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。     

陕南寒武系下部宽川铺组帽球状化石*

陕南寒武纪早期宽川铺组微体球状化石类型多样,如何正确辨别这些球状化石的生物属性是当前早期生命演化古生物学研究中的一个难点。我们在宽川铺组中发现了一类为数众多,具有一个或者多个不规则的帽状隆起结构以及翻边帽沿的帽状化石。研究表明这些帽状化石均为不完整的个体,其完整形态呈不规则的凹球形。根据帽球状化石帽身的数量和相对位置,这类化石可划分为"单帽型"、"双帽型"和"复帽型"三种类型。这些凹球状化石呈双层壳壁,内壁光滑,外壁粗糙。因为凹球状化石形态及其表面小孔与微体藻类、后生动物的胚胎以及壳体化石差异都非常显著,所以推测这类化石可能与带壳原生动物亲缘关系最为紧密。     

Fossil preservation in the Neoproterozoic Doushantuo phosphorite Lagerstätte, South China

Phosphorites of the late Neoproterozoic Doushantuo Formation exposed in the vicinity of Weng'an, Guizhou Province, and Chadian, Shaanxi Province, South China, contain exceptionally well-preserved algal thalli, acritarchs, and globular microfossils interpreted as animal embryos. Combined optical microscopic and SEM observations provide insights into the taphonomy of phosphatized fossils. Algal cells and tissues are variably resistant to decay, and within preserved populations permineralization began at varying stages of degradation. In consequence, there is a spectrum of quality in cellular preservation. Algal cell walls, acritarch vesicles, and embryo envelopes are commonly encrusted by an isopachous rim of apatite, with cell interiors filled by collophane and later diagenetic dolomite. In contrast, blastomere surfaces of animal embryos are encrusted primarily by minute phosphatic spherules and filaments, possibly reflecting an immediately postmortem infestation of bacteria that provided nucleation sites for phosphate crystal growth. Thus, the same processes that gave rise to Phanerozoic phosphatized Lagerstätten - phosphatic encrustation, and impregnation, probably mediated by microbial activity - effected soft-tissue preservation in the Doushantuo Lagerstätte. It remains unclear how phosphatic ions and organic macro-molecules interact at the molecular level and to what extent specific microbial metabolisms or microenvironmental conditions control the phosphatization of soft tissues. New observations of phosphatized Doushantuo fossils include: a second locality (Chadian) for Wengania globosa , interpreted as an algal thallus and previously known only from Weng'an; microtunnels in Weng'an phosphorites interpreted as pyrite trails; and new taxa described from Weng'an: Meghystrichosphaeridium reticulatum (acritarch), Sarcinophycus radiatus (algal thallus), and one unnamed problematic form.     

寒武纪早期宽川铺生物群中圆管螺化石显微结构及显微谱学分析

陕南寒武系底部宽川铺组不仅保存了大量的小壳化石,还以磷酸盐化的方式立体保存了多种软躯体动物的胚胎和成体化石,为研究早期动物矿化模式、个体发育方式以及动物躯体构型的早期辐射过程提供了非常关键的信息。然而宽川铺生物群的研究大多聚焦于化石系统分类及亲缘关系探索,对生物群的沉积背景、埋藏过程和保存模式的分析甚少。本文以宽川铺生物群的核心产出层位——陕西省西乡县大河镇宽川铺组下部磷质碎屑灰岩为研究对象,对其中的一类管状化石圆管螺及其围岩进行了显微结构学和显微谱学综合分析,获得了化石显微结构和关键结构对应的元素、矿物成分等信息,讨论了管状化石圆管螺的埋藏过程和保存模式,并在此基础上对其生物学特征进行了初步探讨。该研究对理解宽川铺生物群的埋藏机制提供了新的线索。     

Fossil preservation in the Neoproterozoic Doushantuo phosphorite Lagerstatte, South China.

Phosphorites of the late Neoproterozoic Doushantuo Formation exposed in the vicinity of Weng'an, Guizhou Province, and Chadian, Shaanxi Province, South China, contain exceptionally well-preserved algal thalli, acritarchs, and globular microfossils interpreted as animal embryos. Combined optical microscopic and SEM observations provide insights into the taphonomy of phosphatized fossils. Algal cells and tissues are variably resistant to decay, and within preserved populations permineralization began at varying stages of degradation. In consequence, there is a spectrum of quality in cellular preservation. Algal cell walls, acritarch vesicles, and embryo envelopes are commonly encrusted by an isopachous rim of apatite, with cell interiors filled by collophane and later diagenetic dolomite. In contrast, blastomere surfaces of animal embryos are encrusted primarily by minute phosphatic spherules and filaments, possibly reflecting an immediately postmortem infestation of bacteria that provided nucleation sites for phosphate crystal growth. Thus, the same processes that gave rise to Phanerozoic phosphatized Lagerstatten--phosphatic encrustation, and impregnation, probably mediated by microbial activity--effected soft-tissue preservation in the Doushantuo Lagerstatte. It remains unclear how phosphatic ions and organic macromolecules interact at the molecular level and to what extent specific microbial metabolisms or microenvironmental conditions control the phosphatization of soft tissues. New observations of phosphatized Doushantuo fossils include: a second locality (Chadian) for Wengania globosa, interpreted as an algal thallus and previously known only from Weng'an; microtunnels in Weng'an phosphorites interpreted as pyrite trails; and new taxa described from Weng'an: Meghystrichosphaeridium reticulatum (acritarch), Sarcinophycus radiatus (algal thallus), and one unnamed problematic form.     

陕南寒武系底部宽川铺组微体化石人工智能识别

我国陕南寒武纪初期的宽川铺组(距今大约535百万年)以盛产磷酸盐化的微体小壳化石和球状动物胚胎化石而闻名于世,是探索早期动物门类起源和寒武纪生命大爆发研究中的一个非常重要的窗口。在宽川铺组中寻找微体化石新类型主要依赖显微镜下人工挑选的传统方法日趋低效,迫切需要新的技术手段来解决这些问题。而微体化石数量庞大,大小接近,形态简单,非常适合于采用机器学习的手段进行人工智能分类。我们在宽川铺组微体化石中尝试使用机器学习的手段来进行化石图像识别和机器分拣。采用方向梯度直方图来提取化石图像的主要向量特征,并设计了二叉树型多分类识别器进行化石数字分类。目前处理了5 000多张微体化石照片的人工智能识别,已经取得了较高的准确率。     

Experimental taphonomy of giant sulphur bacteria: implications for the interpretation of the embryo-like Ediacaran Doushantuo fossils

The Ediacaran Doushantuo biota has yielded fossils interpreted as eukaryotic organisms, either animal embryos or eukaryotes basal or distantly related to Metazoa. However, the fossils have been interpreted alternatively as giant sulphur bacteria similar to the extant Thiomargarita. To test this hypothesis, living and decayed Thiomargarita were compared with Doushantuo fossils and experimental taphonomic pathways were compared with modern embryos. In the fossils, as in eukaryotic cells, subcellular structures are distributed throughout cell volume; in Thiomargarita, a central vacuole encompasses approximately 98 per cent cell volume. Key features of the fossils, including putative lipid vesicles and nuclei, complex envelope ornament, and ornate outer vesicles are incompatible with living and decay morphologies observed in Thiomargarita. Microbial taphonomy of Thiomargarita also differed from that of embryos. Embryo tissues can be consumed and replaced by bacteria, forming a replica composed of a three-dimensional biofilm, a stable fabric for potential fossilization. Vacuolated Thiomargarita cells collapse easily and do not provide an internal substrate for bacteria. The findings do not support the hypothesis that giant sulphur bacteria are an appropriate interpretative model for the embryo-like Doushantuo fossils. However, sulphur bacteria may have mediated fossil mineralization and may provide a potential bacterial analogue for other macroscopic Precambrian remains.     

Unveiling biases in soft‐tissue phosphatization: extensive preservation of musculature in the Cretaceous (Cenomanian) polychaete Rollinschaeta myoplena (Annelida: Amphinomidae)

The process of soft‐tissue phosphatization (the replication of labile tissues by calcium phosphate) is responsible for many instances of high‐resolution soft tissue preservation, often revealing anatomical insights into the animals that so preserved. However, while much work has gone into exploring key issues such as biases and micro‐controls, phosphatization remains poorly understood as a taphonomic process. Here, using camera lucida, plain‐light microscopy and SEM imagery, we address this issue by describing the taphonomy and fidelity of the musculature of Rollinschaeta myoplena Parry et al., a phosphatized annelid from the Cretaceous Konservat‐Lagerstätten of Hakel and Hjoula, Lebanon, with an unprecedented quantity of three‐dimensional soft‐tissue preservation. Analysis highlights two strong, previously recognized biases affecting the process of phosphatization: (1) a taxonomic bias restricted to R. myoplena that triggers unusually extensive phosphatization; and (2) a tissue bias whereby longitudinal and parapodial musculature show markedly higher fidelity in comparison to the musculature of the intestine and body wall circular muscles. Potential explanations for these biases include internal phosphate‐enrichment by relative muscle density, the relative rate of decay and the physiology of musculature. Incongruence between experimental decay series for polychaetes and the prevalence of labile tissue preservation over recalcitrant tissues in R. myoplena exposes the limits of decay experiments for understanding exceptional preservation.     

Organic compositions of the embryo-bearing lowermost Cambrian Kuanchuanpu formation on the northern Yangtze platform, China

We investigated the origin of organic matter and paleoceanographic conditions of the embryo-bearing lowermost Cambrian Kuanchuanpu Formation in Shaanxi Province, China, in terms of sedimentary facies, fossil morphometry, and organic geochemistry, including biomarker compositions. The results indicated the abundance of algae and eukaryotic phytoplankton that fueled a diverse ecosystem including cnidarians and small shelly fossils. The water column contained enough oxygen to provide habitat for benthic cnidarians such as Olivooides. On the other hand, low pristane/phytane ratios, detection of squalane, and hopane composition indicate reductive conditions within the sediments, and that the redox front was likely near the water-sediment interface. Common occurrences of pyrite and barite indicate that redox reactions of sulfur species were activated at the water-sediment interface. Such circumstances and the high phosphate concentration in ambient water may recall the presence of sulfur-oxidizing bacteria that promoted the phosphatization of animal embryos, as suggested for in the Neoproterozoic Doushantuo Formation. Nitrogen was the limiting nutrient for primary productivity, and the high phosphate concentration in the ocean promoted the phosphatization of animal embryos at the reductive water–sediment interface.     

疑源类Tianzhushania在贵州瓮安震旦纪陡山沱组磷块岩中的发现

首次报道贵州瓮安地区震旦系陡山沱组磷块岩中的大型具刺凝源类Tianzhushania spinosa及其新种Tianzhushania tuberifera sp.nov.前者形态特征及内部结构与湖北峡尔地区震旦系陡山沱组隧石结核中发现的典型Tianzhushania spinosa 标本（硅化）完全相同,但多数未遭受重力挤压呈球形保存。新种特征明显,也呈球形保存,但内层表面具有明显瘤突,瘤突顶端向外延伸出单一或多条管状制,另外还探讨大型具刺疑源类Tianzhushania属的特征,已定种的地理分布,地层意义和亲缘关系。     

陕南晚震旦世骨骼化石中的微生物磷酸盐化作用

陕南晚震旦世高家山生物群中骨骼化石的电镜扫描显示,Sinotubulites levis壳壁中具有丰富的磷酸盐化球状和丝状体,可分为3种不同类型的微生物组构：1)由相互缠绕的丝状体所组成的藻席状组构;2)由外表面成莓球状,内部中空的球状体紧密堆叠所形成的组构类型;3)由表面极为光滑的球状体堆叠所形成的组构。它们可能分别代表不同类型的矿化蓝藻,依此推断微生物在S.levis壳壁磷酸盐化过程中起着重要作用。在共生的Sinotubulites baimatuoensis和Cloudina壳壁上,极少或完全不存在这些磷酸盐化组构表明微生物的磷酸盐化作用具有专属性,可能由壳壁微结构所决定,具多层状壁的S.levis可能与具许多有机质纹层的多毛类壳壁关系密切。     

陕南早寒武世具口部的磷酸盐化Punctatus及其胚胎化石

报道陕西南部灯影组宽川铺段地层中发现的Punctatus锥体标本、保留精美软组织特征的口部标本和可能的胚胎标本。对Punctatus软组织及整体形态功能研究表明,Punctatus在分类上可能更接近于腔肠动物的水螅型,代表初具原始触手的腔肠动物早期演化类型。在上万枚与Punctatus共生的球状化石中,发现若干枚可能的原肠期胚胎及胚胎发育晚期标本,在此基础上提出PunctatusemeiensisHe,1980花冠状口部可能经历的胚胎发育序列。     

贵州瓮安新元古代陡山沱组磷酸盐化管柱状微体化石新材料

在用醋酸浸泡处理采自贵州瓮安新元古代陡山沱组磷酸质岩石样品时,获得大量微体动物骨骼化石。文中重点报道部分管柱状微体化石,即：Sinoquadraticus poratus gen.et sp.nov和Sinoquadraticus wenganensis gen. et sp.nov.。并对Sinocyclocylicus的部分特征进行补充。     

Preservational modes of some ichthyosaur soft tissues (Reptilia,Ichthyopterygia) from the Jurassic Posidonia Shale of Germany

Konservat-Lagerstätten, such as the Toarcian (Early Jurassic) Posidonia Shale of southwestern Germany, are renowned for their spectacular fossils. Ichthyosaur skeletons recovered from this formation are frequently associated with soft tissues; however, the preserved material ranges from three-dimensional, predominantly phosphatized structures to dark films of mainly organic matter. We examined soft-tissue residues obtained from two ichthyosaur specimens using an integrated ultrastructural and geochemical approach. Our analyses revealed that the superficially-looking ‘films’ in fact comprise sections of densely aggregated melanosome (pigment) organelles sandwiched between phosphatized layers containing fibrous microstructures. We interpret this distinct layering as representing condensed and incompletely degraded integument from both sides of the animal. When compared against previously documented ichthyosaur fossils, it becomes readily apparent that a range of preservational modes exists between presumed ‘phosphatic’ and ‘carbonized’ soft-tissue remains. Some specimens show high structural fidelity (e.g. distinct integumentary layering), while others, including the fossils examined in this study, retain few original anatomical details. This diversity of soft-tissue preservational modes among Posidonia Shale ichthyosaurs offers a unique opportunity to examine different biostratinomic, taphonomic and diagenetic variables that potentially could affect the process of fossilization. It is likely that soft-tissue preservation in the Posidonia Shale was regulated by a multitude of factors, including decay efficiency and speed of phosphatic mineral nucleation; these in turn were governed by a seafloor with sustained microbial mat activity fuelled by high organic matter input and seasonally fluctuating oxygen levels.     

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。     

The ‘Orsten’—More than a Cambrian Konservat-Lagerstätte yielding exceptional preservation

In several areas of southern Sweden, limestone nodules, locally called Orsten occur within bituminous alum shales. These shales and nodules were deposited under dysoxic conditions at the bottom of what was most likely a shallow sea during the late Middle to Upper Cambrian (ca. 500 million years ago). Subsequently, the name ‘Orsten’ has been referred to particular, mainly arthropod, fossils from such nodules, and, in a wider sense, to the specific type of preservation of minute fossil through secondarily phosphatization. This preservation is exceptional in yielding uncompacted and diagenetically undeformed three-dimensional fossils. ‘Orsten’-type preservation resulted from incrustation of a thin external layer and also by impregnation by calcium phosphate and, therefore, mineralization of the surface of the former animals during early diagenesis. Primarily, this type of preservation seems to have affected only cuticle-bearing metazoans such as cycloneuralian nemathelminths and arthropods. ‘Orsten’ preservation in this sense seems to be limited by size, in having yielded no partial or complete animals larger than 2 mm. On the other end of the scale, even larvae 100 μm long are preserved, often more complete than larger specimens, and details such as setules and pores smaller than 1 μm can be observed. Fossils preserved in such a manner are almost exclusively hollow carcasses, but can be filled secondarily; less common are completely phosphatized compact specimens. The high quality of preservation makes the Swedish ‘Orsten’ a typical Konservat-Lagerstätte. Yet, its special type of preservation is more widespread in time and geographical distribution than assumed initially, and the origin of the phosphate is not necessarily restricted just to one source. Subsequent to the first discoveries of limb fragments of Cambrian arthropods in 1975, animals in this special preservational type have been discovered in several continents and across a broad stratigraphic range including even Proterozoic strata. The latter have yielded early cleavage and metazoan embryonic stages, expanding knowledge on the preservational capacities of the ‘Orsten’. Here, we report the recent status of our research on the ‘Orsten’ and give perspectives for future exploration on a worldwide scale, particularly in light of a recently formed international research group named Center of Orsten Research and Exploration (C.O.R.E.).     

Pyritized preservation of chancelloriids from the Cambrian Stage 3 of South China and implications for biomineralization

The enigmatic Cambrian animal chancelloriids were discovered in a wide range of taphonomic settings; however, preservation of biomineralized sclerite microstructure was solely known from secondarily phosphatized skeletal remains. Here, we investigate a uniquely pyritized chancelloriid from the lower Cambrian Guojiaba Formation in southern Shaanxi Province, China, using a combination of advanced analytic techniques. Results of the energy dispersive spectroscopy (EDS), X-Ray Fluorescence (XRF), and Raman spectrum show that the sclerites and scleritomes are preserved as pyritized internal moulds with a calcitic outer layer. The outer layer enveloping the internal moulds likely represents the recrystallized counterpart of the original biomineralized sclerite wall. Distinctive fibrous microstructures are discovered in the sclerites, which echo the features seen in the phosphatized fossils of chancelloriids. The typical microstructure, along with the recrystallized calcite, corroborate the interpretation that chancelloriid sclerites were originally constructed by fibrous aragonite. The stability of the microstructure and mineral composition in both carbonate and siliciclastic backgrounds indicate that chancelloriids were adapted to exploit aragonitic fibres to build their skeletons regardless of the change of their living environments.     

Origin and early diversification of the phylum Cnidaria Verrill: major developments in the analysis of the taxon's Proterozoic–Cambrian history

Diploblastic eumetazoans of the phylum Cnidaria originated during the Neoproterozoic Era, possibly during the Cryogenian Period. The oldest known fossil cnidarians occur in strata of Ediacaran age and consist of polypoid forms that were either nonbiomineralizing or weakly so. The oldest possible anthozoans, including the genus Ramitubus, may be related to tabulate corals and occur in the Doushantuo Lagerstätte (upper Doushantuo Formation, South China), the age of which is poorly constrained (approximately 585 Ma?). Conulariid scyphozoans may first appear as early as 635–577 Ma (Lantian Formation, South China). A definite conulariid, most similar to Palaeozoic species assigned to the genus Paraconularia, occurs in association with the possible scyphozoan, Corumbella werneri, in the latest Ediacaran (c. 543 Ma) Tamengo Formation of Brazil. Basal Cambrian (c. 540 Ma) phosphorites in the upper Kuanchuanpu Formation (South China) yield solitary polyps of the oldest probable anthozoan (Eolympia pediculata), which appears to have been a stem hexacorallian. This same formation contains fossils interpreted by some authors as pentaradial cubozoan polyps; however, both the oldest known cubozoans and the oldest hydrozoans, all medusae, may actually occur in the Cambrian (Series 3, c. 505 Ma) Marjum Formation (Utah, USA). Although these recently published palaeontological data tend to corroborate the hypothesis that Cnidaria has a relatively deep Neoproterozoic history, the timing of major internal branching events remains poorly constrained, with, for example, the results of some molecular clock analyses indicating that the two cnidarian subphyla (Anthozoaria and Medusozoa) may have originated as many as one billion years ago. Further progress towards elucidating the evolution and early fossil record of cnidarians may accrue from: (1) an intensive search for phosphatized soft parts in possible anthozoans from the Ediacaran Doushantuo Formation; (2) an expanded search for Ediacaran conulariids; and (3) additional detailed analyses of the taphonomy and preservation of Ediacaran and Cambrian cnidarians, including possible pentaradial cubozoan polyps from the Fortunian upper Kuanchuanpu Formation.     

The Ediacaran Aspidella‐type impressions in the Jinxian successions of Liaoning Province,northeastern China

Macroscopic impression fossils from the Xingmincun Formation of the Jinxian Group, Liaoning Province of northeastern China, are identified as members of the Aspidella plexus of Ediacaran age. This is the first recognition of the taxon in the Liaoning Province, although such fossils have been previously recorded in the succession, but were referred to as new species and relegated to an earlier Neoproterozoic age. A revision of the taxonomic interpretation and relative age estimation of the previous record is provided, as well as an evaluation of abiotic vs. biotic processes that could produce similar structures to studied impressions. The mode of preservation of the fossils is considered from a biochemical point of view and along with the properties of organic matter in the integument of soft‐bodied metazoans. The selective preservation of the Ediacaran organisms, including metazoans, as impressions (moulds and casts) against the organically preserved contemporaneous cyanobacterial and algal microfossils, and an exceptionally small number of terminal Ediacaran metazoan fossils (Sabellidites, Conotubus and Shaanxilithes), demonstrates the non‐resistant characteristics and the very different biochemical constitution of the Ediacaran metazoans compared with those that evolved in the Cambrian and after. The refractory biomacromolecules in cell walls of photosynthesizing microbiota (bacterans, cutans, algaenan and sporopollenin groups) and in the chitinous body walls of Sabellidites contrast sharply with the labile biopolymers in Ediacaran metazoans known only from impressions. The newly emerging biosynthesis of resistant biopolymers in metazoans (chitin and collagen groups) initiated by the annelids at the end of Ediacaran and fully evolved in Cambrian metazoans, considered with the ability to biomineralize, made their body preservation possible. The Chengjiang and Burgess Shale metazoans show evidence of this new biochemistry in body walls and cuticles, and not only because of the specific taphonomic window that enhanced their preservation.