Re‐evaluation of the conodont Iapetognathus and implications for the base of the Ordovician System GSSP

Terfelt, F, Bagnoli, G. & Stouge, S. 2011: Re‐evaluation of the conodont Iapetognathus and implications for the base of the Ordovician System GSSP. Lethaia, Vol. 45, pp. 227–237. In 2000, the International Union of Geological Sciences (IUGS) ratified the decision from the International Working Group on the Cambrian–Ordovician Boundary (COBWG) to place the Global boundary Stratotype Section and Point (GSSP) for the base of the Ordovician System in the Green Point section, Newfoundland, Canada, at a point coinciding with the first appearance of the conodont Iapetognathus fluctivagus. However, a restudy of the conodont successions from Green Point shows that this species is not present at the boundary interval, and as a consequence the section does not fulfil the biostratigraphical requirements of a GSSP. The GSSP horizon as now defined is based on a level part‐way through the range of I. preaengensis– a species with lower first appearance datum (FAD). The true FAD of I. fluctivagus is above the FAD of planktonic graptolites and well above the FAD of I. preaengensis. As a consequence of these problems, a restudy of the GSSP section and the other sections in the Cow Head Group is necessary. A redefinition of the GSSP horizon is suggested. The following four alternative horizons have potential as new horizons for the GSSP level: the FAD of Cordylodus intermedius; the FAD of Cordylodus andresi; the FAD of Eoconodontus notchpeakensis; and the FAD of the agnostoid Lotagnostus americanus. □Boundary, Cambrian, conodont, Global boundary Stratotype Section Point, Iapetognathus, Ordovician.     

Status of Divisions of the International Geologic Time Scale

Each chronostratigraphic unit of the International Geologic Time Scale will be defined at its base by a Global Stratotype Section and Point (GSSP) or Global Standard Stratigraphic Age (GSSA). Nearly 50 GSSPs and 10 GSSAs have now been ratified. Ideally, the GSSP coincides with events having a global correlation potential. The international stage divisions of some systems, such as the Jurassic or Neogene, are similar to traditional usage in European geology. However, in order to utilize global correlation horizons, the international stage divisions of other systems, such as the Ordovician or Permian, have required assembling new stage nomenclatures or hybrids of different regional stages. A reference table by the International Commission on Stratigraphy itemizes the current or potential GSSP and GSSA definitions of all international geologic time units.     

Global Standard Stratotype-section and Point (GSSP) of the Furongian Series and Paibian Stage (Cambrian)

The Global Standard Stratotype-section and Point (GSSP) of the Furongian Series (uppermost series of the Cambrian System) and the Paibian Stage (lowermost stage of the Furongian Series), has been recently defined and ratified by the International Union of Geological Sciences (IUGS). The boundary stratotype is 369 metres above the base of the Huaqiao Formation in the Paibi section, northwestern Hunan Province, China. This point coincides with the first appearance of the cosmopolitan agnostoid trilobite Glyptagnostus reticulatus, and occurs near the base of a large positive carbon isotopic excursion (SPICE excursion).     

A proposed new global stratotype for Aeronian Stage of the Silurian System: Hlásná Třebaň section,Czech Republic

The current Global Stratotype Section and Point (GSSP ) for the Aeronian Stage (Llandovery Series, Silurian System), on Trefawr track in the Llandovery area of Wales, is an inadequate marker for precise, global, correlation. The International Subcommission on Silurian Stratigraphy has, therefore, undertaken the selection of a new GSSP for this level. The lowest occurrence of the graptolite Demirastrites triangulatus , 1.38 m above the base of the black‐shale succession of the ?elkovice Formation at the Hlásná T?ebaň section in Central Bohemia, is proposed to mark the base of the Aeronian Stage. The section, which fulfils all formal requirements for stratotype of a chronostratigraphical unit, should be considered as a candidate for the new GSSP . An abundant, well‐preserved, diverse graptolite fauna occurs through the section along with common chitinozoans, which indicate that the Spinachitina maennili Biozone spans the boundary interval. The section comprises the lower–middle Aeronian (D. triangulatus–Lituigraptus convolutus graptolite biozones) along with underlying Rhuddanian (Akidograptus ascensus–Coronograptus cyphus biozones) and Hirnantian strata. Several graptolite genera of primary importance in global correlation (Demirastrites , Petalolithus , Rastrites and Campograptus ) first appear in the lower part of the triangulatus Biozone. The structurally simple section is somewhat condensed, but there is a uniform succession of black shale without any evidence of disconformity in the broad boundary interval. The C_org isotope record exhibits a minor positive excursion just above the base of the triangulatus Biozone, whereas TOC and N isotope and elemental geochemical records provide evidence for uninterrupted sedimentation under stable, anoxic conditions.     

MACROEVOLUTION AND MACROECOLOGY THROUGH DEEP TIME

Abstract:  The fossil record documents two mutually exclusive macroevolutionary modes separated by the transitional Ediacaran Period. Despite the early appearance of crown eukaryotes and an at least partially oxygenated atmosphere, the pre-Ediacaran biosphere was populated almost exclusively by microscopic organisms exhibiting low diversity, no biogeographical partitioning and profound morphological/evolutionary stasis. By contrast, the post-Ediacaran biosphere is characterized by large diverse organisms, bioprovinciality and conspicuously dynamic macroevolution. The difference can be understood in terms of the unique escalatory coevolution accompanying the early Ediacaran introduction of eumetazoans, followed by their early Cambrian (Tommotian) expansion into the pelagic realm. Eumetazoans reinvented the rules of macroecology through their invention of multitrophic food webs, large body size, life-history trade-offs, ecological succession, biogeography, major increases in standing biomass, eukaryote-dominated phytoplankton and the potential for mass extinction. Both the pre-Ediacaran and the post-Ediacaran biospheres were inherently stable, but the former derived from the simplicity of superabundant microbes exposed to essentially static, physical environments, whereas the latter is based on eumetazoan-induced diversity and dynamic, biological environments. The c . 100-myr Ediacaran transition (extending to the base of the Tommotian) can be defined on evolutionary criteria, and might usefully be incorporated into the Phanerozoic.     

Metacommunity analyses show an increase in ecological specialisation throughout the Ediacaran period

The first animals appear during the late Ediacaran (572 to 541 Ma); an initial diversity increase was followed reduction in diversity, often interpreted as catastrophic mass extinction. We investigate Ediacaran ecosystem structure changes over this time period using the “Elements of Metacommunity Structure” framework to assess whether this diversity reduction in the Nama was likely caused by an external mass extinction, or internal metacommunity restructuring. The oldest metacommunity was characterised by taxa with wide environmental tolerances, and limited specialisation or intertaxa associations. Structuring increased in the second oldest metacommunity, with groups of taxa sharing synchronous responses to environmental gradients, aggregating into distinct communities. This pattern strengthened in the youngest metacommunity, with communities showing strong environmental segregation and depth structure. Thus, metacommunity structure increased in complexity, with increased specialisation and resulting in competitive exclusion, not a catastrophic environmental disaster, leading to diversity loss in the terminal Ediacaran. These results reveal that the complex eco-evolutionary dynamics associated with Cambrian diversification were established in the Ediacaran.

This study shows that the eco-evolutionary dynamics of metazoan diversification known from the Cambrian Period started earlier in the Ediacaran Period with the Avalon assemblage and increased in complexity towards the Phanerozoic as new anatomical innovations appeared, culminating in the “Cambrian Explosion."     

Proposed GSSP for the base of Cambrian Stage 7, coinciding with the first appearance of Lejopyge laevigata, Hunan, China

The Luoyixi section, exposed in a roadcut along the Youshui River (Fengtan Reservoir), Guzhang County, Hunan Province, China, is proposed as the stratotype for the base of an unnamed stage boundary (base of the Cambrian stage provisionally termed Stage 7). The proposed position of the GSSP is 121.3 m above the base of the Huaqiao Formation, at a horizon coinciding with the first appearance of the cosmopolitan agnostoid trilobite Lejopyge laevigata. The section fulfills all the requirements for a GSSP, and the horizon can be constrained not only with the primary stratigraphic marker (L. laevigata) but also with secondary biostratigraphic, sequence-stratigraphic, and chemostratigraphic correlation tools. The first appearance of L. laevigata is one of the most readily recognizable levels in the Cambrian, and can be correlated with precision to all paleocontinents.     

牙形刺的鉴定与乐平统的底界

Clarkina postbitteri和C.dukouensis两个种的正确鉴定是确定乐平统底界的关键.Mei和Wardlaw(1994b,c)在建立这两个种时就已明确指出,C.postbitteri不同于C.dukouensis主要是有浑圆而不是钝圆的齿台后端,并有明显的后边缘(brim),并不是依据齿脊特征区别的.金玉 (Jin,2000a)与Henderson(2001)强调,C.postbitteri的中后部细齿分离是有别于C.dukouensis的主要特征.比较C.postbitteri与C.dukouensis的正模和Mei et al .(1 994c)所鉴定的C.dukouensis(图la-c),可以清楚地看到,两者的齿脊几乎相同.Mei et al .(1994c)所鉴定的C.dukouensis齿脊的细齿甚至比C.postbitteri的还分离(图1a,b).两个种的区别,应保持原来的定义主要依齿台后端形状和后边缘的有无. Clarkina dukouensis由C.postbitteri逐渐过渡而来,Henderson(2001)认为,由于是逐渐过渡,C.dukouensis首次出现的点位就是任意选择的.然而,泥盆系一石炭系界线的定义是在Siphonodella presulcata-S.sulcata连续演化谱系中S.sulcata的首次出现,无人认为,由于是连续演化谱系这个界线点位就是任意的.所有泥盆系的阶的金钉子都是以祖先种到定义种的演化谱系中定义种的首次出现为定义的,也无人认为这些金钉子的点位是任意的,因为每个阶的定义种都有明确的定义.C.dukouensis也有明确的定义,只要齿台后端变钝圆,后边缘消失,就由C.postbitteri进化到C.dukouensis.这个位置就在LPD115-6k,并非可任意选择. 金玉开(Jin,2000a),Henderson(2001)指出,他们对种的鉴定采用了居群概念.在确定金钉子点位时,必须考虑定义种在首次出现(FAD)时的居群特征,这就是a.个体很少;b.形态类型单调;c与先驱种共存;d.在几个剖面上可以证实.LPD115-6k中的C.dukouensis个体很少,形态单调,与先驱种C.postbitteri共存,完全具有此种早期居群的特征.相反,在LPD115 6j和6i上部的C.postbitteri个体数量多,形态类型多样,含有自C. postbitteri到C.dukouensis的过渡类型,代表C.postbitteri演化谱系的高级阶段,反映此种中晚期的居群特征.同时,不能扩大居群的概念,不能把不同的种包括到同一居群中去.金玉 (Jin,2000a)和Henderson(2001)认为C.dukouensis在LPD11 4-7e才开始出现,笔者认为,他们将LPD114-7b中的典型的Clarkina leveni,C.niuzhuanensis,C.bizarrensis也不恰当地归入Clarkina postbitteri居群中.LPD114-7b应属C.leveni带. Henderson(2001)强调,Jinogondolella granti是C.postbitteri的祖先,两者没有重叠,形态差别大.金玉 (Jin,2000a)和Henderson(2001)用点断平衡演化来解释这一差别.但真正的点断平衡演化都有先驱种和后继种的重叠,新的分子不是在先驱分子消失后才出现的,而是在一定的时限内与先驱种共存,并且都是在连续的地层中出现的.没有证实的点断平衡事件,从来不用来做GSSP的建立依据.国际地层规范强调的是,GSSP要建立在连续的地层中,定义种必须有清楚的演化关系.没有任何形态上的证据可以证明C.postbitteri是由Jinogondollella granti演化而来. Henderson(2001)还强调,LPD115-6i上下之间有一小间断(diastem),这一界线是一层序界线,是最理想的乐平统的底界.但小间断的存在,正表明这是一条不理想的生物地层界线,因为根据国际地层规范,GSSP必须建立在连续的单一岩相的地层中.层序地层界线不适合建立GSSP,因为所有层序地层界线都意味着可能有间断.最理想的生物地层界线是在层序界面或初始海泛面之上的第一广泛分布的生物带的底界(Wang&Su,2000).C.dukouen-sis正是这样的生物带,它比层序地层界线高21cm,比岩石地层界线低21cm.层序地层界线是非常好的辅助参考标准,但不是统或阶的界线本身.     

Potential Global Standard Stratotype-section and Point (GSSP) for a Cambrian stage boundary defined by the first appearance of the trilobite Ptychagnostus atavus, Drum Mountains, Utah, USA

The base of the Ptychagnostus (or Acidusus) atavus Zone is one of the most clearly recognizable horizons on an intercontinental scale in the Cambrian System, and would serve as an excellent position for the base of a new stage-level chronostratigraphic subdivision. Among well-exposed, readily accessible sections in Laurentia, the “Stratotype Ridge” section, Drum Mountains, western Utah, USA, is perhaps the most suitable for a Global Standard Stratotype-section and Point (GSSP) defined by the first appearance datum (FAD) of the cosmopolitan agnostoid trilobite P. atavus. In the “Stratotype Ridge” section, the FAD of P. atavus occurs near the base of a calcisiltite bed 62 m above the base of the Wheeler Formation. A position corresponding closely to this horizon can be recognized with precision in Gondwana, Siberia, Kazakhstan, and Baltica using a combination of stratigraphic tools, the most useful of which are trilobite biostratigraphy, conodont biostratigraphy, and sequence stratigraphy. Brachiopod biostratigraphy and chemostratigraphy provide general constraints on the position of the horizon intercontinentally.     

Upper Silurian microplankton of the Leintwardine Group, Ludlow Series, in the type Ludlow area and adjacent regions

The acritarchs and prasinophyte algae from the Upper Bringewood, Lower Leintwardine, Upper Leintwardine and Lower Whitcliffe formations of the Ludlow type area and surrounding regions are described. The following new taxa are proposed: Cymatiosphaera pumila sp. nov., Melikeriopalla pustula sp. nov., Cheleutochroa beechenbankensis sp. nov., Cymbosphaeridium molyneuxii sp. nov., Flammulasphaera bella gen. et sp. nov., Percultisphaera incompta sp. nov., Salopidium aldridgei sp. nov. and Umbellasphaeridium? wicanderi sp. nov. A holotype is defined for Lophosphaeridium galeatum Hill and a further 34 new taxa are described under open nomenclature. Two biozones, identified by the first appearance of the zone taxon, are defined in the Sunnyhill section, Ludlow [Global Stratotype Section and Point (GSSP) for the base of the Ludfordian Stage, Ludlow Series]. The base of the Leoniella vilis Biozone is identified at 4.52m below the top of the Upper Bringewood Formation at Beechenbank, Aymestrey. The base of the Triangulina sanpetrensis Biozone is identified at 15.49m above the base of the Lower Leintwardine Formation and Ludfordian Stage at the Sunnyhill section. These biozones allow correlation with sections in north–west Spain, Podolia and Gotland. The possibility of using Visbysphaera whitcliffense and U.? wicanderi as biozonal indicators is suggested.     

Correlating the global Cambrian–Ordovician boundary: Precise comparison of the Xiaoyangqiao section,Dayangcha, North China with the Green Point GSSP section,Newfoundland, Canada

The Cambrian–Ordovician boundary interval exposed at the Xiaoyangqiao section, North China is presented. The distribution of stratigraphically important fossils in the Xiaoyangqiao section revealed several nearly coeval graptolite, conodont, trilobite, and acritarch bioevents in the uppermost Cambrian–lowermost Ordovician carbonate-siliciclastic sedimentary sequence. The precise correlation to the Green Point GSSP section, western Newfoundland, Canada allows for the identification of the corresponding GSSP level in the Xiaoyangqiao section. The combined data from the Xiaoyangqiao section and the Green Point GSSP section provide a series of events that all can be applied as proxies for identification of the Cambrian–Ordovician boundary horizon outside the GSSP. Based on this, the Xiaoyangqiao section, Dayangcha, is here strongly recommended as a candidate for an Auxiliary Boundary Stratigraphic Section and Point section (ASSP) for the base of the Ordovician System, because it provides one of the best and most complete Cambrian–Ordovician transitions in the world and because the first planktic graptolites are from the Xiaoyangqiao section.     

Microfossils from the Krol ‘A’ of the Lesser Himalaya,India: Additional supporting data for its early Ediacaran age

An assemblage of microfossils of moderate diversity, with the remarkable occurrence of ECAP acritarchs, is reported from the Ediacaran Krol ‘A’ (= the Mahi Formation) succession of Lesser Himalaya, India. Microfossils occur in the chert nodules exposed in Solan district, Himachal Pradesh. Two microfossils, Barogophycus symmetricus n. gen. n. sp. and Botominella lineata are new to the well-established Krol assemblage. The paper concentrates predominantly on fossil eukaryotic filamentous and coccoidal micro-organism of simple morphology. The assemblage is dominated by remains of prokaryotic cyanobacteria and demonstrates the diversity achieved by microorganisms at the beginning of the Ediacaran Period in the aftermath of the Marinoan glaciation. Filamentous and coccoidal microorganisms differ in taxonomic composition from the Mesoproterozoic microbiotas. The Krol microfossils assemblage has biostratigraphic potential and usefulness in broadly demarcating the different levels of the Ediacaran strata. Because of the presence of ECAP acritarchs, along with the appearance of other eukaryotic microorganisms of filamentous and coccoidal morphology, the general level of the Krol ‘A’ assemblage is considered as Ediacaran.     

Conodonts from the Early Triassic microbialite of Guangxi (South China): implications for the definition of the base of the Triassic System

We describe a new Early Triassic (Griesbachian) succession of conodont faunas from a high‐resolution sampling of the basal Early Triassic microbial limestone and the base of the overlying unit at the Wuzhuan section (Nanpanjiang Basin, Guangxi, South China). The microbial limestone records the earliest phase of the Early Triassic biotic recovery after the end‐Permian mass extinction. For the first time, rich conodont faunas are reported from within the microbialite. The faunas from Wuzhuan are largely dominated by anchignathodontids, including several Isarcicella species, which were previously documented only from strata above the microbialite. A total of 14 conodont species assigned to three genera is recorded from the Wuzhuan section. Starting from the base of the microbialite upwards, several species are sequentially added to the conodont assemblage. The alpha diversity peaks at the top of the microbialite. The conodont record in the considered microbialite interval at Wuzhuan is presumably unaffected by local ecological changes. It therefore more likely represents an evolutionary rather than an ecological pattern. We compare the Wuzhuan's conodont record with a well‐supported phylogenetic model and suggest that the sequence of first occurrences at Wuzhuan is the closest to the ‘true’ sequence of evolutionary events that took place during this Griesbachian radiation of anchignathodontids. Based on comparisons with the GSSP section at Meishan, we suggest further that the first occurrence of Hindeodus parvus in Meishan does not correspond to its first appearance datum.     

The base of the Middle Ordovician in China with special reference to the succession at Hengtang near Jiangshan, Zhejiang Province, southern China

Extensive work during the last decade has led to the recent decision by the International Subcommission on Ordovician Stratigraphy that the base of the Middle Ordovician Series should be placed at the base of the Baltoniodus triangularis Conodont Biozone in the Huanghuachang GSSP (Global Stratotype Section and Point) section near Yichang, Hubei Province, China. A review of the biostratigraphy of successions in many parts of China shows that for various reasons, it is currently difficult to recognize the precise boundary level in many regions, and additional studies are clearly needed. A newly exposed, previously poorly known, condensed section in deeperwater facies at Hengtang near Jiangshan in the Chiangnan (Jiangnan) belt has yielded a substantial number of important graptolites and conodonts through the boundary interval. It provides a more informative illustration of the relations between the ranges of several key taxa of these groups than any other section in China, and the level of the base of the Middle Ordovician appears to be within an about 1.8 m thick interval of Ningkuo Formation, and lies in the lower Isograptus caduceus imitatus Biozone.     

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.     

Characterization and astronomically calibrated age of the first occurrence of Turborotalia frontosa in the Gorrondatxe section, a prospective Lutetian GSSP: implications for the Eocene time scale

The Gorrondatxe section, a prospective Lutetian Global Stratotype Section and Point (GSSP), has recently been used as the master reference section to reassess the correlation between Eocene magnetostratigraphic and calcareous planktonic biostratigraphic scales. However, the exact calibration of some events remained ill defined, as they were thought to be missing in Gorrondatxe due to a fault. The most important missing events were the first occurrence of the planktonic foraminifera Turborotalia frontosa and the C22n/C21r chron boundary. Either might be a reliable correlation criterion for the Ypresian/Lutetian boundary, as both approach the age of the original Lutetian Stratotype. New studies allowed the identification of the former event 9 m above the Gorrondatxe fault, within magnetic polarity Chron C21r and calcareous nannofossil Zone CP12a. Distinctive test features that characterize the most primitive morphotype of T. frontosa are described. Despite the high turbidite content, recurrent pelagic limestone–marl couplets and bundles occur, whose formation was driven by precession and eccentricity astronomical cycles. The first occurrence of T. frontosa was found 27 couplets and 5.5 bundles (60 m) below the first occurrence of the calcareous nannofossil Blackites inflatus , which is dated at 48 Ma. Hence, the age of the first occurrence of T. frontosa is estimated at 48.55 Ma, confirming that it is the most suitable planktonic foraminiferal correlation criterion for the Ypresian/Lutetian boundary. These results show that the stratigraphic interval missing due to the Gorrondatxe fault cannot be greater than a few metres and reinforce the value of this section as a prospective Lutetian GSSP.     

History, philosophy, and application of the Global Stratotype Section and Point (GSSP)

The history, philosophy, and application of the concept of the Global Stratotype Section and Point (GSSP) are reviewed. Geochronologic units defined by GSSPs serve as practical classificatory pigeonholes for the subdivision of geologic time. Accordingly, the main factor involved in the definition of GSSPs must be global correlatability. Early opposition to the GSSP concept centered around the desire for a traditional biochronologic time scale defined conceptually in terms of palaeobiological events, but such time scales are inherently unstable and thus unsuitable for the use of all geoscientists. The GSSP concept is also generally incompatible with the desire for 'natural' geochronologic boundaries. GSSPs have been defined mainly on the basis of biostratigraphic guiding criteria, but magnetic polarity reversals and chemostratigraphic and cyclostratigraphic horizons are now playing an important role. Most primary guiding criteria used to place a 'golden spike' will eventually become problematical in some way, so GSSPs should be defined so as to be correlatable by as many different lines of age-significant information as possible. The 'Global Standard Stratigraphic Age' (better renamed 'Standard Global Numerical Age') is a numerical analogue of the golden spike. Numerical definitions are currently appropriate for the formal subdivision of the Precambrian, and perhaps also for the Pleistocene/Holocene boundary. Recent suggestions to abandon chronostratigraphic terms (system, series, stage) in favor of geochronologic terms (period, epoch, age) are logically defensible, but could perpetuate the continuing confusion between various stratigraphic categories.     

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.     

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.