贵州寒武系杷榔组杷榔化石库埋藏学初步研究———以剑河辣子寨剖面为例*

贵州剑河辣子寨村辣子寨剖面杷榔组的杷榔动物群是近期发现的一个化石丰度较高且较复杂多样的动物群。该产地的化石库含有两个化石富集的层位,即下化石层和上化石层。通过对两个化石富集层的岩石结构、构造、沉积特征、生物类群组成及化石丰度和保存特征等方面的分析研究表明,下化石层与上化石层内化石组合和埋藏特征存在较大差异。下化石层类群组成多样、化石丰度较高,以掘头虫类三叶虫、三叶形虫Naraoia、大型双瓣壳节肢动物Isoxys、Tuzoia及古蠕虫、海绵等为主,且化石主要保存在事件沉积层内,保存质量较好,为外陆棚沉积环境下大量生物个体发生原位快速沉积埋藏作用形成。而上化石层内化石以具硬壳的三叶虫为主,有较大的埋藏量,少量或碎片状的Naraoia,Isoxys,且这类化石数量少,主要在背景层中保存,可能为静水沉积时,沉积界面积累了大量生物遗体并暴露于基底表面一定时间后,再被沉积物逐渐堆积掩埋而形成,为生物遗壳的原位或近原位埋藏。     

山东莱阳晚白垩世鸭嘴龙动物群化石特异埋藏初步研究

近年来对位于莱阳棘鼻青岛龙发现地点(1号化石地点)以东新发现的2号化石地点进行大规模发掘,已发现5个化石富集层,赋存的化石均以鸭嘴龙科为主,一部分个体可能代表栉龙亚科的成员,而另一部分个体归于赖氏龙亚科的棘鼻青岛龙。这些鸭嘴龙化石从个体大小上分别代表成年、亚成年和幼年晚期个体。化石富集层以灰绿色和褐红色含砂砾泥岩沉积为主,具有典型的泥石流沉积特征和骨骼埋藏特征,并具有两种主要的死亡和埋藏模式,即恐龙群体活着时遭遇泥石流被吞没集群死亡后,肢体在短距离搬运中被撕裂并迅速沉积掩埋,骨骼分散但完整,同一个体的骨骼相距不远;恐龙正常或经泥石流导致其非正常死亡后,部分遗体未被掩埋,经暴露软组织腐烂,被后续发生的泥石流卷入并二次搬运,最终埋藏,表现为分散但较完整的骨骼与破碎呈砾石磨圆状的骨骼共生保存。这些富集层的化石及其埋藏特征反映了生活在河湖边的鸭嘴龙动物群,在鸭嘴龙幼年晚期刚刚加入成年鸭嘴龙动物群后,被卷入突发的洪水泥石流导致其集群死亡并快速埋藏的事件。     

贵州中寒武世凯里化石库的埋藏学和生态学初步研究*

在详细的地层剖面和化石标本野外观察和室内研究的基础上 ,初步分析了贵州中寒武世凯里化石库的埋藏环境、化石保存状态和保存方式、以及矿物成分。认为凯里化石库形成于较深水的陆棚环境 ,海底位于风暴浪基面之下。由于有频率较高的间歇性沉积事件的侵入 ,沉积速率高 ,海底附近的生物群落的发展经常被沉积事件所中断 ,因此 ,化石库内很少有群落较长时间演替而形成的多类别生物尸体堆积的硬底层。正常堆积的化石层通常由一些发育时间较短的群落演替序列构成。一些能量较强的沉积事件对一些逃跑能力差的生物或原位快速掩埋 (如始海百合类、水母状动物等 ) ,或被卷入沉积物流体内后被快速埋藏 (如浮游型的三叶虫、水母状动物等 ) ,为凯里化石库中特异化石的形成提供了先决条件。沉积事件普遍较弱是凯里化石库内特异化石种类单调的最重要原因。底内生物的缺乏、极细颗粒的粘土沉积物对封闭在泥内的生物的完整保存起到了保护作用。早期成岩矿物 (黄铁矿 )对阻止软体腐烂和复制软体组织起到了关键性的作用。     

贵州遵义松林寒武纪早期牛蹄塘生物群埋藏学特征初探

通过系统发掘发现遵义松林寒武纪早期牛蹄塘生物群自下而上存在3个化石富集层:锐虾Perspicaris富集层、海绵动物富集层和带有澄江生物群色彩的三叶虫、楔叶形虫Sphenothallus富集层。牛蹄塘生物群化石保存完整、分选性差、无明显的定向性、缺少软躯体化石和底内生物,结合岩性、地球化学、构造等特征综合认为牛蹄塘生物群是弱水动力作用下缓慢沉积的原地埋藏群落。牛蹄塘组沉积时期曾有过多次充氧事件,水体氧化时间的长短以及底层水氧化-还原界面的差异导致了不同的生物组合。     

埋藏群落

指被沉积物覆盖而保存下来的残体群落。同一残体群落中的成分不一定都能被沉积物所覆盖保存，其中有的在未被覆盖之前即已腐烂毁坏、搬运迁移或为其他生物所食，而不同残体群落的成分也可被搬运至同一地点埋藏保存。埋藏群落虽被沉积物覆盖，但仍然继续发生变化，没有硬体部分的生物及由易溶物质组成的外壳等常遭破坏而不能保存，故埋藏群落的内容远较残体群落单调而贫乏。埋藏群落可以分为未经搬运的原地埋藏群落及经过搬运的异地埋藏群落。经过化石作用而保存为化石的部分埋藏群即为化石群落。埋藏群落     

贵州寒武纪杷榔组古蠕虫类埋藏特征及意义*

贵州剑河寒武纪杷榔组辣子寨剖面含有丰度较高的古蠕虫类化石。化石鉴定为Wronascolex geyiensis, 主要保存在三个层位。在对三个层位的岩石、沉积特征、化石成因及埋藏特征探讨分析后, 证实W. geyiensis化石保存于快速的沉积事件导致的浊流沉积层内, 而化石埋藏的完美程度受到成岩过程中埋藏位置、沉积物中矿物结晶和充填方式的影响, 这些后期的成岩、成矿作用对先期形成的化石精细结构产生了直接和间接的破坏作用。本文通过对W. geyiensis化石体上黏土矿物、草莓状黄铁矿、自形–半自形黄铁矿的成因分析后, 获得最适合保存软躯体化石的环境应该为缺氧且生物体与孔隙水中的硫酸盐无接触的原地和近原地埋藏条件。     

贵州台江中寒武世凯里生物群的埋藏序列分析*

在贵州台江凯里组中发现一系列的风暴沉积韵律层 ,风暴韵律层明显地控制着早期后生动物的埋藏、保存。根据风暴韵律层与化石埋藏、保存特征 ,建立了化石埋藏序列A—D4层。A层为向上变细的、富产潜穴的泥质粉砂岩 ;B层为平行纹层发育的、层面上富产三叶虫化石Pagetia头尾分离的粉砂质泥岩、泥质粉砂岩 ;C层富产软体化石的具弱水平纹理的泥岩 ,凯里化石库就产于此层 ;D层为块状泥岩层 ,仅含少量保存完好的三叶虫化石。     

华南板块上扬子陆表海志留系远岸区韩家店组的生物沉积特征

上扬子区志留系兰多维列统特列奇阶下部韩家店组以杂色泥岩-粉砂岩、细砂岩为主,大化石含量偏少,局部层段发育碳酸盐岩薄层或透镜体。本文划分黔北道真巴渔剖面韩家店组生物沉积类型并确认环境指标。滇黔桂古陆以北远岸区韩家店组底栖生物类型与其赋存的岩相存在相关性,泥质粉砂岩层面上常见生物潜穴,腕足类Nucleospira被水动力搬运后密集堆积形成贝壳滩;原地生长保存的苔藓虫Fistulipora群落形成厚度约0.2 m的薄层生物层。薄层碳酸盐岩层微相序列显示底栖生物多样性组成,以细生屑颗粒占优势,化石类型包括床板珊瑚、皱壁珊瑚、苔藓虫、腕足类、三叶虫、介形类、海百合茎以及钙藻等,均为栖居于陆表海缓坡富氧海底的常见类型;出现少量极浅水灰泥坪、云坪和内碎屑颗粒滩环境。韩家店组生物沉积特征指示该区海底深度维持在最大浪基面之上,为基底稳定沉降与沉积补偿大致均衡的构造背景。在扬子上升后至早二叠世海进的漫长古陆阶段,韩家店组地层产状未受到强烈褶皱变形。     

辽西四合屯及周边地区义县组下部地层层序与脊椎动物化石层位

辽西四合屯发掘和野外考察表明,四合屯及周边地区义县组下部地层由三段组成：第一段冲积相粉碎屑岩;第二段中基性火山熔岩;第三段滨一浅湖、半深湖二深湖相砂岩和页岩构成2个正旋回．其中第三段有上、下2个脊椎动物化石层位：尖山沟上部脊椎动物化石层和四合屯下部脊椎动物化石层．在四合屯厚度gm的发掘剖面上肯定了10个含鱼层、1个含蛙层、3个含龟层、5个含恐龙层和5个含鸟层．四合屯以鸟类和爬行类为代表的化石群埋藏于厚度166cm,富集于厚度84cm的湖相页岩中,记录了佛罗一白垩纪界线附近重大生物演化事件和脊椎动物集群死亡事件．孔子鸟和中华龙鸟不存在厚度14m的上下层位关系．湖相层凝灰岩和重力流事件沉积,反映盆地曾经为一水域宽阔、火山活动频繁的泛盆地．     

贵州寒武纪杷榔动物群的古生态学初探*

贵州剑河辣子寨村辣子寨剖面杷榔组的杷榔动物群是近期发现的一个化石丰度较大的产地。目前已发现了海绵动物、刺细胞动物、曳鳃动物、腕足动物、软舌螺动物、节肢动物、棘皮动物、藻类等8大类群的化石代表,计有63属80种。本文依据辣子寨剖面杷榔化石库下化石层的生物组合及各类群的生态特性、埋藏特征,探讨杷榔动物群的生态系及生活环境。杷榔动物群的生物群落,包括了底栖固着、内栖钻孔、表栖游移、漂游和游泳的生物类群,且生物群生活在赤道地区以北开阔海盆的透光区至弱透光区、水域含氧、海底有氧—弱氧、盐分正常、快速沉积事件频发的陆棚较深水环境。其古生态特点为:生物种类丰富,具有滤食、食草、食泥、食肉和食腐的动物;而捕食-被捕食的食物网的关系较为复杂,具有明显的金字塔型的营养结构。     

Ichnology of deep‐sea fan overbank deposits of the Ganei slates (Eocene,Switzerland)— a classical flysch trace fossil locality studied first by Oswald Heer

Ganei (Switzerland) is a classical locality for trace fossils. At this site, Heer (1877) described a large number of trace fossils, several of which were new taxa. The trace fossils occur in thin‐bedded turbidites in which the basal divisions of the Bouma sequence are typically absent; the turbidites are assigned to the Ganei Slates and are Eocene in age. They are interpreted to have been deposited in an overbank environment within an upper to middle fan area distal to a channel. Two trace‐fossil associations occur: the first (I) is characterized by bulldozing organisms producing biodeformational structures, Scolica, and Nereites irregularis; the second (II) association shows a distinct tiering pattern with near‐surface graphoglyptids and a mixed layer with simple tubes such as cf. Palaeophycus and Planolites, plus patterned tubes such as Nereites cirrinalis, and Chondrites. Deeper turbidite layers were colonized by Chondrites and Gyro‐phyllites. All trace fossils show a normal size spectrum compared to previously studied trace‐fossil associations, so the degree of oxygenation probably did not influence the composition of either trace‐fossil association. Seafloor sediment was probably soft and did not affect the trace‐fossil associations. Sedimentation rate and event frequency did not change and are estimated to have been in a range of 5–10 cm/1000 years and 2–5 events per 1000 years, respectively. The composition of trace‐fossil associations I and II is therefore interpreted to have been controlled by the benthic food content being higher for trace‐fossil association I than for II.     

Soils,time, and primate paleoenvironments

Soils are the skin of the earth. From both poles to the equator, wherever rocks or sediment are exposed at the surface, soils are forming through the physical and chemical action of climate and living organisms. The physical attributes (color, texture, thickness) and chemical makeup of soils vary considerably, depending on the composition of the parent material and other variables: temperature, rainfall and soil moisture, vegetation, soil fauna, and the length of time that soil–forming processes have been at work. United States soil scientists¹ have classified modern soils into ten major groups and numerous subgroups, each reflecting the composition and architecture of the soils and, to some extent, the processes that led to their formation. The physical and chemical processes of soil formation have been active throughout geologic time; the organic processes have been active at least since the Ordovician.² Consequently, nearly all sedimentary rocks that were deposited in nonmarine settings and exposed to the elements contain a record of ancient, buried soils or paleosols. A sequence of these rocks, such as most ancient fluvial (stream) deposits, provides a record of soil paleoenvironments through time. Paleosols are also repositories of the fossils of organisms (body fossils) and the traces of those organisms burrowing, food–seeking, and dwelling activities (ichnofossils). Indeed, most fossil primates are found in paleosols. Careful study of ancient soils gives new, valuable insights into the correct temporal reconstruction of the primate fossil record and the nature of primate paleoenvironments.     

Taphonomy of dense ophiuroid accumulations from the Middle Triassic of Poland

The taphonomy of three Middle Triassic (Muschelkalk) monospecific ophiuroid taphocoenoses, comprising Aspiduriella similis (Eck), from different regions in Poland (Holy Cross Mountains, Upper Silesia and North-Sudetic Basin) has been investigated. The majority of specimens (88.5% of a total of 428 individuals) are partially disarticulated, having only proximal and median portions of their arms preserved (Taphonomic Group 2). Pristine specimens, with only faint traces of disarticulation (Taphonomic Group 1), as well as those that preserve the disc only, or which have only proximal stumps of arms preserved (Taphonomic Group 3), are much rarer (6.5% and 5%, respectively). Moreover, most specimens (76.4%) are oral side up. Only 19.2% of specimens are preserved in life position, and a small fraction (4.4%) are preserved oblique to bedding. All ophiuroids studied occur in thin, pelitic layers devoid of any trace fossils. Associated body fossils, such as bivalves, gastropods or crinoids, are very rare. Taking into account ophiuroid taphonomy, as well as sedimentological characteristics of the thin layer (burial layer), it is clear that all assemblages were transported prior to burial. The predominance of articulated skeletons indicates that the burial event (obrution event) was not only rapid, but also single – subsequent events would have destroyed the previously buried ophiuroids. Storm-related resuspension of fine-grained material from nearshore environments which covered the ophiuroids is the most likely burial agent. The fact that in all regions the ophiuroid taphofacies studied is identical suggests that the three assemblages underwent the same burial history, and that a single event might have been responsible.     Muschelkalk , ophiuroids , Poland , taphonomy , Triassic .     

山西兴县中奥陶统马家沟组五5亚段遗迹化石组合及其古环境意义

鄂尔多斯盆地东缘关家崖剖面奥陶系马家沟组五5亚段地层中保存大量无脊椎动物遗迹化石,可识别出居住迹、牧食迹、觅食迹三大类,共7个遗迹属、14个遗迹种,分别是Arenicolites isp.,Cylindricumisp.,Helminthopsis abeli,Helminthopsis isp.1,Helminthopsis isp.2,Lorenziniaisp.,Planolites isp.,Planolites beverleyensis,Planolites montanus,Teichichnus rectus,Thalassinoides isp.1,Thalassinoides isp.2,Thalassinoides isp.3,Thalassinoides isp.4。根据遗迹化石特征及其沉积环境,可划分3个遗迹组合:1、Helminthopsis-Planolites组合,该组合以觅食迹为主,部分为居住迹,出现在潮间带和潮下带;2、Thalassinoides-Teichichnus组合,该组合遗迹化石种类较单一,主要为觅食迹,分布于潮间带和潮上带;3、Thalassinoides-Helminthopsis组合,该组合出现在潮间带。基于遗迹化石组合的分布特点,提出山西兴县奥陶系马家沟组五5亚段的沉积环境模式。     

内蒙古海拉尔盆地南部地区晚白垩世青元岗组介形类

内蒙古海拉尔盆地南部探井青元岗组红色碎屑岩中发现较丰富的介形类化石,即下段的Ahanicypris obesa-Talicypridea triangulata组合和上段的Chinocypridea augusta-Talicypridea qingyuangangensis组合,这些介形类化石地方性色彩强烈,表现为以具冠状壳喙类型的Ahanicypris,Talicypridea,Chinocypridea和网纹发育的Harbinia等属繁盛为特征,反映了中国东北地区晚白垩世晚期介形类动物群面貌。根据介形类化石组合特征及其分布规律,可以与松辽盆地晚白垩世四方台组的介形类动物群对比,时代为晚白垩世Maastrichtian期。     

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.     

Taphonomic windows and molluscan preservation

Recent studies on silicified fossil biotas have suggested that substantial skewing of the molluscan record resulted from early aragonite dissolution in mid-outer carbonate ramp settings. If those rare skeletal lagerstätten are representative, then the quality and completeness of the molluscan record are thrown into doubt. Yet database studies suggest that the bivalve fossil record is actually relatively complete. If so, then biodiversity must be captured by other processes that preserved shells vulnerable to early dissolution, and which operated on a relatively high frequency, i.e., less than the species duration for bivalves.Storm beds, shell plasters and submarine hardgrounds are identified as fossil deposits that can preserve the labile aragonitic component of the fauna and thus represent potential taphonomic windows. Many storm event beds include rich accumulations of shelly benthos. Differences between storm bed faunas and those of the background facies could reflect transportation effects. However, some storm bed assemblages are rich in originally aragonitic infaunal bivalves that are not represented in background facies or more proximal shelf equivalents, and here rapid burial and removal of organic matter by winnowing may be the keys to aragonite shell preservation. Despite Palaeozoic to Cenozoic changes in the thickness and frequency of shell beds that reflect the predominant bioclast producers, shallow infaunas are commonly concentrated together with epifauna in such deposits.Some low energy, organic-rich mud-dominated settings are associated with preservation of aragonitic molluscs. Infaunal bivalves are a prominent component of shell plasters or pavements in such settings, linked to episodic bottom water anoxia. Decaying algal blooms drew the redox boundary up above the sediment–water interface, and brought populations of infaunal bivalves to the surface where they died. Isolated from the oxic taphonomically active zone, the shells were not dissolved and were buried as thin shell layers. In similar settings, aragonitic shells were preserved as moulds through early pyritisation, or even through preservation of original shell aragonite.In oxic environments, bioturbational reworking of surface sediment destroyed moulds of aragonitic shells after early dissolution. In some hardgrounds, these delicate moulds were preserved due to synsedimentary cementation, probably using carbonate released by aragonite dissolution. The examples included here come from both intervals of “calcite” and “aragonite” seas, and it is not possible to assess whether the saturation state (with respect to aragonite) of the ambient sea water played a role in the selective removal of aragonitic shells.While taphonomic windows may have captured the diversity of individual groups, it is clear from quantitative data involving skeletal lagerstätten that the scale of loss from early aragonite dissolution has drastically altered the trophic composition of some fossil assemblages commonly used as the basis for reconstructions of past communities.     

Trace and soft body fossils from the Pedroche Formation (Ovetian, Lower Cambrian of the Sierra de Córdoba, S Spain) and their relation to the Pedroche event

The low Lower Cambrian rocks from the Sierra de Córdoba contain one of the best successions in Europe, which consists of well exposed mixed facies with abundant fossil assemblages showing long stratigraphical ranges throughout the Pedroche Formation. These assemblages include diverse Ovetian archaeocyaths, trilobites, small shelly fossils, calcimicrobes, trace fossils and stromatolites. Trace fossils are still poorly known, and thus they are the main objective of this work. Ichnological data are obtained from the Arroyo de Pedroche 1, Arroyo de Pedroche 2 and Puente de Hierro sections. Trace fossils include the ichnogenera Bergaueria, aff. Bilinichnus, Cochlichnus, aff. Cosmorhaphe?, Cylindrichnus, Dactyloidites, Dimorphichnus, Diplichnites, Monocraterion, Palaeophycus, aff. Phycodes, Planolites, Psammichnites, Rusophycus, Skolithos, Torrowangea and Treptichnus, as well as faecal pellets, meniscate trace fossils and others. They are abundant in shales and sandstones, and indicate important changes in the benthic conditions with respect to the underlying Torreárboles Formation. Changes in fossil assemblages within Member I of the Pedroche Formation indicate palaeoecological disruptions, which led to the disappearance of numerous archaeocyath species and the decrease of stromatolite biodiversity. This was followed by dominance of trilobite and brachiopod assemblages, accompanied by trace fossils of the Psammichnites ichnosp. A ichnoassociation. This biotic turnover (Pedroche event) occurred at the lower part of the archaeocyath Zone III, within the Bigotina bivallata biozone. The diagnoses of the ichnospecies Cochlichnus anguineus and Dactyloidites cabanasi are emended.     

Ediacaran microbial colonies

Enigmatic discoidal fossils are common in Neoproterozoic sedimentary sequences and in the stratigraphic record pre-date the first appearance of diverse Ediacaran fossil assemblages. Termed 'medusoids', these Neoproterozoic discoidal fossils have generally been interpreted as coelenterate-grade organisms implying a radially symmetrical body plan for ancestral eumetazoans. Analysis of exceptionally preserved discoidal fossils from the White Sea area, however, indicates that most of these discoidal forms represent colonial microbes. Localized pyritization, for example, reveals the presence of a conspicuous filamentous substructure in Ediacaria , whereas concentric rings, radial sectors and central structures in Cyclomedusa and Paliella compare directly with Recent microbial colonies growing in a nutritionally heterogeneous environment. At least some Ediacaran discoids can be compared with extant concentric ring-shaped microbial colonies that grow in hypersaline microbial mats. Insofar as most of the remaining record of Ediacaran discoids can be attributed to the holdfast structures of non-radiate modular organisms, there is no support from the fossil record for identifying a radiate ancestry for the Metazoa.     

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.