Xi-guang Zhang

An epipodite-bearing crown-group crustacean from the Lower Cambrian.

Crown-group crustaceans (Eucrustacea) are common in the fossil record of the past 500 million years back to the early Ordovician period, and very rare representatives are also known from the late Middle and Late Cambrian periods. Finds in Lower Cambrian rocks of the Phosphatocopina, the fossil sister group to eucrustaceans, imply that members of the eucrustacean stem lineage co-occurred, but it remained unclear whether crown-group members were also present at that time. ‘Orsten’-type fossils are typically tiny embryos and cuticle-bearing animals, of which the cuticle is phosphatized and the material is three-dimensional and complete with soft parts. Such fossils are found predominantly in the Cambrian and Ordovician and provide detailed morphological and phylogenetic information on the early evolution of metazoans. Here we report an Orsten-type Konservat-Lagerstätte from the Lower Cambrian of China that contains at least three new arthropod species, of which we describe the most abundant form on the basis of exceptionally well preserved material of several growth stages. The limb morphology and other details of this new species are markedly similar to those of living cephalocarids, branchiopods and copepods and it is assigned to the Eucrustacea, thus representing the first undoubted crown-group crustacean from the early Cambrian. Its stratigraphical position provides substantial support to the proposition that the main cladogenic event that gave rise to the Arthropoda was before the Cambrian. Small leaf-shaped structures on the outer limb base of the new species provide evidence on the long-debated issue of the origin of epipodites: they occur in a set of three, derive from setae and are a ground-pattern feature of Eucrustacea.

Middle Cambrian Arthropod Embryos with Blastomeres

A phosphatized Middle Cambrian (∼510 million years ago) fauna from Duyun, southern China, has yielded fossil embryos that may be of arthropod affinity and could belong to the co-occurring eodiscid trilobite Pagetia sp. The shell was most likely flexible and possessed at least two thin layers. Four embryos reveal blastomeres, and two embryonic stages are represented. These embryos demonstrate that the basic paradigm for the growth of the invertebrate embryo has not changed in more than half a billion years.

Specialized appendages in fuxianhuiids and the head organization of early euarthropods

The organization of the head provides critical data for resolving the phylogenetic relationships and evolutionary history of extinct and extant euarthropods. The early Cambrian-period fuxianhuiids are regarded as basal representatives of stem-group Euarthropoda, and their anterior morphology therefore offers key insights for reconstructing the ancestral condition of the euarthropod head. However, the paired post-antennal structures in Fuxianhuia protensa remain controversial; they have been interpreted as both ‘great appendages’ and as gut diverticulae. Here we describe Chengjiangocaris kunmingensis sp. nov. and Fuxianhuia xiaoshibaensis sp. nov. from a new early Cambrian (Stage 3) fossil Lagerstätte in Yunnan, China. Numerous specimens of both species show a unique ‘taphonomic dissection’ of the anterodorsal head shield, revealing the cephalic organization in detail. We demonstrate the presence of a pair of specialized post-antennal appendages (SPAs) in the fuxianhuiid head, which attach at either side of the posteriorly directed mouth, behind the hypostome. Preserved functional articulations indicate a well-defined but restricted range of limb movement, suggestive of a simple type of sweep feeding. The organization of the SPAs in fuxianhuiids is incompatible with the (deutocerebral) anterior raptorial appendages of megacheirans, and argue against the presence of protocerebral limbs in the fuxianhuiids. The positions of the fuxianhuiid antennae and SPAs indicate that they are segmentally homologous to the deutocerebral and tritocerebral appendages of crown-group Euarthropoda respectively. These findings indicate that antenniform deutocerebral appendages with many podomeres are a plesiomorphic feature of the ancestral euarthropod head.

Evidence for a single median fin-fold and tail in the Lower Cambrian vertebrate, Haikouichthys ercaicunensis

In this study, we illustrate an exceptionally well‐preserved Haikouichthys ercaicunensis from the Lower Cambrian Chengjiang fauna that displays complete single dorsal, ventral and caudal fins. This 530‐million‐year old vertebrate is fish‐shaped and characterized by a single median fin‐fold, which is an essential trait of the initial vertebrate chordates. The radially orientated ray‐like structures in its dorsal fin somewhat resemble but are probably not real radials seen in basal vertebrates, such as hagfishes and lampreys. The unique design of primitive fins and fin structures provides additional insights into the early evolution of vertebrates.

A Eucrustacean Metanauplius from the Lower Cambrian

A new eucrustacean arthropod, Wujicaris muelleri gen. et sp. nov, is represented by a Lower Cambrian early metanauplius of strikingly modern morphology despite being the oldest known fossil of such an early immature crustacean larva. The morphology of the metanauplius closely mirrors that of corresponding developmental stages of living barnacles and copepods, and it is likely that its appendages had a similar function for feeding and locomotion. The metanauplius larva demonstrates remarkable stasis in morphology, life history, and lifestyle of (small) eucrustaceans over 525 million years, probably as a result of adaptation to a long-lasting physical niche and regime involving low Reynolds numbers and laminar current flow.

Early Cambrian palaeoscolecid cuticles from Shaanxi, China

Phosphatized, three-dimensional cuticle fragments of the palaeoscolecids Houscolex lepidotus new genus and new species, Houscolex species indeterminate, and an unidentified form are described from the Lower Cambrian Qiongzhusi Formation of Shaanxi, China. These segmented worms, characterized by regularly arranged simple plates and platelets, small circular pores and fine pits, and irregularly distributed nipplelike protuberances, are unique among known palaeoscolecids. The occurrence of such cuticles as seemingly persistent elements of phosphatized faunas suggests that wormlike organisms achieved a fairly high diversity and widespread geographic distribution during the Early and Middle Cambrian.

Fuxianhuiid ventral nerve cord and early nervous system evolution in Panarthropoda.

Significance Understanding the evolution of the CNS is fundamental for resolving the phylogenetic relationships within Panarthropoda (Euarthropoda, Tardigrada, Onychophora). The ground pattern of the panarthropod CNS remains elusive, however, as there is uncertainty on which neurological characters can be regarded as ancestral among extant phyla. Here we describe the ventral nerve cord (VNC) in Chengjiangocaris kunmingensis, an early Cambrian euarthropod from South China. The VNC reveals extraordinary detail, including condensed ganglia and regularly spaced nerve roots that correspond topologically to the peripheral nerves of Priapulida and Onychophora. Our findings demonstrate the persistence of ancestral neurological features of Ecdysozoa in early euarthropods and help to reconstruct the VNC ground pattern in Panarthropoda. Panarthropods are typified by disparate grades of neurological organization reflecting a complex evolutionary history. The fossil record offers a unique opportunity to reconstruct early character evolution of the nervous system via exceptional preservation in extinct representatives. Here we describe the neurological architecture of the ventral nerve cord (VNC) in the upper-stem group euarthropod Chengjiangocaris kunmingensis from the early Cambrian Xiaoshiba Lagerstätte (South China). The VNC of C. kunmingensis comprises a homonymous series of condensed ganglia that extend throughout the body, each associated with a pair of biramous limbs. Submillimetric preservation reveals numerous segmental and intersegmental nerve roots emerging from both sides of the VNC, which correspond topologically to the peripheral nerves of extant Priapulida and Onychophora. The fuxianhuiid VNC indicates that ancestral neurological features of Ecdysozoa persisted into derived members of stem-group Euarthropoda but were later lost in crown-group representatives. These findings illuminate the VNC ground pattern in Panarthropoda and suggest the independent secondary loss of cycloneuralian-like neurological characters in Tardigrada and Euarthropoda.

A superarmored lobopodian from the Cambrian of China and early disparity in the evolution of Onychophora

Significance Paleozoic lobopodians constitute a diverse assemblage of worm-like organisms that are known from various exceptional fossil deposits and were among the earliest animals to develop skeletonized body parts for protection. Here, we describe Collinsium ciliosum gen. et sp. nov., an armored lobopodian from the early Cambrian Xiaoshiba Lagerstätte (South China). Collinsium belongs to an extinct clade of superarmored lobopodians characterized by supernumerary dorsal spines, and specialized limbs for filter feeding; collectively, these fossil taxa represent a well-defined group within the lineage leading to extant velvet worms (Onychophora). Despite their greater morphological variety and appendage complexity compared with other lobopodians and extant velvet worms, Collinsium and its close relatives embodied a unique, yet ultimately failed, autoecology during the Cambrian explosion. We describe Collinsium ciliosum from the early Cambrian Xiaoshiba Lagerstätte in South China, an armored lobopodian with a remarkable degree of limb differentiation including a pair of antenna-like appendages, six pairs of elongate setiferous limbs for suspension feeding, and nine pairs of clawed annulated legs with an anchoring function. Collinsium belongs to a highly derived clade of lobopodians within stem group Onychophora, distinguished by a substantial dorsal armature of supernumerary and biomineralized spines (Family Luolishaniidae). As demonstrated here, luolishaniids display the highest degree of limb specialization among Paleozoic lobopodians, constitute more than one-third of the overall morphological disparity of stem group Onychophora, and are substantially more disparate than crown group representatives. Despite having higher disparity and appendage complexity than other lobopodians and extant velvet worms, the specialized mode of life embodied by luolishaniids became extinct during the Early Paleozoic. Collinsium and other superarmored lobopodians exploited a unique paleoecological niche during the Cambrian explosion.

Embryonic Development of a Middle Cambrian (500 Myr Old) Scalidophoran Worm

Abstract Extraordinarily rare phosphatized embryos liberated from fossiliferous limestone of the Middle Cambrian (about 500 Myr old) Gaotai Formation of Duyun, southern China, are assigned to Markuelia qianensis n. sp. Several specimens with increasing numbers of blastomeres may represent four successive cleavage stages, seemingly exhibiting a radial holoblastic cleavage pattern. No subsequent stages showing gastrulation were observed. However, several specimens are late, pre-hatching stages, each with a vermiform shape coiled in either left- or right-handed directions within the fertilization envelope. These specimens indicate that the intuitively assumed difference in preservation potential between early cleavage and late pre-hatching stages is probably not valid. This new material clarifies the affinity of the first fossilized invertebrate embryos ever described, from the same rocks, which were originally attributed to arthropods, presumably trilobites. Instead, they belong to scalidophorans, and this finding infers likely diversified cleavage patterns for stem members of this group, and yields fresh insight into the embryogenesis of early metazoans as a whole.

Appendages of early Cambrian Eoredlichia (Trilobita) from the Chengjiang biota, Yunnan, China

The early Cambrian Chengjiang biota in Yunnan, southern China has yielded many articulated exoskeletons of the spiny redlichiid trilobite Eoredlichia intermedia , of which some have their appendages exceptionally well preserved. Both of the paired uniramous antennae of a medium-sized holaspis consist of 46–50 short segments (articles), each of which bears a fine spine near its inner edge. Behind the antennae there are twenty-one pairs of biramous limbs: three pairs are situated underneath the cephalon, one pair underneath each of the fifteen thoracic segments, and probably three pairs underneath the small pygidium. The endopod consists of a broad basis and seven podomeres, of which the last is divided into three terminal spines. The exopod is blade-like, and according to one interpretation, is dorsally hinged to the basis of the endopod; an alternative suggestion being that both the endopod and exopod are split from the basis, the latter being independent and not forming part of the endopod. The exopod has a prominent anterior rim, and possesses about forty long filaments along the posterior margin, and short setae along the rounded distal lobe. The basic appendage features of the redlichiid trilobites, and likewise the gut, are comparable to those of other known Cambrian polymerid trilobites that belong to more distantly related clades.