Abstract A head region with specialised appendages, sclerotization, and segmentation of the trunk (arthrodization), and its appendages (arthropodization) represent three key innovations in arthropod evolution. Two scenarios have been proposed for acquisition of these innovations either in a synchronous or a sequential mode. Here we describe a new species Sunella dimorphismus sp. nov., from the Chengjiang biota (ca. 518 Ma), which displays a bivalved carapace, raptorial frontal appendages, and an arthrodized trunk with a series of biramous arthropodized appendages revealed by new specimens and new observations with assistance of computed tomography. Phylogenetic analyses placed Sunella as the earliest diverging deuteropods besides Erratus. Ancestral state reconstructions refine our understanding of the gain and loss of key characters in the euarthropod. The results demonstrate that trunk limb arthropodization preceded trunk arthrodization, with both prior to the evolution of a six-segmented functional head, while the trunk arthrodization is found to be lost in isoxyiids.

Euarthropods are characterized by segmented, sclerotized, and articulated bodies and appendages, expressed through arthrodization and arthropodization. Both are regarded as defining features of Euarthropoda and have been proposed to evolve in a specific sequence, during which arthropodization preceded arthrodization, and initiated in frontal appendages and was subsequently co-opted into other appendages. Here, we investigate their evolution across Ecdysozoa using morphological comparison, ancestral-state reconstruction (ASR), and semi-quantitative methods. Morphological comparisons and ASR reveal repeated gains, reductions, losses, and regional decoupling of segmentation, sclerotization, and articulation in both body and appendicular systems. Semi-quantitative indices indicate variable degrees of arthrodization and arthropodization across ecdysozoan lineages and do not show a progressive increase from the basal panarthropods and the stem-group euarthropods to crown-group euarthropods. Together, these results support a modular, convergent evolution of arthrodization and arthropodization in different lineages, probably through repeated modification, reduction, or redeployment of the ancestral developmental toolkit.

The Artiopoda, exemplified by trilobites, represents a diverse group of euarthropods that thrived during the early Paleozoic. Acanthomeridion serratum, an early diverging artiopodan with a detailed exoskeleton record in the lower Cambrian (Series 2, Stage 3) Chengjiang Lagerstatte, presents challenges in understanding its soft tissues, particularly its post-antennal cephalic appendages. In this study, new micro-computed tomography (micro-CT) analysis reveals that these appendages feature a transversely elongated protopod with teeth-like gnathobasic spines, a delicate endopod without a prominent walking claw and a multisegmented exopod with widely spaced lamellae. Smaller sub-gnathobasic spines alternate with the larger teeth-like gnathobasic spines. The robust protopod, characterized by medially opposed gnathal edges, suggests a feeding function, indicative of a durophagous lifestyle. Phylogenetic analyses place A. serratum as a sister taxon to Zhiwenia, occupying a basal position in the Artiopoda. These findings provide direct evidence of durophagous predation in the Cambrian, likely driving the evolution of biomineralized shells or hard exoskeletons in various metazoan groups.

Chuandianella ovata, a waptiid “bivalved” arthropod occurred in strara of Cambrian Series 2 and Stage 3 (ca. 521–514.5 Ma) along the western margin of the Yangtze plate, has been reported in numerous publications, whereas many morphological features still require further investigation. Herein, based on 1459 finely preserved specimens from eight localities of the Chengjiang biota, seventeen new characteristics are described and documented, and five previously described traits are revised. In our reconstruction, the cephalon bore a pair of stalked eyes with setae along the convex margin and annuli at the basal part of the eye lobes, a median triangular sclerite, one pair of biramous antennules, and three pairs of uniramous appendages that may represent the mandibules, maxillules and maxillae, respectively. According to microtomography and fossil imaging, the thorax houses nine pairs of appendages, each identified as a quadripartite, comprising a protopodite, a reduced endopodite, flap-like setose exopodite, and annulated epipodite with numerous lamellae. This appendicular morphology provides additional evidence for the presence of exites (epipodites) in Palaeozoic arthropods. Re-examination of the trunk limbs of Cambrian euarthropods indicates that exites may be a common feature that evolved deep in arthropod phylogeny.

The origin and evolution of trilobated body plan of the Artiopoda, a group of epibenthic euarthropods from Cambrian Lagerstätten, remain unclear. Here we examine old and new specimens of Urokodia aequalis, one of euarthropods from the Chengjiang biota, revealing new morphological details and revising its taxonomy. Urokodia possesses an elongate body with a five-segmented head, a thorax with 13-15 tergites, and a three-segmented pygidium with well-defined axial region. The ventral morphology includes paired stalked eyes, one fleshy antenna pair, the following homogeneous head and thoracic appendages, each with an annular proximal-element, an articulated stenopodous branch and a lamellar flap, and the pygidial appendages solely consisting of lamellar flaps. Cladistic analyses resolved Urokodia as the basal-most member of the Artiopoda, offering a hypothesis of the initial origin of trilobation in the pygidium. The new data, in conjunction with the presence of the elongated body plan across major lineages of euarthropods, suggest a convergent evolution of this trait.

Radiodonts, a diverse clade of early Palaeozoic stem-group euarthropods, have provided critical information for understanding the evolution of this phylum, and were important constituents of marine ecosystems at this time. The well-known Chengjiang Lagerst & auml;tte (Cambrian Stage 3, c. 518 Ma) has yielded the highest known diversity of radiodonts of any Cambrian Konservat-Lagerst & auml;tten, and represents a crucial deposit for radiodont research. One important but generally overlooked Chengjiang radiodont taxon, previously identified as Anomalocaris sp. or Radiodont C, is herein designated as the type species of a new monotypic genus, Shucaris ankylosskelos gen. et sp. nov., based on dozens of specimens. Shucaris is distinctive for its combination of several characters, including the presence of two pairs of endites on its proximal-most claw podomere, posteriorly inward curved endites on claw podomeres 2-6, and, most strikingly, the coexistence of gnathobase-like structures and an oral cone. New material of Ramskoeldia consimilis records the first known oral cone in this species, confirming that oral cones and gnathobase-like structures co-occur in multiple radiodont taxa. Phylogenetic analyses retrieve Shucaris as either an early diverging member of Anomalocarididae or as sister to the clade Anomalocarididae + Amplectobeluidae. Moreover, our phylogenetic analyses also support the divergence between hurdiid and non-hurdiid radiodonts. https://zoobank.org/urn:lsid:zoobank.org:pub:6989C81B-06B2-4AED-8297-3B200A6F8D69

The Cambrian (Series 2, Stage 3) Qingjiang Lagerstatte of South China is one of the most diverse Burgess Shale-type deposits around the world, yielding abundant non-biomineralized fossils. Radiodonta, a taxonomically and ecologically diverse stem-euarthropod group, has been generally thought to represent the largest consumers in early Palaeozoic marine ecosystems. Here we describe several new radiodont specimens from the Qingjiang Lagerstatte assigned to various groups, including Stanleycaris qingjiangensis sp. nov., a new type of hurdiid head carapace, one possible Hurdia carapace, and two partial appendages with uncertain affinities. These specimens not only extend the geographic and stratigraphic range of these taxa, they also illuminate the diversity of radiodonts (in particular hurdiids) in their early evolutionary history. Radiodont palaeobiogeographic patterns are visualized using network analysis. Laurentia and South China share many members at the genus level, Anomalocaris is the most cosmopolitan taxon, but most genera are endemic. Radiodonts show a high initial diversity that declines through the early Palaeozoic, enabling three diversification phases of radiodonts to be recognized: the thriving phase (Cambrian Series 2), declining phase (Cambrian Miaolingian) and terminal phase (Cambrian Furongian to Ordovician Floian).

lsoxys is a worldwide distributed bivalved arthropod known almost exclusively from Cambrian Burgess Shale-type Lagerstätten. Outline analyses using 34 specimens of the iconic large bivalved arthropod Isoxys minor from the Cambrian Stage 3 (~518Ma) Qingjiang biota and the Cambrian Stage 4 Guanshan biota, interpret that they are the same species and there is a very slight difference in the shape of the outlines of the carapaces between the two biotas. This suggests that environment might be driving intraspecific variation. Quantitative analysis of shape changes during growth using 51 specimens of l. minor from the Qingjiang biota, reveals that its valves gradually elongate and the ratio of cardinal spines and spherical eyes relative to the valve length significantly decreases during postembryonic development. I. minor has proportionally large cardinal spines and eyes in the earliest stages, and this allometric growth is beneficial for self-protection and foraging, which may have improved the survival rate of individuals with these characters. In addition, two of the specimens document the evidence of brood care in l. minor, and the egg cluster occupies almost the entire dorsal region under the carapace. Compared to other early Paleozoic egg-carrying arthropods, l. minor broods have the highest number (~300 per clutch) of small (0, ~0.5mm) eggs. Since the ovigerous individuals are almost half the size of the adults, l. minor may have possessed reproductive ability during the early life stage. The results indicate that spines played an antipredatory role for l. minor, and that it followed a r-strategy of reproducing with many individuals at an early stage. l. minor also represents the earliest diverging arthropod from which brood care has been documented.

Segmentation and tagmatization have contributed to the preeminent success of arthropods since their first appearance in the Cambrian. However, the exact mechanism of segmentogenesis is still insufficiently known in living and extinct groups. Here, we describe the postembryonic development of a Waptiid arthropod Chuandianella ovata from the early Cambrian Chengjiang biota, South China. The new data illuminate a complex dynamic pattern of anamorphosis and epimorphosis, and a three-step process of segmentogenesis, i.e., the elongation of the terminal segment, delineation of an incipient segment, and full separation of a new segment. Compensatory growth is accomplished by rapid growth of new segments and/or generation of additional segments, which results in the trimorphism of the posterior tagma. Such complex developmental dynamics has rarely been known in the arthropod fossil record and its presence in early history helps to understand the rapid diversification of arthropods in the early Cambrian.

Fossil Lagerstätten similar to the Cambrian Burgess Shale have a crucial role in revealing the origin and early evolution of arthropods. Tuzoiidae, one of the most important groups of bivalved arthropods, occupied an important ecological niche in the Cambrian marine ecosystem. We describe a new taxon, Duplapex anima gen. et. sp. nov., in the Family Tuzoiidae on the basis of four exceptionally preserved specimens from the early Cambrian (Stage 3) Qingjiang biota of Hubei, South China. Duplapex is characterized by an ornamented bivalved carapace, a ventral notch and doublure spine on the valve and a pair of compound eyes connected by fleshy annulated eye stalks. Despite having an unusual morphology for the group, the new taxon is recognized as a tuzoiid arthropod and indicates that these problematic euarthropods had a greater degree of morphological disparity than previously considered. D. anima , as well as the newly reported Tuzoia. sp. from the Fandian biota, represent the oldest occurrence of Tuzoiidae, extending its stratigraphic range to Cambrian Stage 3 and expanding the palaeogeographical distribution of the group to the northern border of the Yangtze Platform. Thematic collection: This article is part of the Advances in the Cambrian Explosion collection available at: https://www.lyellcollection.org/cc/advances-cambrian-explosion

Ediacaran embryo‐like spherical fossils exhibit diverse cell adhesion patterns resembling partial cleavage‐stage embryos of living animals. Two three‐celled specimens characterized by a pair of small cells overlying a large cell have been recovered from the Ediacaran Zhenba microfossil assemblage. Their cell adhesion pattern is highly comparable to a phenomenon reported from the Weng'an biota that was interpreted as fossil embryos undergoing discoidal cleavage. However, our specimens contain fewer cells and thus probably represent developmental precursors of the Weng'an counterparts. Additionally, new material shows several anatomical features that are inconsistent with an embryo interpretation, including (1) an unusually large volume of “blastomeres,” (2) a putative nucleus preserved within the large “yolk cell,” and (3) completely separated cells. Collectively, the Zhenba embryo‐like specimens permit a reconstruction of the consecutive developmental sequence from single‐celled individuals to the three‐celled individuals, leading us to interpret the newly found specimens as products of abnormal development of Ediacaran embryo‐like organisms whose affinity remains unresolved.

双瓣壳类节肢动物在全球寒武纪海洋中广泛分布,是寒武纪时期布尔吉斯页岩型化石生物群的重要组成类群和典型代表.它们的起源、演化及生态学研究是对早期后生动物演化研究的重要内容.本文详细描述了峡东地区寒武系第二统第三阶水井沱组下段两种双瓣壳类节肢动物,Caudicaella bispinata(Cui and Huo,1990)comb.nov.和Sunella grandis Huo,1965.依据新建的Caudicaella属征,对孙氏虫科的鉴定特征进行了修订.首次研究了孙氏虫科两属种壳瓣的生长发育模式,揭示了壳瓣高长比的等速生长和C.bispinata基刺相对于壳长的异速生长关系.元素扫描分析研究表明,长阳地区天柱山村剖面化石为碳质压膜保存.功能形态学研究认为,C.bispinata是远洋浮游型生物,其广泛的古地理分布有望为寒武纪早期全球生物地层对比提供新的化石依据.

Arthropods other than trilobites or bradoriids constitute a major component of many Cambrian Burgess‐Shale‐type Lagerstätten. Owing to their scarcity in other less well‐preserved deposits, they are generally regarded as lightly sclerotized but non‐mineralized. Here we show that Chuandianella ovata , one of the typical lightly sclerotized arthropods from the Chengjiang biota, reinforced its carapace with phosphatic mineralization. Multiple methods reveal a pitted microstructure and organophosphatic composition of the carapace. Nanogranules of amorphous calcium phosphate (ACP) on the fossil carapace are comparable to those of recent crustacean microstructures, implying that arthropods may have a conserved mechanism of phosphatic mineralization since the Cambrian. The fossil record indicates that the mineralization of ecdysozoans, preceded by the mineralization of lophotrochozoans in the Terreneuvian (~541–521 Ma), appeared in Cambrian Stage 3 (~521–514 Ma) and surpassed the range of living counterparts in phylogenetic coverage. This phenomenon strengthens the view that biomineralization has evolved multiple times independently within the Metazoa. The sudden appearance of mineralized skeletons in the early history of metazoan evolution is frequently attributed to external environment drivers. However, this viewpoint cannot explain the minerology and appearance time of metazoan lineages with mineralized hard parts during the Cambrian explosion.

A treasure trove of Cambrian secrets Animal life exploded in diversity and form during the Cambrian period about 500 million years ago. Fu et al. describe an early Cambrian fossil site in China that contains a variety of specimens, more than half of which are previously undescribed (see the Perspective by Daley). The site rivals previously described Cambrian sites, such as the Burgess Shale, and should help to elucidate biological innovation and diversification during this period. Science , this issue p. 1338 ; see also p. 1284