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The early Cambrian Kylinxia zhangi occupies a pivotal position in arthropod evolution, branching from the euarthropod stem lineage between radiodonts (Anomalocaris and relatives) and "great-appendage" arthropods. 1 , 2 Its combination of appendage and exoskeletal features is viewed as uniquely bridging the morphologies of so-called "lower" and "upper" stem-group euarthropods. 3 , 4 Microtomographic study of new specimens of Kylinxia refines and corrects previous interpretation of head structures in this species. Phylogenetic analyses incorporating new data reinforce the placement of Kylinxia in the euarthropod stem group but support new hypotheses of head evolution. The head of Kylinxia is composed of six segments, as in extant mandibulates, e.g., insects. 5 In Kylinxia, these are an anterior sclerite associated with an unpaired median eye and paired lateral eyes (thus three rather than five eyes as was previously described 1 ), deutocerebral frontal-most appendages, and four pairs of biramous appendages (rather than two pairs of uniramous appendages). Phylogenetic trees suggest that a six-segmented head in the euarthropod crown group was already acquired by a common ancestor with Kylinxia. The segmental alignment and homology of spinose frontal-most appendages between radiodonts and upper stem-group euarthropods 6 , 7 , 8 , 9 , 10 is bolstered by morphological similarities and inferred phylogenetic continuity between Kylinxia and other stem-group euarthropods., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2023 Elsevier Inc. All rights reserved.)

Among extant animals, Lophotrochozoa accounts for the majority of phyla. 1 This bilaterian clade radiated rapidly during the Cambrian explosion, obfuscating its phylogenetic relationships and rendering many aspects of its early evolution uncertain. Many early lophotrochozoans are known only from isolated skeletal microfossils, "small shelly fossils," often derived from larger animals with complex multi-element skeletons. 2 The discovery of articulated fossils has revealed surprising insights into the animals from which these skeletal pieces were derived, such as paired shells in the mollusc Halkieria. 3 Tommotiids are a key group of phosphatic early skeletal fossils that first appear in the late early Cambrian. 4 , 5 Although their affinities were previously obscure, discoveries of partial scleritomes and investigations of growth and microstructure 6 provide links with Brachiopoda 7 , 8 and Phoronida, 9 two of the lophophorate phyla. By contrast, the body plan of camenellan tommotiids remains a palaeontological mystery, with hypothetical reconstructions representing motile, benthic, dorsally armored worms. 4 , 10 Here, we describe an articulated camenellan (Wufengella bengtsoni gen. et sp. nov.) from the Cambrian Chengjiang Biota, China, revealing the morphology of the scleritome and the first soft tissues from an adult tommotiid. Wufengella carries two dorsal rows of sclerites in a highly asymmetric arrangement, flanked by smaller, cap-shaped sclerites. The scleritome was fringed by iterated fascicles of chaetae and two layers of flattened lobes. Phylogenetic analysis confirms that camenellans occupy a deep branch in lophophorate phylogeny, prior to the acquisition of a sessile lifestyle. Wufengella reveals direct evidence for a metameric body plan reminiscent of annelids early in the evolutionary history of lophophorates. 11 , 12 ., Competing Interests: Declaration of interests O.B. declares possession of shares in FlyDome, the equipment used to generate the RTI datasets., (Copyright © 2022 The Authors. Published by Elsevier Inc. All rights reserved.)

Variability in segment numbers in the world's most-leggy millipede adds support to a multiplicative mode of segment generation in myriapods., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2022 Elsevier Inc. All rights reserved.)

Facivermis yunnanicus [1, 2] is an enigmatic worm-like animal from the early Cambrian Chengjiang Biota of Yunnan Province, China. It is a small (<10 cm) bilaterian with five pairs of spiny anterior arms, an elongated body, and a swollen posterior end. The unusual morphology of Facivermis has prompted a history of diverse taxonomic interpretations, including among annelids [1, 3], lophophorates [4], and pentastomids [5]. However, in other studies, Facivermis is considered to be more similar to lobopodians [2, 6-8]-the fossil grade from which modern panarthropods (arthropods, onychophorans, and tardigrades) are derived. In these studies, Facivermis is thought to be intermediate between cycloneuralian worms and lobopodians. Facivermis has therefore been suggested to represent an early endobenthic-epibenthic panarthropod transition [6] and to provide crucial insights into the origin of paired appendages [2]. However, the systematic affinity of Facivermis was poorly supported in a previous phylogeny [6], partially due to incomplete understanding of its morphology. Therefore, the evolutionary significance of Facivermis remains unresolved. In this study, we re-examine Facivermis from new material and the holotype, leading to the discovery of several new morphological features, such as paired eyes on the head and a dwelling tube. Comprehensive phylogenetic analyses using parsimony, Bayesian inference, and maximum likelihood all support Facivermis as a luolishaniid in a derived position within the onychophoran stem group rather than as a basal panarthropod. In contrast to previous studies, we therefore conclude that Facivermis provides a rare early Cambrian example of secondary loss to accommodate a highly specialized tube-dwelling lifestyle., Competing Interests: Declaration of Interests The authors declare no competing interests., (Copyright © 2020 The Authors. Published by Elsevier Inc. All rights reserved.)

Arthropods are the most diverse animal phylum, and their phylogenetic relationships have been debated for centuries. With the advent of molecular phylogenetics, arthropods were found to be monophyletic and placed within a clade of molting animals, the ecdysozoans, with nematodes and six other phyla. Molecular phylogenetics also provided a new framework for relationships between the major arthropod groups, such as the clade Pancrustacea, which comprises insects and crustaceans. Phylogenomics based on second-generation genomics and transcriptomics has further resolved puzzles such as the exact position of myriapods or the closest crustacean relatives of hexapods. It is now broadly recognized that extant arthropods are split into chelicerates and mandibulates, and relationships within the two mandibulate clades (myriapods and pancrustaceans) are stabilizing. Notably, the phylogeny of insects is now understood with considerable confidence, whereas relationships among chelicerate orders remain poorly resolved. The evolutionary history of arthropods is illuminated by a rich record of fossils, often with exquisite preservation, but current analyses conflict over whether certain fossil groups are stem- or crown-group arthropods. Molecular time-trees calibrated with fossils estimate the origins of arthropods to be in the Ediacaran, while most other deep nodes date to the Cambrian. The earliest stem-group arthropods were lobopodians, worm-like animals with annulated appendages. Confidently placing some key extinct clades on the arthropod tree of life may require less ambiguous interpretation of fossil structures and better integration of morphological data into the phylogeny., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

The origin of ctenophores (comb jellies) is obscured by their controversial phylogenetic position, with recent phylogenomic analyses resolving either sponges or ctenophores as the sister group of all other animals. Fossil taxa can provide morphological evidence that may elucidate the origins of derived characters and shared ancestries among divergent taxa, providing a means to "break" long branches in phylogenetic trees. Here we describe new fossil material from the early Cambrian Chengjiang Biota, Yunnan Province, China, including the putative cnidarian Xianguangia, the new taxon Daihua sanqiong gen et sp. nov., and Dinomischus venustus, informally referred to as "dinomischids" here. "Dinomischids" possess a basal calyx encircled by 18 tentacles that surround the mouth. The tentacles carry pinnules, each with a row of stiff filamentous structures interpreted as very large compound cilia of a size otherwise only known in ctenophores. Together with the Cambrian tulip animal Siphusauctum and the armored Cambrian scleroctenophores, they exhibit anatomies that trace ctenophores to a sessile, polypoid stem lineage. This body plan resembles the polypoid, tentaculate morphology of cnidarians, including a blind gastric cavity partitioned by mesenteries. We propose that comb rows are derived from tentacles with paired sets of pinnules that each bear a row of compound cilia. The scleroctenophores exhibit paired comb rows, also observed in Siphusauctum, in addition to an organic skeleton, shared as well by Dinomischus, Daihua, and Xianguangia. We formulate a hypothesis in which ctenophores evolved from sessile, polypoid suspension feeders, sharing similarities with cnidarians that suggest either a close relationship between these two phyla, a striking pattern of early convergent evolution, or an ancestral condition for either metazoans or eumetazoans., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Cambrian fossils underpin a new hypothesis for body plan evolution in the deepest branching lineages of jawed arthropods, the mandibulates., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

The discovery of fossilized brains and ventral nerve cords in lower and mid-Cambrian arthropods has led to crucial insights about the evolution of their central nervous system, the segmental identity of head appendages and the early evolution of eyes and their underlying visual systems. Fundamental ground patterns of lower Cambrian arthropod brains and nervous systems correspond to the ground patterns of brains and nervous systems belonging to three of four major extant panarthropod lineages. These findings demonstrate the evolutionary stability of early neural arrangements over an immense time span. Here, we put these fossil discoveries in the context of evidence from cladistics, as well as developmental and comparative neuroanatomy, which together suggest that despite many evolved modifications of neuropil centers within arthropod brains and ganglia, highly conserved arrangements have been retained. Recent phylogenies of the arthropods, based on fossil and molecular evidence, and estimates of divergence dates, suggest that neural ground patterns characterizing onychophorans, chelicerates and mandibulates are likely to have diverged between the terminal Ediacaran and earliest Cambrian, heralding the exuberant diversification of body forms that account for the Cambrian Explosion., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

The record of arthropod body fossils is traceable back to the "Cambrian explosion," marked by the appearance of most major animal phyla. Exceptional preservation provides crucial evidence for panarthropod early radiation. However, due to limited representation in the fossil record of internal anatomy, particularly the CNS, studies usually rely on exoskeletal and appendicular morphology. Recent studiesshow that despite extreme morphological disparities, euarthropod CNS evolution appears to have been remarkably conservative. This conclusion is supported by descriptions from Cambrian panarthropods of neural structures that contribute to understanding early evolution of nervous systems and resolving controversies about segmental homologies. However, the rarity of fossilized CNSs, even when exoskeletons and appendages show high levels of integrity, brought into question data reproducibility because all but one of the aforementioned studies were based on single specimens. Foremost among objections is the lack of taphonomic explanation for exceptional preservation of a tissue that some see as too prone to decay to be fossilized. Here we describe newly discovered specimens of the Chengjiang euarthropod Fuxianhuia protensa with fossilized brains revealing matching profiles, allowing rigorous testing of the reproducibility of cerebral structures. Their geochemical analyses provide crucial insights of taphonomic pathways for brain preservation, ranging from uniform carbon compressions to complete pyritization, revealing that neural tissue was initially preserved as carbonaceous film and subsequently pyritized. This mode of preservation is consistent with the taphonomic pathways of gross anatomy, indicating that no special mode is required for fossilization of labile neural tissue., (Copyright © 2015 The Authors. Published by Elsevier Ltd.. All rights reserved.)

An anomalocaridid from the Ordovician exposes a second set of body flaps and reopens the question of how the two branches of arthropod legs evolved., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

The near-simultaneous appearance of most modern animal body plans (phyla) ~530 million years ago during the Cambrian explosion is strong evidence for a brief interval of rapid phenotypic and genetic innovation, yet the exact speed and nature of this grand adaptive radiation remain debated. Crucially, rates of morphological evolution in the past (i.e., in ancestral lineages) can be inferred from phenotypic differences among living organisms-just as molecular evolutionary rates in ancestral lineages can be inferred from genetic divergences. We here employed Bayesian and maximum likelihood phylogenetic clock methods on an extensive anatomical and genomic data set for arthropods, the most diverse phylum in the Cambrian and today. Assuming an Ediacaran origin for arthropods, phenotypic evolution was ~4 times faster, and molecular evolution ~5.5 times faster, during the Cambrian explosion compared to all subsequent parts of the Phanerozoic. These rapid evolutionary rates are robust to assumptions about the precise age of arthropods. Surprisingly, these fast early rates do not change substantially even if the radiation of arthropods is compressed entirely into the Cambrian (~542 mega-annum [Ma]) or telescoped into the Cryogenian (~650 Ma). The fastest inferred rates are still consistent with evolution by natural selection and with data from living organisms, potentially resolving "Darwin's dilemma." However, evolution during the Cambrian explosion was unusual (compared to the subsequent Phanerozoic) in that fast rates were present across many lineages., (Copyright © 2013 Elsevier Ltd. All rights reserved.)