埃迪卡拉纪至寒武纪苗岭世刺细胞动物化石研究现状与展望

刺细胞动物是一类具有刺细胞的水生无脊椎动物,分布在世界各地的海洋和淡水中。作为后生动物最早分化出的一支,刺细胞动物对研究后生动物的起源和早期演化具有极其重要的意义,也为研究后生动物系统发育、地层对比和古地理恢复等方面提供了重要的科研线索。本文简要介绍了刺细胞动物早期(埃迪卡拉纪至寒武纪苗岭世)的化石记录和研究现状,将刺细胞动物的早期演化分为四个时期,分别为前寒武纪、寒武纪纽芬兰世、寒武纪第二世和寒武纪苗岭世,并对上述四个时期的刺细胞动物进行论述。目前的研究表明:1)前寒武纪的刺细胞动物化石记录虽然比较丰富,但充满争议,目前尚未有比较确切的刺细胞动物记录;2)寒武纪纽芬兰世如宽川铺生物群中保存了较多的刺细胞动物胚胎及发育序列化石,是研究刺细胞动物演化的重要窗口;3)寒武纪第二世,以澄江生物群为代表的布尔吉斯页岩型化石保存了软躯体的刺细胞动物。与此同时,在第二世中已经出现了最早的珊瑚化石记录;4)寒武纪苗岭世刺细胞动物主要以布尔吉斯页岩型化石保存(如布尔吉斯页岩生物群和凯里生物群),同时期在美国犹他州发现的刺细胞动物印痕化石表明了这个时期几乎所有的刺细胞动物大类已经出现。本文根据上述时期的研究,对今后刺细胞动物化石的研究提供一些建议和讨论。     

Early Cambrian Pentamerous Cubozoan Embryos from South China

Background

Extant cubozoans are voracious predators characterized by their square shape, four evenly spaced outstretched tentacles and well-developed eyes. A few cubozoan fossils are known from the Middle Cambrian Marjum Formation of Utah and the well-known Carboniferous Mazon Creek Formation of Illinois. Undisputed cubozoan fossils were previously unknown from the early Cambrian; by that time probably all representatives of the living marine phyla, especially those of basal animals, should have evolved.

Methods

Microscopic fossils were recovered from a phosphatic limestone in the Lower Cambrian Kuanchuanpu Formation of South China using traditional acetic-acid maceration. Seven of the pre-hatched pentamerous cubozoan embryos, each of which bears five pairs of subumbrellar tentacle buds, were analyzed in detail through computed microtomography (Micro-CT) and scanning electron microscopy (SEM) without coating.

Results

The figured microscopic fossils are unequivocal pre-hatching embryos based on their spherical fertilization envelope and the enclosed soft-tissue that has preserved key anatomical features arranged in perfect pentaradial symmetry, allowing detailed comparison with modern cnidarians, especially medusozoans. A combination of features, such as the claustrum, gonad-lamella, suspensorium and velarium suspended by the frenula, occur exclusively in the gastrovascular system of extant cubozoans, indicating a cubozoan affinity for these fossils. Additionally, the interior anatomy of these embryonic cubozoan fossils unprecedentedly exhibits the development of many new septum-derived lamellae and well-partitioned gastric pockets unknown in living cubozoans, implying that ancestral cubozoans had already evolved highly specialized structures displaying unexpected complexity at the dawn of the Cambrian. The well-developed endodermic lamellae and gastric pockets developed in the late embryonic stages of these cubozoan fossils are comparable with extant pelagic juvenile cubomedusae rather than sessile cubopolyps, whcih indicates a direct development in these fossil taxa, lacking characteristic stages of a typical cnidarian metagenesis such as planktonic planula and sessile polyps.     

Tiny sea anemone from the Lower Cambrian of China

Background

Abundant fossils from the Ediacaran and Cambrian showing cnidarian grade grossly suggest that cnidarian diversification occurred earlier than that of other eumetazoans. However, fossils of possible soft-bodied polyps are scanty and modern corals are dated back only to the Middle Triassic, although molecular phylogenetic results support the idea that anthozoans represent the first major branch of the Cnidaria. Because of difficulties in taxonomic assignments owing to imperfect preservation of fossil cnidarian candidates, little is known about forms ancestral to those of living groups.

Methods and Findings

We have analyzed the soft-bodied polypoid microfossils Eolympia pediculata gen. et sp. nov. from the lowest Cambrian Kuanchuanpu Formation in southern China by scanning electron microscopy and computer-aided microtomography after isolating fossils from sedimentary rocks by acetic acid maceration. The fossils, about a half mm in body size, are preserved with 18 mesenteries including directives bilaterally arranged, 18 tentacles and a stalk-like pedicle. The pedicle suggests a sexual life cycle, while asexual reproduction by transverse fission also is inferred by circumferential grooves on the body column.

Conclusions

The features found in the present fossils fall within the morphological spectrum of modern Hexacorallia excluding Ceriantharia, and thus Eolympia pediculata could be a stem member for this group. The fossils also demonstrate that basic features characterizing modern hexacorallians such as bilateral symmetry and the reproductive system have deep roots in the Early Cambrian.     

陕南早寒武世宽川铺生物群沉积环境及埋藏机制研究

陕南早寒武世宽川铺生物群三维立体保存了大量结构精美的磷酸盐化胚胎及小壳化石,为后生动物演化及化石胚胎学的研究提供了珍贵的实证材料。目前的研究主要集中在化石分类学及演化生物学上,而对沉积环境及埋藏机制的研究却很少涉及。文中通过对大量野外岩石照片、岩石薄片、小壳化石SEM照片的综合研究后认为,宽川铺生物群不是原地埋藏的产物,而是事件沉积的结果。该研究对于探讨宽川铺生物群化石的埋藏环境及保存机制具有一定的意义。     

Revision of the Lower Cambrian brachiopod Heliomedusa Sun & Hou, 1987

Jin Yugan & Wang Huayu 1992 01 15: Revision of the Lower Cambrian brachiopod Heliomedusa Sun & Hou, 1987. Lethaia , voi. 25. pp. 35–49. Oslo. ISSN 0024–1164.
Fossils referred to Heliomedusa , from the Chiungchussu Formation of Early Cambrian age in Yunnan. are extraordinarily preserved. They were previously considered to represent jellyfish. and have been suspected more recently of being an obolellid brachiopod. A study of the morphology of this genus indicates that it is related closely lo the non-pediculate craniopsids. which occurred from Early Ordovician to Carboniferous time. The soft parts of this brachiopod. such as setae and nerves, which usually are not preserved in fossils also are described. Brachiopoda, Inarticulata, Craniapsidue, anatomy, setae, nervous system, mantle canals, Early Cambrian .     

现代丰年虾卵的腐解实验——对早期化石胚胎研究的启示

早寒武世和埃迪卡拉纪中的球状化石,一些已被归入可能的后生动物胚胎化石,由于具较为完好的三维保存方式以及近乎完美的胚胎发育序列,为早期后生动物的起源、分类、谱系演化及发育生物学提供了难得的实证材料。然而随着研究的深入,多数寒武纪胚胎的生物学分类位置未定;而数量异常巨大、又有独自的保存方式的晚元古代陡山沱组胚胎的真伪和生物学归属,更是争议未消。通过对现生生物胚胎的实验埋藏研究,可以揭示出各类生物胚胎在腐解、埋藏各阶段的保存潜力,而现代胚胎在各实验埋藏阶段形态、结构的变化,也能为化石胚胎的研究提供重要的实证材料。本文就是通过对虾卵胚胎各发育阶段腐解保存潜力的实验模拟研究,试图为球状化石的形成机制和化石归属提供一些实验室依据。     

PALAEOBIOLOGY OF THE CLIMACTICHNITES TRACEMAKER

Abstract:  Trace fossils such as Climactichnites offer rare insights into the palaeobiology of Cambrian soft-bodied animals, especially those that inhabited emergent sand flats and are not known from body fossils. Analysis of field and museum Climactichnites , together with experiments on the preservation of similar modern trails, indicates that the tracemaker was an elongate, bilaterally symmetrical, dorsoventrally flattened, soft-bodied animal with a muscular foot. These characteristics are consistent with the tracemaker being a primitive mollusc or mollusc-like animal. Unlike most Neoproterozoic and Cambrian molluscs, the tracemaker could reach considerable size; at up to c . 69 cm long, it was one of the largest Cambrian animals of its time. When moving on the sediment surface, locomotion resulted from muscular waves generated along the sole of its flexible foot; the foot was extended and then clamped onto the substrate. Contraction of pedal muscles then pulled the body forward. Sedimentary structures associated with Climactichnites  wilsoni , such as polygonal desiccation cracks, raindrop impressions, adhesion structures and gas escape structures demonstrate that the animal inhabited intermittently subaerially exposed environments. The tracemaker's method of locomotion is similar to that employed by modern intertidal gastropods, which make Climactichnites -like trails on exposed sand flats. However, these modern trails are not preserved because of erosion by wind, waves, tides and subsequent bioturbation. Abundant microbial sedimentary structures are associated with C. wilsoni , and together with low levels of vertical bioturbation, intimate that microbial binding may have mediated the preservation of these early mollusc trails.     

龙凤山藻在澄江早寒武世生物群中的发现

主要报道了产自云南昆明海口耳材村早寒武世筇竹寺组玉案山段澄江生物群中可能为漂浮生长的宏观藻类化石-心型龙凤山藻（新种）（Longfengshania cordata sp.nov.)和中华豆芽藻（新属、新种）（lantulaformis sinensis gen.et sp.nov)。这些化石的发现进一步显示了澄江生物群种物多样性,为揭示该生物群爆发性演化提供了新的证据。通过对龙凤山藻属亲缘关系的深入比较研究,进一步证明该类化石应归属宏观藻类以及它们遗传上的稳定性、演化上的保守性和环境上的适应性。     

龙凤山藻在澄江早寒武世生物群中的发现

主要报道了产自云南昆明海口耳材村早寒武世筇竹寺组玉案山段澄江生物群中可能为自由漂浮生长的宏观藻类化石--心型龙凤山藻(新种)(Longfengshania cordata sp. nov.)和中华豆芽藻(新属、新种)(Plantulaformis sinensis gen. et sp. nov.).这些化石的发现进一步显示了澄江生物群物种多样性,为揭示该生物群爆发性演化提供了新的证据.通过对龙凤山藻属亲缘关系的深入比较研究,进一步证明该类化石应归属宏观藻类以及它们遗传上的稳定性、演化上的保守性和环境上的适应性.     

Larval development in Cnidaria: A connection to bilateria?

Among the basal animal phyla, the Cnidaria display many characteristics similar to the Bilateria (the higher Metazoa). However, the relation of that outgroup phyla to the Bilateria is still equivocal. Additionally to morphological and genetic data, studies on cnidarian embryogenesis are essential to clarify the Cnidaria-Bilateria relationship. We analyzed cellular differentiation during planula larvae development of the jellyfish Podocoryne carnea. Within 24 to 30 h postfertilization, the diploblastic body structure and all cell types found in polyps have already differentiated in the larva. Whereas the differentiating smooth muscles, RFamide-positive nerve cells, or nematocytes (stinging cells) express no axial polarity, a newly discovered tyrosine-tubulin-positive nervous system develops gradually in repetitive patterns from anterior to posterior. These data demonstrate that part of the cnidarian nervous system develops from anterior to posterior in serially repeated patterns. This developmental mechanism seems to follow the bilaterian pattern and would have antedated the Cambrian explosion.     

Disparity, decimation and the Cambrian "explosion": comparison of early Cambrian and present faunal communities with emphasis on velvet worms (Onychophora)

The controversy about a Cambrian "explosion" of morphological disparity (followed by decimation), cladogenesis and fossilization is of central importance for the history of life. This paper revisits the controversy (with emphasis in onychophorans, which include emblematic organisms such as Hallucigenia), presents new data about the Chengjiang (Cambrian of China) faunal community and compares it and the Burgess Shale (Cambrian of Canada) with an ecologically similar but modern tropical marine site where onychophorans are absent, and with a modern neotropical terrestrial onychophoran community. Biovolume was estimated from material collected in Costa Rica and morphometric measurements were made on enlarged images of fossils. Cambrian tropical mudflats were characterized by the adaptive radiation of two contrasting groups: the vagile arthropods and the sessile poriferans. Arthropods were later replaced as the dominant benthic taxon by polychaetes. Vagility and the exoskeleton may explain the success of arthropods from the Cambrian to the modern marine and terrestrial communities, both in population and biovolume. Food ecological displacement was apparent in the B. Shale, but not in Chengjiang or the terrestrial community. When only hard parts were preserved, marine and terrestrial fossil deposits of tropical origin are even less representative than deposits produced by temperate taxa, Chengjiang being an exception. Nutrient limitations might explain why deposit feeding is less important in terrestrial onychophoran communities, where carnivory, scavenging and omnivory (associated with high motility and life over the substrate) became more important. Fossil morphometry supports the interpretation of "lobopod animals" as onychophorans, whose abundance in Chengjiang was equal to their abundance in modern communities. The extinction of marine onychophorans may reflect domination of the infaunal habitat by polychaetes. We conclude that (1) a mature ecological community structure was generalized during the Cambrian, and even biodiversity and equitability indices were surprisingly close to modern values; (2) the morphological diversity and geographic distribution of onychophorans indicate a significant pre-Cambrian evolutionary history which does not support the "explosion" hypothesis; (3) disparity among phyla was not as important as the explosion-decimation model predicts, but in the case of onychophorans, disparity within the phylum was greater than it is today and its reduction may have been associated with migration into the sediment when large predators evolved.     

A new rhynchonelliform brachiopod Longtancunella with soft-part preservation from the Hongjingshao Formation (Cambrian Stage 3) in Yunnan,South China

Rhynchonelliform brachiopods made their first appearance in early Cambrian, and became a major group within the palaeozoic evolutionary fauna since late Cambrian. Exceptionally preserved fossils from the early Cambrian Lagerstätten provide valuable chances to investigate their phylogeny and ecology. Longtancunella is one of the most interesting early rhynchonelliforms, and has been mainly recovered from the Chengjiang Lagerstätte (Series 2, Stage 3). Here, we report a new rhynchonelliform Longtancunella xiazhuangensis n. sp. from the lower Hongjingshao Formation (upper Stage 3) in Yunnan Province, China. These specimens were well preserved with soft parts, including pinnate mantle canal system and a pedicle. It is identified as a new species based mainly on its difference in shell ornamentations, pinnate mantle canals and pedicle morphology from the type species. Its pedicle looks unusually stout with distinct annulated lamellae on the surface, and reveals crucial evidence in illustrating its ecology and settling strategy as an early marine epifauna. The ecological interaction between L. xiazhuangensis and other marine animals also provides insights into the food web structure in the early Cambrian.     

Beyond the Burgess Shale: Cambrian microfossils track the rise and fall of hallucigeniid lobopodians

Burgess Shale-type deposits are renowned for their exquisite preservation of soft-bodied organisms, representing a range of animal body plans that evolved during the Cambrian ‘explosion’. However, the rarity of these fossil deposits makes it difficult to reconstruct the broader-scale distributions of their constituent organisms. By contrast, microscopic skeletal elements represent an extensive chronicle of early animal evolution—but are difficult to interpret in the absence of corresponding whole-body fossils. Here, we provide new observations on the dorsal spines of the Cambrian lobopodian (panarthropod) worm Hallucigenia sparsa from the Burgess Shale (Cambrian Series 3, Stage 5). These exhibit a distinctive scaly microstructure and layered (cone-in-cone) construction that together identify a hitherto enigmatic suite of carbonaceous and phosphatic Cambrian microfossils—including material attributed to Mongolitubulus, Rushtonites and Rhombocorniculum—as spines of Hallucigenia-type lobopodians. Hallucigeniids are thus revealed as an important and widespread component of disparate Cambrian communities from late in the Terreneuvian (Cambrian Stage 2) through the ‘middle’ Cambrian (Series 3); their apparent decline in the latest Cambrian may be partly taphonomic. The cone-in-cone construction of hallucigeniid sclerites is shared with the sclerotized cuticular structures (jaws and claws) in modern onychophorans. More generally, our results emphasize the reciprocal importance and complementary roles of Burgess Shale-type fossils and isolated microfossils in documenting early animal evolution.     

Unlocking the early fossil record of the arthropod central nervous system

Extant panarthropods (euarthropods, onychophorans and tardigrades) are hallmarked by stunning morphological and taxonomic diversity, but their central nervous systems (CNS) are relatively conserved. The timing of divergences of the ground pattern CNS organization of the major panarthropod clades has been poorly constrained because of a scarcity of data from their early fossil record. Although the CNS has been documented in three-dimensional detail in insects from Cenozoic ambers, it is widely assumed that these tissues are too prone to decay to withstand other styles of fossilization or geologically older preservation. However, Cambrian Burgess Shale-type compressions have emerged as sources of fossilized brains and nerve cords. CNS in these Cambrian fossils are preserved as carbon films or as iron oxides/hydroxides after pyrite in association with carbon. Experiments with carcasses compacted in fine-grained sediment depict preservation of neural tissue for a more prolonged temporal window than anticipated by decay experiments in other media. CNS and compound eye characters in exceptionally preserved Cambrian fossils predict divergences of the mandibulate and chelicerate ground patterns by Cambrian Stage 3 (ca 518 Ma), a dating that is compatible with molecular estimates for these splits.     

Cambrian anemones with preserved soft tissue from the Chengjiang biota, China

The group Cnidaria includes 'jellyfish', soft-bodied anemone and anemone-like forms and calcified corals. These diploblastic organisms have a fossil record extending back to the earliest metazoans of the Neoproterozoic; however certain cnidarians of the subclass Zoantharia, characterized by soft-bodied anemone-like forms, are absent or poorly represented in the fossil record. Despite the paucity of fossils, it is thought that calcification by soft anemone-like animals was responsible for producing the skeleton that allowed the preservation of the first corals. We report discovery of an abundant assemblage of in situ soft-bodied polyps with tissues. They are preserved in exquisite detail and come from the well-known Lower Cambrian Chengjiang biota of Yunnan, China. The soft-bodied polyps display a simple anatomy that is comparable to some extant anemones of the order Actinaria. The new fossils are assigned to Archisaccophyllia kunmingensis n. gen. et n. sp. Their simple and conservative form suggests that these fossils may represent some kind of ancestral rootstock. The preserved life assemblage provides a unique snapshot of Lower Cambrian anemone life and provides clues for relationships with extant actiniarians as well as calcified corals.     

The evolutionary history of crustacean segmentation: a fossil-based perspective

The evolution of segmentation in Crustacea, that is, the formation of sclerotized and jointed body somites and arrangement of somites into tagmata, is viewed in light of historical traits and functional constraints. The set of Early to Late Cambrian 'Orsten' arthropods have informed our current views of crustacean evolution considerably. These three-dimensionally preserved fossils document ancient morphologies, as opposed to purely hypothetical models and, because of the unusual preservation of larval stages, provide us with unparalleled insight into the morphogenesis of body somites and their structural equipment. The variety of evolutionary levels represented in the 'Orsten' including lobopodians, tardigrades, and pentastomids also allows phylogenetic interpretations far beyond the Crustacea. The 'Orsten' evidence and data from representatives of the Lower Cambrian Chengjiang biota in southwestern China, including phylogenetically earlier forms, form the major source of our morphology-based review of structural and functional developments that led toward the Crustacea. The principal strategy of arthropods is the simultaneous development of head somites, as expressed in a basal "head larva," and a successive addition of postcephalic somites from a preterminal budding zone with progressive maturation of metameric structures. This can be recognized in the developmental patterns of extant and fossil representatives of several euarthropod taxa, particularly crustaceans, trilobites, and chelicerates (at least basally). The development of these taxa points to an early somite-poor and free-living hatching stage. Embryonic development to a late stage within an egg, as occurring in recent onychophorans and certain in-group euarthropods, is regarded as achieved several times convergently.     

Thecate stem medusozoans (Cnidaria) from the early Cambrian Chengjiang biota

Cnidarians are phylogenetically located near the base of the ‘tree of animals’, and their early evolution had a profound impact on the rise of bilaterians. However, the early diversity and phylogeny of this ‘lowly’ metazoan clade has hitherto been enigmatic. Fortunately, cnidarian fossils from the early Cambrian could provide key insights into their evolutionary history. Here, based on a scrutiny of the purported hyolith Burithes yunnanensis Hou et al. from the early Cambrian Chengjiang biota in South China, we reveal that this species shows characters distinct from those typical of hyoliths, not least a funnel-shaped gastrovascular system with a single opening, a whorl of tentacles surrounding the mouth, and the lack of an operculum. These characters suggest a great deviation from the original definition of the genus Burithes, and a closer affinity with cnidarians. We therefore reassign the material to a new genus: Palaeoconotuba. Bayesian inference of phylogeny based on new anatomical traits identifies a new clade, including Palaeoconotuba and Cambrorhytium, as a stem group of sessile medusozoan cnidarians that are united by the synapomorphies of developing an organic conical theca and a funnel-like gastrovascular system. This study unveils a stem lineage of medusozoans that evolved a lifelong conical theca in the early Cambrian.     

寒武纪早期岩家河生物群:研究进展和展望*

埃迪卡拉纪-寒武纪转换时期动物的起源、演化和"寒武纪大爆发"一直是国际古生物学界研究的热点问题,其中寒武纪早期小壳化石群与埃迪卡拉纪化石群和寒武纪早期澄江化石库之间的内在关系是古生物学界研究的难题,其主要原因是寒武纪早期与小壳化石群伴生的宏体动、植物化石的缺乏。发现于峡东地区的寒武纪早期岩家河生物群填补了这一缺失环节,该生物群包含宏体动物、宏观藻类、小壳化石、球形化石(可能的胚胎化石)、微古植物和蓝菌类等化石,部分宏体化石显示了从埃迪卡拉纪向寒武纪过渡色彩。化石保存方式有碳质膜、黄铁矿化、磷酸盐化、硅化。因此对岩家河生物群生物多样性和埋藏学进行综合研究,将可提供纽芬兰世(梅树村期)碳酸盐台地—碳酸盐台地内部的局部凹陷盆地相的一个较完整的生物景观图,对探索"寒武纪大爆发主幕"前夕生物的辐射、演化模式及保存机制具有重要的科学意义。     

First evidence of Ediacaran–Fortunian elliptical body fossils in the Brioverian series of Brittany,NW France

Body fossils are described for the first time from the Upper Brioverian (Ediacaran–Fortunian) deposits of Central Brittany (NW France). The material consists of a dozen of specimens preserved on two slates, recently collected in a quarry in Saint‐Gonlay. The fossils, centimetric in size, have an elliptical and acuminated outline, with a peripheral bulge but without conspicuous concentric or radial lines. These body fossils are preserved in slates that are locally rich in grazing traces, sometimes associated with microbial mats. The sedimentological characteristics of these deposits correspond to a shallow marine shelf environment, with a low to moderate hydrodynamism. Such findings, in concomitance with previous ichnological discoveries, unravel the potential of the Brioverian strata from Brittany as a new locality to provide answers on Ediacaran and early Cambrian biocenoses.