Discovery of diverse Pectocaris species at the Cambrian series 2 Hongjingshao formation Xiazhuang section (Kunming, SW China) and its ecological, taphonomic, and biostratigraphic implications.

Pectocaris species are intermediate- to large-sized Cambrian bivalved arthropods. Previous studies have documented Pectocaris exclusively from the Cambrian Series 2 Stage 3 Chengjiang biota in Yu'anshan Formation, Chiungchussu Stage in SW China. In this study, we report Pectocaris paraspatiosa sp. nov., and three other previously known Pectocaris from the Xiazhuang section in Kunming, which belongs to the Hongjingshao Formation and is a later phase within Cambrian Stage 3 than the Yu'anshan Formation. The new species can be distinguished from its congeners by the sparsely arranged endopodal endites and the morphologies of the abdomen, telson, and telson processes. We interpret P. paraspatiosa sp. nov. as a filter-feeder and a powerful swimmer adapted to shallow, agitated environments. Comparison among the Pectocaris species reinforces previous views that niche differentiation had been established among the congeneric species based on morphological differentiation. Our study shows the comprehensive occurrences of Pectocaris species outside the Chengjiang biota for the first time. With a review of the shared fossil taxa of Chengjiang and Xiaoshiba biotas, we identify a strong biological connection between the Yu'anshan and Hongjingshao Formations.

Three-dimensional morphology of the biramous appendages in Isoxys from the early Cambrian of South China, and its implications for early euarthropod evolution.

Early euarthropod evolution involved a major transition from lobopodian-like taxa to organisms featuring a segmented, well-sclerotized trunk (arthrodization) and limbs (arthropodization). However, the precise origin of a completely arthrodized trunk and arthropodized ventral biramous appendages remain controversial, as well as the early onset of anterior-posterior limb differentiation in stem-group euarthropods. New fossil material and micro-computed tomography inform the detailed morphology of the arthropodized biramous appendages in the carapace-bearing euarthropod Isoxys curvirostratus from the early Cambrian Chengjiang biota. In addition to well-developed grasping frontal appendages, I. curvirostratus possesses two batches of morphologically and functionally distinct biramous limbs. The first batch consists of four pairs of short cephalic appendages with robust endites with a feeding function, whereas the second batch has more elongate trunk appendages for locomotion. Critically, our new material shows that the trunk of I. curvirostratus was not arthrodized. The results of our phylogenetic analyses recover isoxyids as some of the earliest branching sclerotized euarthropods, and strengthens the hypothesis that arthropodized biramous appendages evolved before full body arthrodization.

Distinct lake sedimentary imprints of earthquakes, floods and human activities in the Xiaojiang Fault zone: Towards a quantitative paleoseismograph in the southeastern Tibetan Plateau.

Lake sediments that widely distributed in the active and complicated fault zones have been recently showing great potential for paleoseismic reconstruction. However, flood events and human activities may make the seismic signal unrecognizable. In this study, high-resolution analyses of sedimentary structure, physical and chemical proxies, as well as absolutely radioactive dating were conducted on seven representative sediment cores from the depocenter, nearshore and inlet areas of Yangzong Lake, a typical fault lake in the Xiaojiang Fault zone, southeastern Tibetan Plateau (TP). These new data were calibrated by historical documents, suggesting that seismically induced mass-transport deposits (MTDs, i.e., turbidites) were massive and/or amalgamated (earthquake doublet), became fining and thickening towards the lake center (without changing lake morphology), and occasionally exhibited soft sediment deformation structures (SSDs, i.e., microfaults). These sediments were relatively poorly sorted and instantaneously deposited from slope failures within the lake. An extremely strong earthquake could cause coseismic subsidence of the lake basin and destruct the local hydrological system, resulting in exceptionally high Mn and total inorganic carbon (TIC) contents in the lake center. In contrast, flood deposits were thinner with horizontal beddings, had higher terrestrial organic matter (higher C/N ratios), and distributed locally in the lake inlet area. Human activities-induced sediments were inversely graded, poorly sorted and gradually deposited, had horizontal beddings and no erosive base, and exhibited high carbon, Pb and Zn contents and low C/N ratios. In addition, macroseismic investigations and statistical results from intensity prediction equations (IPEs) provided a conservative threshold of ∼8 Modified Mercalli Intensities (MMI) for triggering turbidites, and a ∼ 10 MMI for inducing coseismic subsidence and hydrological destruction. This study was among the first attempts to establish a quantitative lacustrine paleoseismograph in the southeastern TP, and the new results would greatly improve the valid assessment of geohazard risks.

Ostracods from the Kunming area of SW China, with description of two new species and male records of Cypridopsis vidua (O.F. Mller, 1776).

Previous studies of the Recent non-marine ostracods of Yunnan Province in southwestern China yielded 42 species, among which only 18 species were reported based on living material. Such a low number is likely to underestimate the species diversity of ostracods in Yunnan, considering the geographical disparity among its different parts. In this study, we report 11 nominal species and one species left in open nomenclature from 22 samples collected from the Kunming area in eastern-central part of Yunnan. Among these species, seven are new to the province, including Notodromas semiovata n. sp., Chrissia acuminata n. sp., and the first records of the male of Cypridopsis vidua (O.F. Mller, 1776) outside the USA. A review of previous literature indicates that, with the exception of the two new species, all the other 23 species have Palaearctic (PA) distributions, while only 14 have Oriental (OL) distributions. Such a composition contradicts previous vertebrate-based zoogeographical subdivisions of China in which Yunnan was included in the OL region, and adds to the argument that the geographical distributions of various invertebrate groups need to be investigated to map the PAOL boundary in China more appropriately.

Before trilobite legs: Pygmaclypeatus daziensis reconsidered and the ancestral appendicular organization of Cambrian artiopods.

The Cambrian Stage 3 Chengjiang biota in South China is one of the most influential Konservat-Lagerstätten worldwide thanks to the fossilization of diverse non-biomineralizing organisms through pyritization. Despite their contributions to understanding the evolution of early animals, several Chengjiang species remain poorly known owing to their scarcity and/or incomplete preservation. Here, we use micro-computed tomography to reveal in detail the ventral appendage organization of the enigmatic non-trilobite artiopod Pygmaclypeatus daziensis-one of the rarest euarthropods in Chengjiang-and explore its functional ecology and broader evolutionary significance. Pygmaclypeatus daziensis possesses a set of uniramous antennae and 14 pairs of post-antennal biramous appendages, the latter of which show an unexpectedly high degree of heteronomy based on the localized differentiation of the protopodite, endopodite and exopodite along with the antero-posterior body axis. The small body size (less than 2 cm), the presence of delicate spinose endites and well-developed exopodites with multiple paddle-shaped lamellae on the appendages of P. daziensis indicate a nekto-benthic mode of life and a scavenging/detritus feeding strategy. Pygmaclypeatus daziensis shows that appendage heteronomy is phylogenetically widespread within Artiopoda-the megadiverse clade that includes trilobites and their relatives with non-biomineralizing exoskeletons-and suggests that a single exopodite lobe with paddle-like lamellae is ancestral for this clade. This article is part of the theme issue 'The impact of Chinese palaeontology on evolutionary research'.

Ilyocyprisleptolinea Wang & Zhai, sp. nov., an ostracod (Ostracoda, Crustacea) from the late Quaternary of Inner Mongolia, northern China.

Ilyocyprisleptolinea Wang & Zhai, sp. nov. is described from late Quaternary sediments in central-eastern Inner Mongolia, northern China. The new species, which has a carapace shape and pitted surface typical of the genus, is characterised by double rows of fine, densely arranged marginal ripplets, separated by an inner list, along both anterior and posterior calcified inner lamellae in the left valve. Outline analysis and Principal Component Analysis indicate that its morpho-space overlaps with I.bradyi Sars, 1890, I.japonica Okubo, 1990, and I.mongolica Martens, 1991, which have living or fossil representatives in Inner Mongolia, but it is clearly discriminated from I.innermongolica Zhai & Xiao, 2013. Judging from the relatively coarse lithology dominated by silt and sand, and the lack of accompanying brackish-water ostracods, I.leptolinea Wang & Zhai, sp. nov. may have lived in a relatively shallow freshwater lake. It perhaps can be added to the list of species that went extinct during the Quaternary, but the timing and process of extinction await further investigation.

Three new species of non-marine ostracods (Crustacea, Ostracoda) from small water bodies of northern China.

Three new species, Pseudocandonacheni sp. nov., Cyclocyprispangi sp. nov., and Tonnacyprisrectangularis sp. nov., collected from northern China, are described in this study. Pseudocandonacheni, of the compressa group, is difficult to be distinguished from other members of the same group by carapace morphology alone, but can be readily recognised by the structure of the hemipenis comprised of a long lobe a, distally inflated lobe h, and exteriorly pointed lobe b, as well as thick trunks of the male fifth limb endopodites. Cyclocyprispangi, despite its similarity in carapace morphology to its congeners, can be identified based on the morphology of the hemipenis, which bears a slender, slightly curved lobe h, and an S-shaped process accompanying lobe b, in addition to the rectangular trunk of the male fifth limb endopodite. On the other hand, Tonnacyprisrectangularis, described on the basis of females only, can be distinguished from other Tonnacypris Diebel & Pietrzeniuk, 1975 representatives by its distinct sub-rectangular carapace alone. Other differences include the length of the swimming setae, the length of the distal claws on the second antennae, and the morphology of the pincer organ on the seventh limb. This study contributes to the poorly known extant non-marine ostracod fauna of Inner Mongolia and Beijing, and generally to the central-eastern Palaearctic region. In addition, the known distribution range of Tonnacypris is extended eastwardly by T.rectangularis. The valve-morphology data are useful for identifying fossil/sub-fossil representatives.

Exites in Cambrian arthropods and homology of arthropod limb branches.

The last common ancestor of all living arthropods had biramous postantennal appendages, with an endopodite and exopodite branching off the limb base. Morphological evidence for homology of these rami between crustaceans and chelicerates has, however, been challenged by data from clonal composition and from knockout of leg patterning genes. Cambrian arthropod fossils have been cited as providing support for competing hypotheses about biramy but have shed little light on additional lateral outgrowths, known as exites. Here we draw on microtomographic imaging of the Cambrian great-appendage arthropod Leanchoilia to reveal a previously undetected exite at the base of most appendages, composed of overlapping lamellae. A morphologically similar, and we infer homologous, exite is documented in the same position in members of the trilobite-allied Artiopoda. This early Cambrian exite morphology supplements an emerging picture from gene expression that exites may have a deeper origin in arthropod phylogeny than has been appreciated.

A juvenile-rich palaeocommunity of the lower Cambrian Chengjiang biota sheds light on palaeo-boom or palaeo-bust environments.

The fossil record, including the record of Burgess Shale-type deposits, is biased towards late ontogenetic stages. Larval stages, juvenile and subadult specimens exist but are very rare and often preserved as phosphatic fossils, resulting in biased population structures. Here, we report a new Burgess Shale-type Lagerstätte from Haiyan, China. The Haiyan palaeocommunity is extraordinary in that it is rich in fossils of early and middle ontogenetic stages of various phyla, with eggs also commonly found in the studied interval. This Lagerstätte also hosts a considerable number of new taxa-many related to later biotas of Gondwana and Laurentia. We propose that the deposit may either preserve one of the earliest nurseries in the fossil record or, alternatively, records several attempted invasions. Our study highlights the complexity of biotas and their interactions in the lower Cambrian ocean and calls for a better understanding of the mechanisms responsible for the observed spatial variation of fossil community composition in the Cambrian.

On some modern Ostracoda (Crustacea) from the Tibetan Plateau in SW China, with descriptions of three new species.

Studies of Holocene ostracods of the Tibetan Plateau have mostly centred around valves and carapaces collected from lake sediments, some at or near the substrate surface and others from short cores. Reports from habitats other than lakes are scarce, and few living species found in this region have appendages described, hindering further taxonomic and phylogenetic work. For this study collections of living ostracods were made in mostly ponds, as well as two rivers and one lake. Six species, including three previously undescribed, were recovered: Tonnacypris estonica (Järvekülg, 1960), Arctocypris edita n. sp., Cypris pubera O.F. Müller, 1776, Potamocypris variegata (Brady Norman, 1889), Ilyocypris tibeta n. sp., and Fabaeformiscandona monticulus n. sp. Specimens of Tonnacypris estonica include males, previously unknown for this species, and suggesting it is a geographical parthenogen. A revision of the fifth limb morphology of the genus Arctocypris, and a subsequent amended diagnosis of the genus are given. Based on carapace and appendage features, Eucypris mareotica (Fischer, 1855) is transferred to this genus: Arctocypris mareotica (Fischer, 1855) com. nov. There are now ca. 100 ostracod species reported from modern or subsurface sediments on the Tibetan Plateau, but only 19 of those can be confirmed as alive when collected (i.e. had appendages intact). The families Limnocytheridae and Ilyocyprididae are both relatively diverse on the plateau compared with the Palaearctic zoogeographical region generally. About one third of ostracod species reported from the Tibetan Plateau are only known from there, suggesting a potentially very high rate of endemism.

Moving legs: A workflow on how to generate a flexible endopod of the 518 million-year-old Chengjiang arthropod Ercaicunia multinodosa using 3D-kinematics (Cambrian, China).

Understanding the functional morphology and mobility of appendages of fossil animals is important for exploring ecological traits such as feeding and locomotion. Previous work on fossils from the 518 million-year-old Chengjiang biota of China was based mainly on two-dimensional information captured from the surface of the specimens. Only recently, µCT techniques started to reveal almost the entire, though flattened and compressed, three-dimensionally preserved morphologies of the arthropods from Chengjiang. This allows more accurate work on reconstructing the possible movement of certain structures such as the appendages. Here, we present a workflow on how to reconstruct the mobility of a limb of the early Chengjiang arthropod Ercaicunia multinodosa from the famous Chinese fossil site. Based on µCT scans of the fossil, we rendered surface models of the 13th-15th right endopods using the 3D visualization and 3D-rendering software Amira. The 3D objects then were postprocessed (Collapse Hierarchy, Unify Normals) in SAP 3D Visual Enterprise Author before being imported into the 3D animation program Autodesk Maya 2020. Using the add-on tool X_ROMM in Maya, we illustrate step-by-step on how to make the articles of the limbs swing-in toward each other. Eventually, we propose several possible limb movements of E. multinodosa, which helps to understand how this early arthropod could have moved its endopods.

Computed tomography sheds new light on the affinities of the enigmatic euarthropod Jianshania furcatus from the early Cambrian Chengjiang biota.

BACKGROUND: The Chengjiang biota is one of the most species-rich Cambrian Konservat-Lagerstätten, and preserves a community dominated by non-biomineralized euarthropods. However, several Chengjiang euarthropods have an unfamiliar morphology, are extremely rare, or incompletely preserved. RESULTS: We employed micro-computed tomography to restudy the enigmatic euarthropod Jianshania furcatus. We reveal new morphological details, and demonstrate that the specimens assigned to this species represent two different taxa. The holotype of J. furcatus features a head shield with paired anterolateral notches, stalked lateral eyes, and an articulated tailspine with a bifurcate termination. The other specimen is formally redescribed as Xiaocaris luoi gen. et sp. nov., and is characterized by stalked eyes connected to an anterior sclerite, a subtrapezoidal head shield covering three small segments with reduced tergites, a trunk with 15 overlapping tergites with a well-developed dorsal keel, and paired tail flukes. CONCLUSIONS: The presence of antennae, biramous appendages with endopods composed of 15 articles, and multiple appendage pairs associated with the trunk tergites identify X. luoi nov. as a representative of Fuxianhuiida, an early branching group of stem-group euarthropods endemic to the early Cambrian of Southwest China. X. luoi nov. represents the fifth fuxianhuiid species described from the Chengjiang biota, and its functional morphology illuminates the ecological diversity of this important clade for understanding the early evolutionary history of euarthropods.

A Cambrian crown annelid reconciles phylogenomics and the fossil record.

The phylum of annelids is one of the most disparate animal phyla and encompasses ambush predators, suspension feeders and terrestrial earthworms1. The early evolution of annelids remains obscure or controversial2,3, partly owing to discordance between molecular phylogenies and fossils2,4. Annelid fossils from the Cambrian period have morphologies that indicate epibenthic lifestyles, whereas phylogenomics recovers sessile, infaunal and tubicolous taxa as an early diverging grade5. Magelonidae and Oweniidae (Palaeoannelida1) are the sister group of all other annelids but contrast with Cambrian taxa in both lifestyle and gross morphology2,6. Here we describe a new fossil polychaete (bristle worm) from the early Cambrian Canglangpu formation7 that we name Dannychaeta tucolus, which is preserved within delicate, dwelling tubes that were originally organic. The head has a well-defined spade-shaped prostomium with elongated ventrolateral palps. The body has a wide, stout thorax and elongated abdomen with biramous parapodia with parapodial lamellae. This character combination is shared with extant Magelonidae, and phylogenetic analyses recover Dannychaeta within Palaeoannelida. To our knowledge, Dannychaeta is the oldest polychaete that unambiguously belongs to crown annelids, providing a constraint on the tempo of annelid evolution and revealing unrecognized ecological and morphological diversity in ancient annelids.

A Reduced Labrum in a Cambrian Great-Appendage Euarthropod.

The euarthropod head is a highly versatile and functionally specialized body region composed of multiple appendage-bearing segments and whose complex evolution has been scrutinized through anatomical, developmental, and paleontological approaches [1-4]. Exceptionally preserved Cambrian fossils have allowed for the reconstruction of critical stages of the evolutionary history of the head, such as the origin of the labrum-an anteromedian flap-like structure that overlies the mouth opening in almost all extant representatives-from an ancestral pair of pre-ocular (protocerebral) appendages [3-5]. The highly conserved position of the labrum makes it a valuable anatomical landmark for understanding the anterior segmental organization among extant and extinct euarthropods [2]. However, the labrum is seemingly absent in the megacheirans, a major extinct group characterized by enlarged raptorial "great appendages" with a central role in competing hypotheses on the early evolution of the head [1-3, 6-8]. Here, we used micro-computed tomography to demonstrate the presence of a three-dimensionally preserved labrum associated with the mouth opening in juvenile specimens of the megacheiran Leanchoilia illecebrosa from the early Cambrian Chengjiang biota, Southwest China. The position of the labrum relative to the pre-oral great appendages of L. illecebrosa indicates that these limbs correspond to the deutocerebral segment and are therefore serially homologous with the first appendage pair of extant euarthropods [1, 2, 4, 6, 8]. The reduced labrum and deutocerebral great appendages of L. illecebrosa also strengthen the affinities of megacheirans as stem-group chelicerates, in line with recent paleoneurological fossil data from the early to mid-Cambrian of China and North America [6, 9].

Fine-scale appendage structure of the Cambrian trilobitomorph Naraoia spinosa and its ontogenetic and ecological implications.

Trilobitomorphs are a species-rich Palaeozoic arthropod assemblage that unites trilobites with several other lineages that share similar appendage structure. Post-embryonic development of the exoskeleton is well documented for some trilobitomorphs, especially trilobites, but little is known of the ontogeny of their soft parts, limiting understanding of their autecology. Here, we document appendage structure of the Cambrian naraoiid trilobitomorph Naraoia spinosa by computed microtomography, resulting in three-dimensional reconstructions of appendages at both juvenile and adult stages. The adult has dense, strong spines on the protopods of post-antennal appendages, implying a predatory/scavenging behaviour. The absence of such gnathobasic structures, but instead tiny protopodal bristles and a number of endopodal setae, suggests a detritus-feeding strategy for the juvenile. Our data add strong morphological evidence for ecological niche shifting by Cambrian arthropods during their life cycles. A conserved number of appendages across the sampled developmental stages demonstrates that Naraoia ceased budding off new appendages by the mid-juvenile stage.

Ilyocypris (Crustacea: Ostracoda) species in North East Asian rice fields; description of one new species, and redescriptions of Ilyocypris dentifera Sars, 1903 and Ilyocypris japonica Okubo, 1990.

Ilyocypris species are abundant and widespread in North East Asian rice fields, but for many species their taxonomy is poorly defined, which negatively impacts ecological studies on rice field ecosystems. Herein, two of these species, Ilyocypris dentifera Sars, 1903 and Ilyocypris japonica Okubo, 1990, are redescribed, and a third species, Ilyocypris incus sp. nov., is described and named. This latter species has previously been reported from Japan and possibly Korea under the names Ilyocypris angulata Sars, 1903 and I. dentifera, but study of the hemipenes demonstrates that it is a separate species. Ilyocypris nipponica Okubo, 2004 nomen nudum previously reported from Japan is herein considered to be a junior synonym of I. dentifera. Tuberculated and non-tuberculated forms of both I. dentifera and I. japonica were recovered during this study and the presence or absence of tubercles is postulated to be related to environmental conditions, such as water chemistry or predation pressures. Although tuberculated forms of I. dentifera do occur, we concur with some previous studies that Ilyocypris angulata Sars, 1903 should be treated as a separate species, based on features of the hemipenes and carapaces. Figures of the carapaces and hemipenes of I. angulata, and Ilyocypris mongolica Martens, 1991, which is morphologically very close to I. japonica, are also presented for comparisons. This study confirms that three species of Ilyocypris inhabit North East Asian rice fields: I. dentifera, I. japonica and Ilyocypris incus sp. nov. Extant Ilyocypris species in North East Asia, including those from habitats other than rice fields, can be divided into two groups based on the Zenker organ, hemipenis, and sixth limb morphologies. These groups are postulated to represent two distinct lineages of Ilyocypris in North East Asia.

Variation in appendages in early Cambrian bradoriids reveals a wide range of body plans in stem-euarthropods.

Traditionally, the origin and evolution of modern arthropod body plans has been revealed through increasing levels of appendage specialisation exhibited by Cambrian euarthropods. Here we show significant variation in limb morphologies and patterns of limb-tagmosis among three early Cambrian arthropod species conventionally assigned to the Bradoriida. These arthropods are recovered as a monophyletic stem-euarthropod group (and sister taxon to crown-group euarthropods, i.e. Chelicerata, Mandibulata and their extinct relatives), thus implying a radiation of stem-euarthropods where trends towards increasing appendage specialisation were explored convergently with other euarthropod groups. The alternative solution, where bradoriids are polyphyletic, representing several independent origins of a small, bivalved body plan in lineages from diverse regions of the euarthropod and mandibulate stems, is only marginally less parsimonious. The new data reveal a previously unknown disparity of body plans in stem-euarthropods and both solutions support remarkable evolutionary convergence, either of fundamental body plans or appendage specialization patterns.

The appendicular morphology of Sinoburius lunaris and the evolution of the artiopodan clade Xandarellida (Euarthropoda, early Cambrian) from South China.

BACKGROUND: Artiopodan euarthropods represent common and abundant faunal components in sites with exceptional preservation during the Cambrian. The Chengjiang biota in South China contains numerous taxa that are exclusively known from this deposit, and thus offer a unique perspective on euarthropod diversity during the early Cambrian. One such endemic taxon is the non-trilobite artiopodan Sinoburius lunaris, which has been known for approximately three decades, but few details of its anatomy are well understood due to its rarity within the Chengjiang, as well as technical limitations for the study of these fossils. Furthermore, the available material does not provide clear information on the ventral organization of this animal, obscuring our understanding of phylogenetically significant details such as the appendages. RESULTS: We employed X-ray computed tomography to study the non-biomineralized morphology of Sinoburius lunaris. Due to the replacement of the delicate anatomy with pyrite typical of Chengjiang fossils, computed tomography reveals substantial details of the ventral anatomy of Sinoburius lunaris, and allow us to observe in detail the three-dimensionally preserved appendicular organization of this taxon for the first time. The dorsal exoskeleton consists of a crescent-shaped head shield with well-developed genal spines, a thorax with seven freely articulating tergites, and a fused pygidium with lateral and median spines. The head bears a pair of ventral stalked eyes that are accommodated by dorsal exoskeletal bulges, and an oval elongate ventral hypostome. The appendicular organization of the head is unique among Artiopoda. The deutocerebral antennae are reduced, consisting of only five podomeres, and bear an antennal scale on the second podomere that most likely represents an exite rather than a true ramus. The head includes four post-antennal biramous limb pairs. The first two biramous appendages are differentiated from the rest. The first appendage pair consists of a greatly reduced endopod coupled with a greatly elongated exopod with a potentially sensorial function. The second appendage pair carries a more conventionally sized endopod, but also has an enlarged exopod. The remaining biramous appendages are homonomous in their construction, but decrease in size towards the posterior end of the body. They consist of a basipodite with ridge-like crescentic endites, an endopod with seven podomeres and a terminal claw, and a lamellae-bearing exopod with a slender shaft. Contrary to previous reports, we confirm the presence of segmental mismatch in Sinoburius lunaris, expressed as diplotergites in the thorax. Maximum parsimony and Bayesian phylogenetic analyses support the monophyly of Xandarellida within Artiopoda, and illuminate the internal relationships within this enigmatic clade. Our results allow us to propose a transformation series explaining the origin of archetypical xandarellid characters, such as the evolution of eye slits in Xandarella spectaculum and Phytophilaspis pergamena as derivates from the anterolateral notches in the head shield observed in Cindarella eucalla and Luohuilinella species. In this context, Sinoburius lunaris is found to feature several derived characters within the group, such as the secondary loss of eye slits and a high degree of appendicular tagmosis. Contrary to previous findings, our analyses strongly support close affinities between Sinoburius lunaris, Xandarella spectaculum and Phytophilaspis pergamena, although the precise relationships between these taxa are sensitive to different methodologies. CONCLUSIONS: The revised morphology of Sinoburius lunaris, made possible through the use of computed tomography to resolve details of its three-dimensionally preserved appendicular anatomy, contributes towards an improved understanding of the morphology of this taxon and the evolution of Xandarellida more broadly. Our results indicate that Sinoburius lunaris possesses an unprecedented degree of appendicular tagmosis otherwise unknown within Artiopoda, with the implication that this iconic group of Palaeozoic euarthropods likely had a more complex ecology and functional morphology than previously considered. The application of computer tomographic techniques to the study of Chengjiang euarthropods holds exceptional promise for understanding the morphological diversity of these organisms, and also better reconstructing their phylogenetic relationships and evolutionary history.

Three-Dimensionally Preserved Appendages in an Early Cambrian Stem-Group Pancrustacean.

Pancrustaceans boast impressive diversity, abundance, and ecological impact in the biosphere throughout the Phanerozoic [1]. Molecular clock estimates suggest an early Cambrian divergence for pancrustaceans [2, 3]. Despite the wealth of Palaeozoic exceptional fossiliferous deposits [4-7], the early evolution of Pancrustacea remains elusive given the difficulty of recognizing synapomorphies between Cambrian forms and extant representatives. Although early studies suggested crustacean affinities for Cambrian bivalved euarthropods [8-11], this view has fallen out of favor by recent reappraisals of their morphology [12-16]. The best evidence for total-group pancrustaceans comes from Cambrian microfossils preserved as three-dimensional phosphatic replicates in Orsten-type assemblages [4, 17-19] or as "small carbonaceous fossils" (SCFs) [20, 21]. Although these taphonomic windows capture minute morphology enabling detailed comparisons with extant representatives, these microfossils are limited to larval stages (Orsten) or recalcitrant fragmentary remains (SCFs) restricting their phylogenetic precision [5, 12, 19, 20, 22, 23]. We employed X-ray computed tomography [24] to reveal the three-dimensionally appendage morphology of the Chengjiang bivalved euarthropod Ercaicunia multinodosa [25] from the early Cambrian of China. E. multinodosa possesses characters uniquely shared with extant crustaceans, including differentiated tritocerebral antennae and epipodite-bearing biramous trunk appendages. Similarities between E. multinodosa with clypecaridids [9], waptiids [16] and hymenocarines [11, 14] suggest that these euarthropods may also possess similarly differentiated appendages, but these details are obstructed by the limits of preservation of compacted macrofossils. E. multinodosa illuminates the early evolution of pancrustacean appendage differentiation and represents the oldest unequivocal crown-group mandibulate known from complete macrofossils [22].