clearly shows that oviparity is secondarily derived in velvet worms. Peripatidae finds a sister group relationship between the Southeast Asian Eoperipatus and the West Gondwanan clade, which divides into the African Mesoperipatus and Neopatida. The latter shows a well supported split between the Pacific Oroperipatus (although it is unclear whether they form one or two clades) and a sister clade that includes the members of the genera Peripatus, Epiperipatus, Macroperipatus and representatives of the monotypic genera Cerradopatus, Plicatoperipatus and Principapillatus. However, Peripatus, Epiperipatus and Macroperipatus are not monophyletic, and all the species from the monotypic genera are related to geographically close species. The same goes for the type species of Macroperipatus (from Trinidad, and sister group to other Trinidad and Tobago species of Epiperipatus) and Epiperipatus (from French Guiana, and related to other Guyana shield species of Epiperipatus and Peripatus). Geographic structure within Neopatida is largely obscured by an unresolved backbone, but many well supported instances of generic non-monophyly challenge the current taxonomic framework, which has often relied on anatomical characters that are untested phylogenetically.

Cryptic speciation in a biodiversity hotspot: multilocus molecular data reveal new velvet worm species from Western Australia (Onychophora : Peripatopsidae : Kumbadjena)

There is a yet uncovered multitude of species to be found among Western Australian Onychophora. Kumbadjena, one of the two genera that reside in this region, has been previously suggested to house an extensive species complex. Morphology alone has not been able to elucidate the diversity in this genus and has instead muddled species delineations. Topologies and species delimitation analyses resulting from the sequences of two mitochondrial ribosomal markers (12S rRNA and 16S rRNA), one nuclear ribosomal marker (18S rRNA), and one mitochondrial protein-coding gene (cytochrome c oxidase subunit I) are indicative of several undescribed species. Fixed diagnostic nucleotide changes in the highly conserved sequences of 18S rRNA warrant distinction of three new species of Kumbadjena: K. toolbrunupensis, sp. nov., K. karricola, sp. nov., and K. extrema, sp. nov. The geographic distributions of the proposed species suggest that Kumbadjena is another example of short-range endemism, a common occurrence in the flora and fauna of the region. The extensive biodiversity and endemism in the region necessitates conservation to preserve the species and processes that promote speciation harboured by Western Australia.

Origin and evolution of the panarthropod head - A palaeobiological and developmental perspective

The panarthropod head represents a complex body region that has evolved through the integration and functional specialization of the anterior appendage-bearing segments. Advances in the developmental biology of diverse extant organisms have led to a substantial clarity regarding the relationships of segmental homology between Onychophora (velvet worms), Tardigrada (water bears), and Euarthropoda (e.g. arachnids, myriapods, crustaceans, hexapods). The improved understanding of the segmental organization in panarthropods offers a novel perspective for interpreting the ubiquitous Cambrian fossil record of these successful animals. A combined palaeobiological and developmental approach to the study of the panarthropod head through deep time leads us to propose a consensus hypothesis for the intricate evolutionary history of this important tagma. The contribution of exceptionally preserved brains in Cambrian fossils - together with the recognition of segmentally informative morphological characters - illuminate the polarity for major anatomical features. The euarthropod stem-lineage provides a detailed view of the step-wise acquisition of critical characters, including the origin of a multiappendicular head formed by the fusion of several segments, and the transformation of the ancestral protocerebral limb pair into the labrum, following the postero-ventral migration of the mouth opening. Stem-group onychophorans demonstrate an independent ventral migration of the mouth and development of a multisegmented head, as well as the differentiation of the deutocerebral limbs as expressed in extant representatives. The anterior organization of crown-group Tardigrada retains several ancestral features, such as an anterior-facing mouth and one-segmented head. The proposed model aims to clarify contentious issues on the evolution of the panarthropod head, and lays the foundation from which to further address this complex subject in the future.

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.

Evolution: Velvet Worm Biogeography

The present-day distribution of velvet worms corresponds neatly to the ancient supercontinent Gondwana - except for a puzzling outpost in southeast Asia. Jaw-dropping new fossil material now establishes when and how peripatid onychophorans reached this isolated spot.