A new type of Precambrian megascopic fossils: the Jinxian biota from northeastern China

Precambrian fossils are crucial for our understanding of the evolution of early organisms. Megascopic body fossils are more important because they potentially represent macroorganisms. However, the Precambrian fossil record is sparse and dominated by microfossils and microbial structures. Here we show a new type of megascopic fossils recovered from the Xingmincun Formation (probably Neoproterozoic age), northeastern China. The specimens are flat, flexible (easily corrugated) and discoidal in outline. Concentric or spiral ridges are preserved on both sides. Petrographical thin section examination indicates that the specimen consist of a thin layer of microcrystalline quartz grains (about 20–30 μm thick) wrapped by an outer sheath, composed primarily of chlorites. Field Emission Scanning Electron Microscopy (FE-SEM) coupled with an x-ray energy dispersive spectrometer system (EDX) analysis shows microstructures and relative element abundance of the fossils, but contributes little in solving their biological affinities. The fossils have previously been linked to discoidal impressions of the Ediacara biota. Close examination on new materials indicates that they are radically different from either the Ediacara impressions or any other Precambrian megascopic remains. Concentric or spiral ridges may result from rhythmic growth and the presence of twin specimens may suggest that the organisms undergo asexual reproduction or inhibition of growth in one direction. Referring them to any known fossil or living group has proved to be difficult. We conclude that they represent a distinct group of Precambrian megascopic organisms regardless of their affinities remaining problematic.     

The first named Ediacaran body fossil, Aspidella Terranovica

Aspidella terranovica Billings, 1872 was first described from the late Neoproterozoic Fermeuse Formation (St. John's Group) on the Avalon Peninsula of eastern Newfoundland, approximately 1km stratigraphically above the famous Ediacaran biota at Mistaken Point, and several kilometres below the base of the Cambrian. Aspidella has been reinterpreted perhaps more than any other Precambrian taxon, and has variously been regarded as a fossil mollusc or ‘medusoid’, a gas escape structure, a concretion, or a mechanical suction mark. Our studies indicate that Aspidella includes a wide variety of preservational morphs varying from negative hyporeliefs with a raised rim and ridges radiating from a slit (Aspidella‐type preservation), to flat discs with a central boss and sharp outer ring (Spriggia preservation), to positive hyporeliefs with concentric ornamentation (Ediacaria preservation). Specimens occur in a continuum of sizes, with preservational styles dependent on the size of the specimen and the grain size of the host lithology; the elongation of specimens is tectonic. Aspidella is confirmed as a body fossil from observations of complex radial and concentric ornamentation, mutually deformed borders in clusters of specimens, and occurrence on the same bedding planes as certain distinctive Ediacaran taxa. Aspidella is indistinguishable from, and has priority over, several of the most common genera of late Neoproterozoic discoidal body fossils worldwide. Similar fossils from Australia are interpreted as holdfasts of frond‐like organisms. The density of specimens in the Aspidella beds suggests levels of benthic biomass in the Neoproterozoic that could rival those of modern marine communities. The serial growth forms, PalaeopascichnusIntrites, Neonereites renariusYelovichnus, associated with Aspidella, are interpreted as body fossils of unknown affinities rather than trace fossils. A new, trilobed, Ediacaran body fossil, Triforillonia costellae gen. et sp. nov., is described from the Aspidella beds of the Fermeuse Formation.     

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.     

湖北三峡地区埃迪卡拉系灯影组“蝌蚪状”遗迹化石*

遗迹化石是埃迪卡拉纪存在两侧对称动物最有力的证据。但多数埃迪卡拉纪遗迹化石为简单、水平的表面爬迹或潜穴。在湖北三峡地区灯影组石板滩段含典型埃迪卡拉软躯体化石的地层中新发现一类形态特别的化石,呈蝌蚪状,一端膨大,一端细管状。通过对化石形态、同位素分析以及沉积学特征的研究,说明该化石为遗迹化石,而不是实体化石。蝌蚪状化石为一种复合迹,垂向活动形成球状的膨大端,平行藻席层活动形成近于水平的潜穴,反映了造迹生物垂向切穿藻席层并沿藻席层进行觅食的行为。该发现说明了在埃迪卡拉纪晚期已有两侧对称动物开始形成较为复杂的潜穴。     

Restudy of the worm-like carbonaceous compression fossils Protoarenicola,Pararenicola, and Sinosabellidites from early Neoproterozoic successions in North China

The carbonaceous compression fossils Protoarenicola baiguashanensis Wang, 1982, Pararenicola huaiyuanensis Wang, 1982, and Sinosabellidites huainanensis Zheng, 1980, from the early Neoproterozoic Liulaobei and Jiuliqiao formations in northern Anhui, North China, were previously interpreted as worm-like metazoans, largely on the basis of transverse annulations and purported proboscis structures. If correct, these would be some of the earliest known bilaterian animals and would provide a key paleontological calibration to molecular clock analyses. In this study, we examine a large population of these carbonaceous fossils, clarify their taxonomy, and provide new insights into their morphological, paleoecological, and phylogenetic interpretations.Although all three species are characterized by annulated tubes, P. baiguashanensis bears a bulbous terminal structure at one end of its tube. P. huaiyuanensis is characterized by a constricted opening at one end and a closed termination at the other. The two ends of S. huainanensis tubes are both closed and round. The bulbous terminal structure in P. baiguashanensis was previously interpreted as an animal proboscis, but new observations suggest that it was more likely a holdfast structure analogous to discoidal holdfast structures of the Mesoproterozoic Tawuia-like fossil Radhakrishnania Kumar, 2001, and the frondose Ediacara fossil Charniodiscus Ford, 1958. Furthermore, it is possible that at least P. baiguashanensis and P. huaiyuanensis may represent reproductive or taphonomic fragments of the same organism. This reinterpretation weakens the previous interpretation that P. baiguashanensis and P. huaiyuanensis were worm-like bilaterian animals. Instead, they can be alternatively interpreted as erect epibenthic organisms, possibly coenocytic algae reaching a tiering height of 30 mm. The predominance of discoidal holdfasts, as opposed to rhizoidal holdfasts, in pre-Ediacaran epibenthic organisms was probably related to more stable substrates in the presence of microbial mats and in the absence of bioturbating animals.     

Microbial biofilms and the preservation of the Ediacara biota

Laflamme, M., Schiffbauer, J.D., Narbonne, G.M., & Briggs, D.E.G. 2011: Microbial biofilms and the preservation of the Ediacara biota. Lethaia, Vol. 44, pp. 203–213. The terminal Neoproterozoic Ediacaran Period is typified by the Ediacara biota (ca. 579–542 Ma), which includes the first morphologically complex macroscopic organisms. Both the taphonomic setting that promoted the preservation of the soft‐bodied Ediacara biota in coarse‐grained sediments, and the influence of associated microbial coatings on this process, have generated debate. Specimens of Ediacaran discs (Aspidella) from the Fermeuse Formation of Newfoundland, Canada, were analysed using environmental scanning electron microscopy (ESEM) and focused ion beam electron microscopy (FIB‐EM) to determine the relationship between the fossil specimens and the surrounding sediment. The presence of chemically distinct (Al–Mg–Fe–K‐ and to a lesser extent S‐rich), finer‐grained sediment (with organized iron sulphides) surrounding the upper and lower margins of the Ediacaran fossils is consistent with elemental analyses of well preserved bacterial biofilms from other localities. ESEM analyses reveal a contrast in the composition of the sediment bound within the discs, which contains a higher concentration of Al, Ca and K, and the purer Si‐rich sediment that forms the surrounding matrix. This suggests that the coarse grained sediment was incorporated into the organism during life. Ediacaran discs were likely surrounded by a bacterial biofilm or thin microbial mat composed primarily of extracellular polymeric substances (or exopolysaccharide) during life, which added structural stability to these frond holdfasts, and facilitated their fossilization. Microbially mediated preservation in Fermeuse‐style Ediacaran taphonomy provides an explanation for the dominance of Aspidella holdfasts in these settings, and suggests that preservation of Ediacaran fossils in the round may be much more prevalent than previously recognized. We suggest that the overwhelming dominance of circular to bulbous forms such as Aspidella in Ediacaran biotas around the world is a direct result of the interplay between microbial ecology and microbially mediated taphonomy. □Aspidella, Ediacaran preservation, environmental scanning electron microscopy, focused ion beam electron microscopy, palaeoecology, taphonomic bias.     

Distinguishing geology from biology in the Ediacaran Doushantuo biota relaxes constraints on the timing of the origin of bilaterians

The Ediacaran Doushantuo biota has yielded fossils that include the oldest widely accepted record of the animal evolutionary lineage, as well as specimens with alleged bilaterian affinity. However, these systematic interpretations are contingent on the presence of key biological structures that have been reinterpreted by some workers as artefacts of diagenetic mineralization. On the basis of chemistry and crystallographic fabric, we characterize and discriminate phases of mineralization that reflect: (i) replication of original biological structure, and (ii) void-filling diagenetic mineralization. The results indicate that all fossils from the Doushantuo assemblage preserve a complex mélange of mineral phases, even where subcellular anatomy appears to be preserved. The findings allow these phases to be distinguished in more controversial fossils, facilitating a critical re-evaluation of the Doushantuo fossil assemblage and its implications as an archive of Ediacaran animal diversity. We find that putative subcellular structures exhibit fabrics consistent with preservation of original morphology. Cells in later developmental stages are not in original configuration and are therefore uninformative concerning gastrulation. Key structures used to identify Doushantuo bilaterians can be dismissed as late diagenetic artefacts. Therefore, when diagenetic mineralization is considered, there is no convincing evidence for bilaterians in the Doushantuo assemblage.     

EDIACARAN AND CAMBRIAN INDEX FOSSILS FROM SONORA, MEXICO

Abstract:  The Cambrian index fossil Treptichnus pedum is reported from the Puerto Blanco Formation near Pitiquito, Sonora, Mexico, and new occurrences of the Neoproterozoic index fossil Cloudina are reported from the underlying La Ciénega Formation. Considered together, these fossils constrain the location of the Ediacaran/Cambrian boundary in Mexico. The suite of fossils in this sequence is important because it provides an independent biostratigraphic datum for constraining the position of Laurentia during the onset of metazoan diversification, for testing proposed lithostratigraphic correlations among western North American Neoproterozoic–Cambrian successions, and for testing regional tectonic paradigms such as the Mojave-Sonora Megashear hypothesis.     

The role of iron in the formation of Ediacaran ‘death masks’

The Ediacara biota are an enigmatic group of Neoproterozoic soft-bodied fossils that mark the first major radiation of complex eukaryotic and macroscopic life. These fossils are thought to have been preserved via pyritic “death masks” mediated by seafloor microbial mats, though little about the chemical constraints of this preservational pathway is known, in particular surrounding the role of bioavailable iron in death mask formation and preservational fidelity. In this study, we perform decay experiments on both diploblastic and triploblastic animals under a range of simulated sedimentary iron concentrations, in order to characterize the role of iron in the preservation of Ediacaran organisms. After 28 days of decay, we demonstrate the first convincing “death masks” produced under experimental laboratory conditions composed of iron sulfide and probable oxide veneers. Moreover, our results demonstrate that the abundance of iron in experiments is not the sole control on death mask formation, but also tissue histology and the availability of nucleation sites. This illustrates that Ediacaran preservation via microbial death masks need not be a “perfect storm” of paleoenvironmental porewater and sediment chemistry, but instead can occur under a range of conditions.     

Infaunal augurs of the Cambrian explosion: An Ediacaran trace fossil assemblage from Nevada,USA

Morphologically complex trace fossils, recording the infaunal activities of bilaterian animals, are common in Phanerozoic successions but rare in the Ediacaran fossil record. Here, we describe a trace fossil assemblage from the lower Dunfee Member of the Deep Spring Formation at Mount Dunfee (Nevada, USA), over 500 m below the Ediacaran–Cambrian boundary. Although millimetric in scale and largely not fabric‐disruptive, the Dunfee assemblage includes complex and sediment‐penetrative trace fossil morphologies that are characteristic of Cambrian deposits. The Dunfee assemblage records one of the oldest documented instances of sediment‐penetrative infaunalization, corroborating previous molecular, ichnologic, and paleoecological data suggesting that crown‐group bilaterians and bilaterian‐style ecologies were present in late Ediacaran shallow marine ecosystems. Moreover, Dunfee trace fossils co‐occur with classic upper Ediacaran tubular body fossils in multiple horizons, indicating that Ediacaran infauna and epifauna coexisted and likely formed stable ecosystems.     

Effaced preservation in the Ediacara biota and its implications for the early macrofossil record

Abstract: Ediacaran structures known as ‘pizza discs’ or Ivesheadia have long been considered enigmatic. They are amongst the oldest known members of the Ediacara biota, apparently restricted to the Avalonian successions of Newfoundland and the UK, c. 579–560 Ma. Here, we suggest that these impressions are taphomorphs, resulting from the post‐mortem decay of the frondose Ediacaran biota. Ediacaran fossils range from well‐preserved, high‐fidelity variants to almost completely effaced specimens. The effaced specimens are inferred to have undergone modification of their original morphology by post‐mortem microbial decay on the sea floor, combined with sediment trapping and binding. In this style of preservation, morphological details within the organism became variously subdued as a function of the extent of organic decay prior to casting by overlying sediments. Decay and effacement were progressive in nature, producing a continuum of grades of preservation on Ediacaran bedding planes. Fossils preserved by such ‘effaced preservation’ are those that have suffered these processes to the extent that only their gross form can be determined. We suggest that the lack of detailed morphology in effaced specimens renders such fossils unsuitable for use as type material, as it is possible that several taxa may, upon degradation and burial, generate similar morphological taphomorphs. We here reinterpret the genus Ivesheadia as a taphomorph resulting from extensive post‐mortem decay of frondose organisms. Blackbrookia, Pseudovendia and Shepshedia from beds of comparable age in England are likewise regarded as taphomorphs broadly related to Charnia or Charniodiscus spp. To reflect the suggestion that such impressions are likely to be taphomorphs, and not taxonomically discrete, we propose the term ivesheadiomorphs to incorporate all such effaced taphonomic expressions of Ediacaran macrofossil taxa in Avalonian assemblages. Our recognition of effaced preservation has significant implications for Ediacaran taxonomy, and consequently for measures of Ediacaran diversity and disparity. It is implied that Avalonian assemblages preserve both organisms that were alive and organisms that were already dead at the time of burial. As such, the fossil assemblages cannot be taken to represent census populations of living organisms, as in prior interpretations.     

Oldest fossil ciliates from the Cryogenian glacial interlude reinterpreted as possible red algal spores

The Cryogenian Period experienced two long lived global glaciations known as Snowball Earths. While these events were dramatic, eukaryotic life persisted through them, and fossil evidence shows that eukaryotes thrived during the c. 30-million-year interlude between the glaciations. Carbonate successions have become an important taphonomic window for this interval. One of the most notable examples is the c. 662–635 Ma Taishir Formation (Tsagaan Olom Group, Zavkhan Terrane, Mongolia) which has yielded a number of eukaryotic fossil taxa. Here, we examine more closely the morphology and taxonomic affinity of some of these Taishir fossils previously interpreted as remains of ciliate tintinnid loricae (purportedly the oldest fossil ciliates). New morphological and ultrastructural analyses indicate that these fossils are not ciliate tintinnids. Instead, we propose a new interpretation: that they are algal reproductive structures related to coeval macroscopic organic warty sheets described as putative red algae. We report the first occurrence of these fossils in the earliest Ediacaran Ol Formation, indicating that this taxon persisted through the Marinoan Snowball Earth. A new interpretation of these fossils as putative red algal spores has broad implications for our understanding of biodiversity in the Neoproterozoic Era, specifically during the Cryogenian Period, and for the antiquity of ciliates.     

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.     

A fungal analog for newfoundland ediacaran fossils?

We propose that some of the more conspicuous Ediacaran fossilsfrom the Avalon Peninsula of Newfoundland, including Aspidella,Charnia, and Charniodiscus, were biologically similar to membersof the Kingdom Fungi. These organisms were multicellular ormultinuclear, lived below the photic zone, could not move ordefoul themselves, did not exhibit taphonomic shrinkage, andwere not transported or moved. Aspidella, in particular, appearsto exhibit indeterminate growth without a maximum size constraint,and appears to show growth zonations similar to modern mycelia.Other fossils from this deposit exhibit a fractal-like growthpattern. Together, these features falsify algal, lichen, andmetazoan interpretations of these fossils, yet reflect characteristicsof modern fungal mycelia. We emphasize that although no MistakenPoint fossil appears to be a metazoan, not all of the MistakenPoint taxa, and not all of the Ediacaran organisms in general,can reasonably be interpreted using a fungal analogy. Furthermore,the hypothesis that these fossils were functionally fungus-likeneed not imply that the organisms were members of the crown-groupFungi. We propose further tests for evaluating both this functionalhypothesis and the phylogenetic hypothesis that these organismswere members of the total-group Fungi.     

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.     

Integrin and cadherin clusters: A robust way to organize adhesions for cell mechanics

The evolutionary emergence of animals is one of the most significant episodes in the history of life, but its timing remains poorly constrained. Molecular clocks estimate that animals originated and began diversifying over 100 million years before the first definitive metazoan fossil evidence in the Cambrian. However, closer inspection reveals that clock estimates and the fossil record are less divergent than is often claimed. Modern clock analyses do not predict the presence of the crown‐representatives of most animal phyla in the Neoproterozoic. Furthermore, despite challenges provided by incomplete preservation, a paucity of phylogenetically informative characters, and uncertain expectations of the anatomy of early animals, a number of Neoproterozoic fossils can reasonably be interpreted as metazoans. A considerable discrepancy remains, but much of this can be explained by the limited preservation potential of early metazoans and the difficulties associated with their identification in the fossil record. Critical assessment of both records may permit better resolution of the tempo and mode of early animal evolution.     

The earliest fossil record of the animals and its significance

The fossil record of the earliest animals has been enlivened in recent years by a series of spectacular discoveries, including embryos, from the Ediacaran to the Cambrian, but many issues, not least of dating and interpretation, remain controversial. In particular, aspects of taphonomy of the earliest fossils require careful consideration before pronouncements about their affinities. Nevertheless, a reasonable case can now be made for the extension of the fossil record of at least basal animals (sponges and perhaps cnidarians) to a period of time significantly before the beginning of the Cambrian. The Cambrian explosion itself still seems to represent the arrival of the bilaterians, and many new fossils in recent years have added significant data on the origin of the three major bilaterian clades. Why animals appear so late in the fossil record is still unclear, but the recent trend to embrace rising oxygen levels as being the proximate cause remains unproven and may even involve a degree of circularity.     

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 origin of the animals and a ‘Savannah’ hypothesis for early bilaterian evolution

The earliest evolution of the animals remains a taxing biological problem, as all extant clades are highly derived and the fossil record is not usually considered to be helpful. The rise of the bilaterian animals recorded in the fossil record, commonly known as the ‘Cambrian explosion’, is one of the most significant moments in evolutionary history, and was an event that transformed first marine and then terrestrial environments. We review the phylogeny of early animals and other opisthokonts, and the affinities of the earliest large complex fossils, the so‐called ‘Ediacaran’ taxa. We conclude, based on a variety of lines of evidence, that their affinities most likely lie in various stem groups to large metazoan groupings; a new grouping, the Apoikozoa, is erected to encompass Metazoa and Choanoflagellata. The earliest reasonable fossil evidence for total‐group bilaterians comes from undisputed complex trace fossils that are younger than about 560 Ma, and these diversify greatly as the Ediacaran–Cambrian boundary is crossed a few million years later. It is generally considered that as the bilaterians diversified after this time, their burrowing behaviour destroyed the cyanobacterial mat‐dominated substrates that the enigmatic Ediacaran taxa were associated with, the so‐called ‘Cambrian substrate revolution’, leading to the loss of almost all Ediacara‐aspect diversity in the Cambrian. Why, though, did the energetically expensive and functionally complex burrowing mode of life so typical of later bilaterians arise? Here we propose a much more positive relationship between late‐Ediacaran ecologies and the rise of the bilaterians, with the largely static Ediacaran taxa acting as points of concentration of organic matter both above and below the sediment surface. The breaking of the uniformity of organic carbon availability would have signalled a decisive shift away from the essentially static and monotonous earlier Ediacaran world into the dynamic and burrowing world of the Cambrian. The Ediacaran biota thus played an enabling role in bilaterian evolution similar to that proposed for the Savannah environment for human evolution and bipedality. Rather than being obliterated by the rise of the bilaterians, the subtle remnants of Ediacara‐style taxa within the Cambrian suggest that they remained significant components of Phanerozoic communities, even though at some point their enabling role for bilaterian evolution was presumably taken over by bilaterians or other metazoans. Bilaterian evolution was thus an essentially benthic event that only later impacted the planktonic environment and the style of organic export to the sea floor.     

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。