Testing the phylogenetic position of Cambrian pancrustacean larval fossils by coding ontogenetic stages

Major Revisions in Pancrustacean Phylogeny and Evidence of Sensitivity to Taxon Sampling

The clade Pancrustacea, comprising crustaceans and hexapods, is the most diverse group of animals on earth, containing over 80% of animal species and half of animal biomass. It has been the subject of several recent phylogenomic analyses, yet relationships within Pancrustacea show a notable lack of stability. Here, the phylogeny is estimated with expanded taxon sampling, particularly of malacostracans. We show small changes in taxon sampling have large impacts on phylogenetic estimation. By analyzing identical orthologs between two slightly different taxon sets, we show that the differences in the resulting topologies are due primarily to the effects of taxon sampling on the phylogenetic reconstruction method. We compare trees resulting from our phylogenomic analyses with those from the literature to explore the large tree space of pancrustacean phylogenetic hypotheses and find that statistical topology tests reject the previously published trees in favor of the maximum likelihood trees produced here. Our results reject several clades including Caridoida, Eucarida, Multicrustacea, Vericrustacea, and Syncarida. Notably, we find Copepoda nested within Allotriocarida with high support and recover a novel relationship between decapods, euphausiids, and syncarids that we refer to as the Syneucarida. With denser taxon sampling, we find Stomatopoda sister to this latter clade, which we collectively name Stomatocarida, dividing Malacostraca into three clades: Leptostraca, Peracarida, and Stomatocarida. A new Bayesian divergence time estimation is conducted using 13 vetted fossils. We review our results in the context of other pancrustacean phylogenetic hypotheses and highlight 15 key taxa to sample in future studies.

The problematic Cambrian arthropod Tuzoia and the origin of mandibulates revisited

The origin of mandibulates, the hyperdiverse arthropod group that includes pancrustaceans and myriapods, dates back to the Cambrian. Bivalved arthropod groups such as hymenocarines have been argued to be early mandibulates, but many species are still poorly known, and their affinities remain uncertain. One of the most common and globally distributed Cambrian bivalved arthropods is Tuzoia. Originally described in 1912 from the Burgess Shale based on isolated carapaces, its full anatomy has remained largely unknown. Here, we describe new specimens of Tuzoia from the Canadian Burgess Shale (Wuliuan, Cambrian) showcasing exceptionally preserved soft tissues, allowing for the first comprehensive reconstruction of its anatomy, ecology and evolutionary affinities. The head bears antennae and differentiated cephalic appendages. The body is divided into a cephalothorax, a homonomous trunk bearing ca 10 pairs of legs with heptopodomerous endopods and enlarged basipods, and a tail fan with two pairs of caudal rami. These traits suggest that Tuzoia swam along the seafloor and used its spinose legs for predation or scavenging. Tuzoia is retrieved by a Bayesian phylogenetic analysis as an early mandibulate hymenocarine lineage, exemplifying the rapid diversification of this group in open marine environments during the Cambrian Explosion.

Ordovician opabiniid-like animals and the role of the proboscis in euarthropod head evolution

A crucial step in the evolution of Euarthropoda (chelicerates, myriapods, pancrustaceans) was the transition between fossil groups that possessed frontal appendages innervated by the first segment of the brain (protocerebrum), and living groups with a protocerebral labrum and paired appendages innervated by the second brain segment (deutocerebrum). Appendage homologies between the groups are controversial. Here we describe two specimens of opabiniid-like euarthropods, each bearing an anterior proboscis (a fused protocerebral appendage), from the Middle Ordovician Castle Bank Biota, Wales, UK. Phylogenetic analyses support a paraphyletic grade of stem-group euarthropods with fused protocerebral appendages and a posterior-facing mouth, as in the iconic Cambrian panarthropod Opabinia. These results suggest that the labrum may have reduced from an already-fused proboscis, rather than a pair of arthropodized appendages. If some shared features between the Castle Bank specimens and radiodonts are considered convergent rather than homologous, phylogenetic analyses retrieve them as opabiniids, substantially extending the geographic and temporal range of Opabiniidae.

Ontogeny, Phylotypic Periods, Paedomorphosis, and Ontogenetic Systematics

The key terms linking ontogeny and evolution are briefly reviewed. It is shown that their application and usage in the modern biology are often inconsistent and incorrectly understood even within the “evo-devo” field. For instance, the core modern reformulation that ontogeny not merely recapitulates, but produces phylogeny implies that ontogeny and phylogeny are closely interconnected. However, the vast modern phylogenetic and taxonomic fields largely omit ontogeny as a central concept. Instead, the common “clade-” and “tree-thinking” prevail, despite on the all achievements of the evo-devo. This is because the main conceptual basis of the modern biology is fundamentally ontogeny-free. In another words, in the Haeckel’s pair of “ontogeny and phylogeny,” ontogeny is still just a subsidiary for the evolutionary process (and hence, phylogeny), instead as in reality, its main driving force. The phylotypic periods is another important term of the evo-devo and represent a modern reformulation of Haeckel’s recapitulations and biogenetic law. However, surprisingly, this one of the most important biological evidence, based on the natural ontogenetic grounds, in the phylogenetic field that can be alleged as a “non-evolutionary concept.” All these observations clearly imply that a major revision of the main terms which are associated with the “ontogeny and phylogeny/evolution” field is urgently necessarily. Thus, “ontogenetic” is not just an endless addition to the term “systematics,” but instead a crucial term, without it neither systematics, nor biology have sense. To consistently employ the modern ontogenetic and epigenetic achievements, the concept of ontogenetic systematics is hereby refined. Ontogenetic systematics is not merely a “research program” but a key biological discipline which consistently links the enormous biological diversity with underlying fundamental process of ontogeny at both molecular and morphological levels. The paedomorphosis is another widespread ontogenetic-and-evolutionary process that is significantly underestimated or misinterpreted by the current phylogenetics and taxonomy. The term paedomorphosis is refined, as initially proposed to link ontogeny with evolution, whereas “neoteny” and “progenesis” are originally specific, narrow terms without evolutionary context, and should not be used as synonyms of paedomorphosis. Examples of application of the principles of ontogenetic systematics represented by such disparate animal groups as nudibranch molluscs and ophiuroid echinoderms clearly demonstrate that perseverance of the phylotypic periods is based not only on the classic examples in vertebrates, but it is a universal phenomenon in all organisms, including disparate animal phyla.

The origin and early evolution of arthropods

The rise of arthropods is a decisive event in the history of life. Likely the first animals to have established themselves on land and in the air, arthropods have pervaded nearly all ecosystems and have become pillars of the planet's ecological networks. Forerunners of this saga, exceptionally well-preserved Palaeozoic fossils recently discovered or re-discovered using new approaches and techniques have elucidated the precocious appearance of extant lineages at the onset of the Cambrian explosion, and pointed to the critical role of the plankton and hard integuments in early arthropod diversification. The notion put forward at the beginning of the century that the acquisition of extant arthropod characters was stepwise and represented by the majority of Cambrian fossil taxa is being rewritten. Although some key traits leading to Euarthropoda are indeed well documented along a diversified phylogenetic stem, this stem led to several speciose and ecologically diverse radiations leaving descendants late into the Palaeozoic, and a large part, if not all of the Cambrian euarthropods can now be placed on either of the two extant lineages: Mandibulata and Chelicerata. These new observations and discoveries have altered our view on the nature and timing of the Cambrian explosion and clarified diagnostic characters at the origin of extant arthropods, but also raised new questions, especially with respect to cephalic plasticity. There is now strong evidence that early arthropods shared a homologous frontalmost appendage, coined here the cheira, which likely evolved into antennules and chelicerae, but other aspects, such as brain and labrum evolution, are still subject to active debate. The early evolution of panarthropods was generally driven by increased mastication and predation efficiency and sophistication, but a wealth of recent studies have also highlighted the prevalent role of suspension-feeding, for which early panarthropods developed their own adaptive feedback through both specialized appendages and the diversification of small, morphologically differentiated larvae. In a context of general integumental differentiation and hardening across Cambrian metazoans, arthrodization of body and limbs notably prompted two diverging strategies of basipod differentiation, which arguably became founding criteria in the divergence of total-groups Mandibulata and Chelicerata. The kinship of trilobites and their relatives remains a source of disagreement, but a recent topological solution, termed the 'deep split', could embed Artiopoda as sister taxa to chelicerates and constitute definitive support for Arachnomorpha. Although Cambrian fossils have been critical to all these findings, data of exceptional quality have also been accumulating from other Palaeozoic Konservat-Lagerstätten, and a better integration of this information promises a much more complete and elaborate picture of early arthropod evolution in the near future. From the broader perspective of a total-evidence approach to the understanding of life's history, and despite persisting systematic debates and new interpretative challenges, various advances based on palaeontological evidence open the prospect of finally using the full potential of the most diverse animal phylum to investigate macroevolutionary patterns and processes.

New opabiniid diversifies the weirdest wonders of the euarthropod stem group

Once considered 'weird wonders' of the Cambrian, the emblematic Burgess Shale animals Anomalocaris and Opabinia are now recognized as lower stem-group euarthropods and have provided crucial data for constraining the polarity of key morphological characters in the group. Anomalocaris and its relatives (radiodonts) had worldwide distribution and survived until at least the Devonian. However, despite intense study, Opabinia remains the only formally described opabiniid to date. Here we reinterpret a fossil from the Wheeler Formation of Utah as a new opabiniid, Utaurora comosa nov. gen. et sp. By visualizing the sample of phylogenetic topologies in treespace, our results fortify support for the position of U. comosa beyond the nodal support traditionally applied. Our phylogenetic evidence expands opabiniids to multiple Cambrian stages. Our results underscore the power of treespace visualization for resolving imperfectly preserved fossils and expanding the known diversity and spatio-temporal ranges within the euarthropod lower stem group.

Pancrustacean Evolution Illuminated by Taxon-Rich Genomic-Scale Data Sets with an Expanded Remipede Sampling

The relationships of crustaceans and hexapods (Pancrustacea) have been much discussed and partially elucidated following the emergence of phylogenomic data sets. However, major uncertainties still remain regarding the position of iconic taxa such as Branchiopoda, Copepoda, Remipedia, and Cephalocarida, and the sister group relationship of hexapods. We assembled the most taxon-rich phylogenomic pancrustacean data set to date and analyzed it using a variety of methodological approaches. We prioritized low levels of missing data and found that some clades were consistently recovered independently of the analytical approach used. These include, for example, Oligostraca and Altocrustacea. Substantial support was also found for Allotriocarida, with Remipedia as the sister of Hexapoda (i.e., Labiocarida), and Branchiopoda as the sister of Labiocarida, a clade that we name Athalassocarida (="nonmarine shrimps"). Within Allotriocarida, Cephalocarida was found as the sister of Athalassocarida. Finally, moderate support was found for Hexanauplia (Copepoda as sister to Thecostraca) in alliance with Malacostraca. Mapping key crustacean tagmosis patterns and developmental characters across the revised phylogeny suggests that the ancestral pancrustacean was relatively short-bodied, with extreme body elongation and anamorphic development emerging later in pancrustacean evolution.

Waptia fieldensis Walcott, a mandibulate arthropod from the middle Cambrian Burgess Shale

Waptia fieldensis Walcott, 1912 is one of the iconic animals from the middle Cambrian Burgess Shale biota that had lacked a formal description since its discovery at the beginning of the twentieth century. This study, based on over 1800 specimens, finds that W. fieldensis shares general characteristics with pancrustaceans, as previous authors had suggested based mostly on its overall aspect. The cephalothorax is covered by a flexible, bivalved carapace and houses a pair of long multisegmented antennules, palp-bearing mandibles, maxillules, and four pairs of appendages with five-segmented endopods-the anterior three pairs with long and robust enditic basipods, the fourth pair with proximal annulations and lamellae. The post-cephalothorax has six pairs of lamellate and fully annulated appendages which appear to be extensively modified basipods rather than exopods. The front part of the body bears a pair of stalked eyes with the first ommatidia preserved in a Burgess Shale arthropod, and a median 'labral' complex flanked by lobate projections with possible affinities to hemi-ellipsoid bodies. Waptia confirms the mandibulate affinity of hymenocarines, retrieved here as part of an expanded Pancrustacea, thereby providing a novel perspective on the evolutionary history of this hyperdiverse group. We construe that Waptia was an active swimming predator of soft prey items, using its anterior appendages for food capture and manipulation, and also potentially for clinging to epibenthic substrates.

Finding hot singles: matching males to females in dimorphic spiders (Araneidae: Micrathena) using phylogenetic placement and DNA barcoding

Many orb-weaving spiders exhibit remarkable sexual dimorphism, hampering the matching of males and females in taxonomic studies. This is the case for the spiny Micrathena spiders, a species-rich Neotropical genus with 27% of its species known from a single sex. In this paper we document several undescribed Micrathena specimens, and test whether they belong to some of those incompletely known species. In order to do so, we: (1) tested the phylogenetic position of males and their putative females using a previous morphological dataset; (2) calculated genetic distances among individuals based on a fragment of the mitochondrial gene cytochrome c oxidase subunit I; and (3) examined their geographical distributions. These approaches, isolated or in combination, allowed us to identify and describe the previously unknown males of M. embira Levi, M. reimoseri Mello-Leitão, M. exlinae Levi, M. miles Simon, M. spinulata F.O. Pickard-Cambridge, M. yanomami Magalhães & Santos and M. cornuta (Taczanowski), and the female of M. beta di Caporiacco. We found that the males previously associated with M. bicolor (Keyserling), M. cornuta and M. lata Chickering had been incorrectly matched with females. The latter actually belongs to a hitherto unnamed species, herein described as Micrathena perfida, sp. nov. New geographical data are given for these and other Micrathena species. Our study highlights the importance of using different sources of data for matching the sexes in diverse groups with strong sexual dimorphism. http://zoobank.org/urn:lsid:zoobank.org:urn:lsid:zoobank.org:pub:AFEDA522-D0B4-4902-A747-9721DEED8B89

Paedomorphosis as an Evolutionary Driving Force: Insights from Deep-Sea Brittle Stars

Heterochronic development has been proposed to have played an important role in the evolution of echinoderms. In the class Ophiuroidea, paedomorphosis (retention of juvenile characters into adulthood) has been documented in the families Ophiuridae and Ophiolepididae but not been investigated on a broader taxonomic scale. Historical errors, confusing juvenile stages with paedomorphic species, show the difficulties in correctly identifying the effects of heterochrony on development and evolution. This study presents a detailed analysis of 40 species with morphologies showing various degrees of juvenile appearance in late ontogeny. They are compared to a range of early ontogenetic stages from paedomorphic and non-paedomorphic species. Both quantitative and qualitative measurements are taken and analysed. The results suggest that strongly paedomorphic species are usually larger than other species at comparable developmental stage. The findings support recent notions of polyphyletic origin of the families Ophiuridae and Ophiolepididae. The importance of paedomorphosis and its correct recognition for the practice of taxonomy and phylogeny are emphasized.

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'.

When Ontogeny Matters: A New Japanese Species of Brittle Star Illustrates the Importance of Considering both Adult and Juvenile Characters in Taxonomic Practice

Current taxonomy offers numerous approaches and methods for species delimitation and description. However, most of them are based on the adult characters and rarely suggest a dynamic representation of developmental transformations of taxonomically important features. Here we show how the underestimation of ontogenetic changes may result in long term lack of recognition of a new species of one of the most common ophiacanthid brittle stars (Echinodermata: Ophiuroidea) from the North Pacific. Based on vast material collected predominantly by various Japanese expeditions in the course of more than 50 years, and thorough study of appropriate type material, we revise the complex of three common species of the ophiuroid genus Ophiacantha which have been persistently confused with each other. The present study thus reveals the previously unrecognized new species Ophiacantha kokusai sp.nov. which is commonly distributed off the Pacific coast of Japan. The new species shows developmental differentiation from the closely related species Ophiacantha rhachophora H. L. Clark, 1911 and retains clearly expressed early juvenile features in the adult morphology. Another species, Ophiacantha clypeata Kyte, 1977, which had been separated from O. rhachophora, is in turn shown to be just a juvenile stage of another North Pacific species, Ophiacantha trachybactra H.L. Clark, 1911. For every species, detailed morphological data from both adult and juvenile specimens based on scanning electron microscopy are presented. A special grinding method showing complex internal features has been utilized for the first time. For all three species in this complex, a clear bathymetric differentiation is revealed: O. rhachophora predominantly inhabits shallow waters, 0-250 m, the new species O. kokusai lives deeper, at 250-600 m, and the third species, O. trachybactra, is found at 500-2,000 m. The present case clearly highlights the importance of considering developmental transformations, not only for a limited number of model organisms, but as part of the taxonomic process.