The proterozoic and earliest cambrian trace fossil record; patterns, problems and perspectives

The increase in trace fossil diversity across the Neoproterozoic-Cambrianboundary often is presented in terms of tabulations of ichnogenera.However, a clearer picture of the increase in diversity andcomplexity can be reached by combining trace fossils into broadgroups defined both on morphology and interpretation. This alsofocuses attention on looking for similarites between Neoproterozoicand Cambrian trace fossils. Siliciclastic sediments of the Neoproterozoicpreserve elongate tubular organisms and structures of probablealgal origin, many of which are very similar to trace fossils.Such enigmatic structures include Palaeopascichnus and Yelovichnus,previously thought to be trace fossils in the form of tightmeanders. A preliminary two or tripartite terminal Neoproterozoic tracefossil zonation can be be recognized. Possibly the earliesttrace fossils are short unbranched forms, probably younger thanabout 560 Ma. Typical Neoproterozoic trace fossils are unbranchedand essentially horizontal forms found associated with diverseassemblages of Ediacaran organisms. In sections younger thanabout 550 Ma a modest increase in trace fossil diversity occurs,including the appearance of rare three-dimensional burrow systems(treptichnids), and traces with a three-lobed lower surfaces.     

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。     

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.     

陕西宁强埃迪卡拉纪晚期高家山生物群中黄铁矿化化石的成岩及风化过程

陕南埃迪卡拉纪晚期的高家山生物群中部分化石以黄铁矿化三维立体保存软躯体管状化石为主要特征。对野外识别出的具有不同风化程度的3类粉砂岩进行了详细的手标本特征对比和扫描电子显微镜测试对比研究。锥管虫（Conotubus）的化石成岩学特征表明黄铁矿最先沉淀,随后方解石充填于黄铁矿留下的空隙或空腔中。这些化石所经历的第一期风化作用相对较弱,保留了大量的原始化石成岩学信息;第二期风化作用则可能形成于一个氧化条件较强的环境中。     

Sediments of the Middle Cambrian Burgess Shale, Canada

The Phyllopod Bed of the Burgess Shale, in which Walcott found the famous soft bodied fossils, consists of thin graded beds of calcareous siltstone and mud-stone, which are probably turbidites. The Burgess Shale was deposited on a reef front submarine fan, and the preservation of the fossils is probably due to rapid burial.     

Tomographic techniques for the study of exceptionally preserved fossils

Three-dimensional fossils, especially those preserving soft-part anatomy, are a rich source of palaeontological information; they can, however, be difficult to work with. Imaging of serial planes through an object (tomography) allows study of both the inside and outside of three-dimensional fossils. Tomography may be performed using physical grinding or sawing coupled with photography, through optical techniques of serial focusing, or using a variety of scanning technologies such as neutron tomography, magnetic resonance imaging and most usefully X-ray computed tomography. This latter technique is applicable at a variety of scales, and when combined with a synchrotron X-ray source can produce very high-quality data that may be augmented by phase-contrast information to enhance contrast. Tomographic data can be visualized in several ways, the most effective of which is the production of isosurface-based 'virtual fossils' that can be manipulated and dissected interactively.     

Exceptional fossil preservation and the cambrian explosion

Exceptionally preserved, non-biomineralizing fossils contributeimportantly to resolving details of the Cambrian explosion,but little to its overall patterns. Six distinct "types" ofexceptional preservation are identified for the terminal Proterozoic-Cambrianinterval, each of which is dependent on particular taphonomiccircumstances, typically restricted both in space and time.Taphonomic pathways yielding exceptional preservation were particularlyvariable through the Proterozoic-Cambrian transition, at leastin part a consequence of contemporaneous evolutionary innovations.Combined with the reasonably continuous record of "Doushantuo-typepreservation," and the fundamentally more robust records ofshelly fossils, phytoplankton cysts and trace fossils, thesetaphonomic perturbations contribute to the documentation ofmajor evolutionary and biogeochemical shifts through the terminalProterozoic and early Cambrian. Appreciation of the relationship between taphonomic pathwayand fossil expression serves as a useful tool for interpretingexceptionally preserved, often problematic, early Cambrian fossils.In shale facies, for example, flattened non-biomineralizingstructures typically represent the remains of degradation-resistantacellular and extracellular "tissues" such as chaetae and cuticles,whereas three-dimensional preservation represents labile cellulartissues with a propensity for attracting and precipitating earlydiagenetic minerals. Such distinction helps to identify theacuticular integument of hyolithids, the chaetae-like natureof Wiwaxia sclerites, the chaetognath-like integument of Amiskwia,the midgut glands of various Burgess Shale arthropods, and themisidentification of deposit-feeding arthropods in the Chengjiangbiota. By the same reasoning, putative lobopods in the SiriusPasset biota and putative deuterostomes in the Chengiang biotaare better interpreted as arthropods.     

The study of fossil spider species

The definition of what constitutes a species has been an area of contention in biology since before the time of Darwin. Here, we discuss concepts of species in regards to the Araneae and particularly focus on diagnosing fossils. Spiders are primarily diagnosed by their copulatory organs, which may be difficult to observe in fossils due to a number of confounding factors, thus potentially hindering identification and systematic classification. However, despite potential difficulties, fossils should and must be studied alongside extant Araneae in order to garner a full understanding of the evolutionary history of this megadiverse group.     

以胶原蛋白为例浅述古蛋白质组学研究现状与未来*

蛋白质作为参与构建生物体的重要生物大分子,是生物功能与代谢的物质基础,同时蛋白质的序列信息又源自生物的遗传编码信息,因此是认识生物演化本质的重要研究对象。近年来,随着质谱技术的发展,获取生物化石中的古蛋白质序列信息不再遥不可及,这为在形态学与DNA序列信息缺乏的条件下对古生物的认识提供了一条新途径。胶原蛋白(Collagen)在动物骨骼中极为丰富,又因为其特殊的结构,易于在化石中保存,故已成为古蛋白质组学研究的重要对象。本文将以胶原蛋白为例,对古蛋白质组学现有的研究方法与已经取得的研究成果进行总结,并对古蛋白质组学目前面临的挑战与困难及未来研究趋势进行讨论,旨在展示古蛋白质组学的应用潜力,并探讨其在生物演化研究中的意义。     

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。     

Cnidarian taphonomy and affinities of the Ediacara biota

Plaster impressions and sand casts of extant medusae, a chondrophoran, and a pennatulid share basic structural characteristics with fossils in the Upper Proterozoic Ediacara assemblage. Impressions of extant medusae and Proterozoic circular impressions show general similarities in arrangement and position of radial and concentric structures and a central raised boss. However, annular rings and radial grooves are more numerous in the Proterozoic fossils and strongly folded or deformed fossils are rare as compared with impressions of modem medusae. Recent pennatulids yield impressions that are more deformed and irregular than the Proterozoic genus Charniodiscus. The greater frequency of deformation of most simulated fossil medusoids relative to Precambrian circular impressions implies that Proterozoic medu-soids were substantially stiffer than many modern taxa of comparable sizes. Many fossils with abundant circular rings have no constructional counterparts among the extant forms studied here and their medusoid affinities should remain in doubt. The structural simplicity of impressions of Ediacara organisms and extant cnidarians suggests that their mutual similarities may be due to convergence. However, there is no compelling morphological reason to reject the claim that some Proterozoic fossils may share affinities with living cnidarians. □ Taphonomy. Ediacara biota, cnidarians, phylogenetic relationships.     

古生物多样性统计中的偏差及其校正

地质历史时期生物多样性统计中的偏差是普遍存在的。文中以奥陶纪海洋生物和二叠纪腕足动物分异度为例介绍和讨论古生物多样性统计中的偏差问题。认为在研究某些时段分异度时不能依靠单一的种、属或科的数量来建立分异度模式,而需要用生物分异度、灭绝率和新生率等多种计算方法来综合分析。同时古生物分异度的研究受到化石记录的完整性、研究、保存和采集程度、时间段的不均一性等多种因素影响,因此,大多需要应用稀疏标准化法等对不同时段或化石群在同等量化标准的基础上进行比较和校正后,才能得到比较符合实际的分异度模式。     

A critique of the chronometric evidence for hominid fossils: I. Africa and the Near East 500-50 ka

The chronometric dating evidence for all hominid fossils from Africa and the Near East that have previously been dated to 500-50 ka is critically assessed using the concept of chronometric hygiene, and these dates are revised using Bayesian statistical analyses where possible. Sixteen relevant hominid sites lacking chronometric evidence are briefly discussed. Chronometric evidence from 37 sites is assessed in detail. The dates for many hominid fossils are poorly constrained, with a number dated by comparisons of faunal assemblages-a method that does not have good chronological resolution for much of the last million years. For sites with stratigraphic sequences of dates, it is generally possible to refine the dating, but in some cases, the revised chronology is less precise than previous chronologies. Fossils over 200 ka in age tend to be poorly dated, but for the last 200 kyr, dating is better due to the availability of electron-spin-resonance and thermoluminescence dating. Consideration of the chronologies favored by the proponents of the out-of-Africa and multiregional hypotheses of human evolution shows their selectivity. The chronological assessment of the fossils here is compatible with either hypothesis. If evolutionary schemes that do not rely on the morphology of the hominid fossils to decide the sequence of fossils are to be built, then further dating is required, alongside full publication of existing dates.     

A three-dimensionally preserved fossil polychaete worm from the Silurian of Herefordshire, England.

Polychaete body fossils are rare, and are almost invariably compressed and too poorly preserved for meaningful comparison with extant forms. We here describe Kenostrychus clementsi gen. et sp. nov. from the Silurian Herefordshire Konservat-Lagerstätte of England, in which three-dimensional external morphology is preserved with a fidelity unprecedented among fossil polychaetes. The fossils, which are preserved in calcite, were serially ground and photographed at 30 microm intervals to produce computer-generated reconstructions of the original external surface. The new genus has a generalized polychaete morphology with large biramous parapodia, unspecialized anterior segments and a small prostomium with median and lateral antennae and ventral prostomial palps. Cirriform branchiae arise from the ventral surface of each notopodium, and may be homologous with the inter-ramal branchiae of the extant nephtyids. Through cladistic analysis, Kenostrychus is interpreted as a member of a stem group of either the Phyllodocida or the Aciculata (Phyllodocida + Eunicida). Direct comparison with other fossil forms is difficult, but hints that inter-ramal respiratory structures may be primitive within the Phyllodocida and/or the Aciculata.     

Conical Thecae of Precambrian Macroorganisms

On the basis of new extensive collections, made by the present authors, a group of unsegmented three-dimensional fossil remains from the Late Precambrian of Namibia and the southeastern White Sea area, including such genera as Protechiurus, Vendoglossa, and Vendoconularia, were restudied. It is established that the fossils belong to two morphologically close genera (Protechiurus and Vendoconularia), composing the new family Protechiuridae. It is possible that they represent the same genus or even species, but the incompleteness of the preservation of the Namibian member of the group prevents their integration. The fossils initially represented conical hollow elastic objects, hexagonal in the cross-section, which were open at the wide end and closed at the pointed end. The sculpture of their surface consisted of longitudinal ridges, both single and double, bounding the rows of the short transverse ridges, scalloped along one margin. Vendoconularia had wide plates which were attached to the cone outwardly along the longitudinal unpaired ridges. In general morphology and several details the fossils resemble the thecae of conulariids and anabaritids, known from Late Precambrian and Paleozoic and classified as the scyphozoan cnidarians. The main difference lays in the theca mineralization, which consists of phosphate in conulariids, carbonate in anabaritids, and is absent in the case of protechiurids. It is proposed that the protechiurids, which appeared in the fossil record first, may belong to the basal group of ancient scyphozoans, ancestral to the conulariids on the one hand, and to the anabaritids on the other.     

Digital image treatment applied to ichnological analysis of marine core sediments

Characterization of trace fossils in marine core sediments is, most times, difficult due to the weak differentiation between biogenic structures and the host sediment, especially in pelagic and hemipelagic facies. This problem is accentuated where a high degree of bioturbation is associated with composite ichnofabrics. Simple methods are presented here based on modifications to image features such as contrast, brightness, vibrance, saturation, exposure, lightness, and color balance using the software Adobe Photoshop CS6 (Adobe Systems, San Jose, CA, USA) to enhance visibility and thus allow for a better identification of the trace fossils. Adjustments involving brightness, levels and vibrance generally give better results. This approach was applied to marine cores of pelagic and hemipelagic sediments obtained from the Integrated Ocean Drilling Program Expedition 339, Site U1385. Enhancing the digital images facilitates ichnological analysis through improving the visibility of weakly observed trace fossils, and in some cases revealing traces not detected previously.     

Proximal metatarsal articular surface shape and the evolution of a rigid lateral foot in hominins

This study quantifies the proximal articular surface shape of metatarsal (MT) 4 and MT 5 using three-dimensional morphometrics. Humans and apes are compared to test whether they have significantly different shapes that are skeletal correlates to comparative lateral foot function. In addition, shod and unshod humans are compared to test for significant differences in surface shape. The MT 4 fossils OH 8, Stw 628, and AL 333-160, and the MT 5 fossils AL 333-13, AL 333-78, OH 8, and Stw 114/115 are compared with humans and apes to assess whether they bear greater similarities to humans, which would imply a relatively stable lateral foot, or to apes, which would imply a flexible foot with a midfoot break. Apes have a convex curved MT 4 surface, and humans have a flat surface. The MT 4 fossils show greater similarity to unshod humans, suggesting a stable lateral foot. Unshod humans have a relatively flatter MT 4 surface compared with shod humans. There is much overlap in MT 5 shape between humans and apes, with more similarity between humans and Gorilla. The fossil MT 5 surfaces are generally flat, most similar to humans and Gorilla. Because of the high degree of shape overlap between humans and apes, one must use caution in interpreting lateral foot function from the proximal MT 5 surface alone.     

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.     

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.