Cambrian Substrate Revolution Research Articles

Evolution and palaeoenvironmental migration of a Cambrian undermat miner

Microbial mats sealed the sea bottom, becoming the food supply of many early metazoans, which exploited this resource in multiple ways during the late Ediacaran and earliest Cambrian. The onset of highly penetrative bioturbation in shallow-marine environments later in the early Cambrian removed this seal, resulting in the protracted restriction of microbial mats to marginal- and deep-marine environments. In this study, we demonstrate that the undermat-miner trace fossil Oldhamia followed the pattern of matground restriction to deep-marine environments. This is shown by a dataset compilation of Cambrian Oldhamia based on a comprehensive review of the literature with addition of recently discovered Oldhamia in the Cambrian strata of North and South China. Oldhamia is one of the most distinctive and widely distributed ichnogenera in early to middle Cambrian siliciclastic successions. It consists of semi-permanent, very shallow-tier, undermat-miner structures produced by stationary vermiform organisms. Despite its importance, details of Oldhamia ichnodiversity trajectories at ichnospecies level, environmental trends, and potential palaeogeographic patterns have not been explored. Our results demonstrate that Oldhamia displays high diversification in Fortunian shallow- and deep-marine settings, representing a signal of the Cambrian Information Revolution. Oldhamia alata and O. geniculata only occurred in shallow-marine settings, O. curvata and O. flabellata were restricted to the deep sea, and O. antiqua and O. radiata ranged from shallow- to deep-marine settings. The producers of Oldhamia retreated into deep sea since the end of the Fortunian mirroring the restriction of matgrounds as a response of intense bioturbation in shallow-marine deposits during the Agronomic Revolution. The producers of Oldhamia originated from the Iapetus Ocean and then migrated into other oceans, most likely through dispersal assisted by ocean currents.

Exploring the macroevolutionary impact of ecosystem engineers using an individual‐based eco‐evolutionary simulation

AbstractEcosystem engineers can radically reshape ecosystems by modulating the availability of resources to other organisms through modifying either physical or biological aspects of the environment. The introduction or removal of ecosystem engineers from otherwise stable ecosystems can impact the diversity of co‐occurring species, such as driving local extinctions of native taxa. While these impacts are well established over ecological timescales for a wealth of taxa, the macroevolutionary implications of the onset of ecosystem engineering behaviours are less clear. Despite this uncertainty, ecosystem engineering has been implicated in several major transitions in Earth history including the appearance of extensive bioturbation during the Cambrian substrate revolution and associated Ediacaran–Cambrian turnover, and the Great Oxygenation Event. Whether ecosystem engineers are frequently associated with turnover and extinction in deep time is not known. Here we investigate this with an eco‐evolutionary simulation framework in which we assign lineages the ability to impact the fitness of co‐occurring taxa through phenotype–environment feedback. We explore numerous conditions, including how frequently such feedback occurs, and whether ecosystem engineers modify or create niches. We show that there is no general expected outcome from the introduction of ecosystem engineers. In a minority of runs, ecosystem engineering lineages completely dominate, rendering all others extinct, but in others they persist (but do not dominate), or die out. We suggest that ecosystem engineers have complex impacts, but possess the capacity to profoundly shape diversity, and it is appropriate to consider them alongside other exogenous extinction drivers in deep time.

Adaptations to changing substrates in diploblastic dinomischids from the early Cambrian

The significant changes in seafloor conditions during the Ediacaran-Cambrian transition presumably bear on the autecology of the benthic animals living in this time interval. However, little evidence shows morphological modifications and life strategy transitions in early animals in response to this substrate revolution. Dinomischidae is a family of diploblastic-grade with a sessile, suspension-feeding lifestyle, which includes Xianguangia, Daihua and Dinomischus. Here, we redescribe Calathites spinalis, a poorly known taxon from the early Cambrian (Epoch 2, Age 3, ∼518 Ma), as a new member of this family. Our new material reveals that the body of C. spinalis consists of a stalked calyx and 18 tentacle-sheath complexes, which is a typical trait of dinomischids. Our phylogenetic analyses recover Calathites as a sister group of Xianguangia and Daihua and support dinomischids being a monophyletic group. Members of dinomischids possess unique attachment structures that vary in shape and size, presenting a unique example of morphological adaptations of early sessile animals during the Cambrian substrate revolution. Comparison with early metazoan fossils reveals that dinomischids and other early basal metazoans lagged behind sessile bilaterians in terms of adaptations to substrate changes; the former tends to persist with attachment strategies adapted to Proterozoic-style seafloor conditions that were relatively firm.

Depositional setting and trace fossil suites of the early Cambrian (series 2, stage 4) Khussak Formation, east-central salt range, north-west sub-Himalayas, Pakistan

This is the first integrated ichnologic-sedimentologic study of the Khussak Formation at the Salt Range, Pakistan. This integrated approach led to identifying thirteen lithofacies and six recurring facies associations. Facies association 1 (FA-1) is made up of massive to parallel laminated mudstone with rare interbeds of sandstone. FA-1 is moderately bioturbated with trace fossils belonging to the Cruziana ichnofacies (Cruziana, Planolites, Rusophycus). FA-1 is interpreted to record deposition in the upper intertidal mud flat setting. Facies association 2 (FA-2) consists of thinly interbedded sandstone (wavy and current rippled) and mudstone (massive to parallel-laminated). The heterolithic wavy bedding is the dominant bedding type. FA-2 is variably bioturbated (BI = 0–5). Trace fossils of moderate diversity of the Cruziana ichnofacies are present. FA-2 represents deposition in the middle intertidal mixed flat setting. Facies association 3 (FA-3) comprises gradational-based, laterally extensive, crudely bedded to massive, medium to lower fine-grained sandstone with thin (cm-scale) mudstone beds. FA-3 is intensely bioturbated (BI = 3–5). Robust and diverse trace fossils belonging to the Skolithos ichnofacies are present (Arenicolites, Cylindrichnus, Diplocraterion, Phycodes, Planolites, Siphonichnus, Skolithos, and Treptichnus). FA-3 records deposition in the lower intertidal sand flat setting. Facies association 4 (FA-4) consists of muddy to silty lower-upper fine-grained sandstone with rare mudstone drapes. FA-4 displays inclined heterolithic stratification (IHS). Tidal couplets (mud and mica drapes) and mud clasts are common. Bioturbation is absent to sparse (BI = 0–2) with low-diversity trace fossils of Skolithos ichnofacies (Cylindrichnus, Diplocraterion, Palaeophycus, Planolites, Skolithos, and rare fugichnia). FA-4 is interpreted to represent deposition in an intertidal creek point bar. Facies association 5 (FA-5) consists of well sorted sandstone (upper medium to fine-grained). The sandstone is erosionally based, and laterally extensive. Planar parallel lamination, trough cross-stratification, reactivation surfaces, and mud drapes are present only locally. The degree of biogenic reworking is low to moderate (BI = 0–4), and traces are sporadically distributed. Trace fossils present are of moderate diversity and abundance, including Arenicolites, Cylindrichnus, Diplocraterion, Monocraterion, Palaeophycus, Planolites, Siphonichnus, and Skolithos. FA-5 is interpreted as subtidal bar deposits. Facies association 6 (FA-6) is characterized by bioturbated muddy sandstone, heterolithic sandstone, and massive mudstone. Physical bedforms are commonly obliterated by bioturbation. Planar horizontal laminae, wavy laminae, small ripples, and synaeresis cracks may occur locally. FA-6 is moderate to intensely bioturbated (BI = 3–6). The distribution of traces (within beds and across the FA) is heterogeneous (sporadically) to homogeneous (locally). Robust Bergaueria, Cylindrichnus, Planolites, Rhizocorallium, Siphonichnus, Teichichnus and Zoophycos are present. FA-6 records deposition in the subtidal bay region. The tidal flat deposits of the Khussak Formation show a distinctive ichnofacies trend. The Cruziana ichnofacies characterizes the lower energy proximal deposits (intertidal mixed and mud flat). In contrast, the high-energy distal deposits (lower intertidal sand flat and shallow subtidal) are characterized by Skolithos ichnofacies. The ichnologic variation between the mud flat, mixed flat, sand flat, and subtidal deposits are gradational, but they can be used as facies boundary indicators. The bioturbation intensity and diversity of trace fossils within the lower Cambrian (Series 2, Stage 4) tidal flat deposits of the Khussak Formation suggest elevated levels of bioturbation intensity during stable environmental conditions in the Cambrian. Furthermore, it implies that the Agronomic Revolution was already emplaced in lower Cambrian (Series 2, Stage 4) tidal flat settings.

Increased habitat segregation at the dawn of the Phanerozoic revealed by correspondence analysis of bioturbation

The Agronomic Revolution of the early Cambrian refers to the most significant re-structuration of the benthic marine ecosystem in life history. Using a global compilation of trace-fossil records across the Ediacaran-Cambrian transition, this paper investigates the relationship between the benthos and depositional environments prior to, during, and after the Agronomic Revolution to shed light on habitat segregation via correspondence analysis. The results of this analysis characterize Ediacaran mobile benthic bilaterians as facies-crossing and opportunistic, with low levels of habitat specialization. In contrast, the Terreneuvian and Cambrian Series 2 reveal progressive habitat segregation, parallel to matground environmental restriction. This event was conducive to the establishment of distinct endobenthic communities along the marine depositional profile, showing that the increase in styles of animal-substrate interactions was expressed by both alpha and beta ichnodiversity. Habitat segregation at the dawn of the Phanerozoic may illustrate an early extension of the trophic group amensalism at community scale.

THE OLDEST TRACE FOSSILS IN ASSOCIATION WITH EDIACARA-TYPE BIOTA IN THE UPPER VENDIAN OF THE SOUTH URALS

Trace fossils and palaeopascichnids have been studied from the Basa Formation of the Vendian Asha Group of the South Urals (southwestern periphery of the Suleiman Anticline, Shubino quarry in the Ust’-Katav city, Chelyabinsk Region. The general morphology and preservation of ichnofossils was shown to be similar to the Late Vendian-Early Cambrian subhorizontal bilobate burrows of Didymaulichnus. The paleontological remains were found to be confined to extremely shallow depositional settings and located stratigraphically below the volcanic tuff layer, the zircon from which has a U–Pb isotopic age of 578 ± 7 Ma. Until now, the first occurrence of trace fossils and the associated “Agronomic Revolution” were thought to be timed to the interval of ~560–550 Ma. Our results show that mobile benthos and biological mixing of sediments may have occurred 20–30 Ma earlier and support the hypothesis that the onset of the first “Agronomic Revolution” and the “Cambrian Explosion” were not significantly coincident in time.

The Oldest Trace Fossils in Association with an Ediacara-Type Biota in the Upper Vendian of the South Urals

Trace fossils and palaeopascichnids have been studied in the Basa Formation of the Vendian Asha Group of the South Urals (southwestern periphery of the Suleiman Anticline, Shubino quarry in Ust’-Katav, Chelyabinsk Region of Russia). The general morphology and preservation of ichnofossils have been shown to be similar to the Late Vendian–Early Cambrian subhorizontal bilobate burrows of Didymaulichnus. The paleontological remains have been found to be confined to extremely shallow deposition settings and located stratigraphically below the volcanic tuff layer with a zircon U–Pb isotopic age of 578 ± 7 Ma. Until now, the first occurrence of ichnofossils and the associated “Agronomic Revolution” were dated at ~560–550 Ma. According to our study results, mobile benthos and biological mixing of sediments could have occurred 20–30 Ma earlier. The hypothesis that the onset of the “First Agronomic Revolution” and the “Cambrian Explosion” were far from being close in time was confirmed.

Design of a Non-Back-Drivable Screw Jack Mechanism for the Hitch Lifting Arms of Electric-Powered Tractors

The agricultural sector is constantly evolving. The rise in the world’s population generates an increasingly growing demand for food, resulting in the need for the agroindustry to meet this demand. Tractors are the vehicles that have made a real difference in agriculture’s development throughout history, lowering costs in soil tillage and facilitating activities and operations for workers. This study aims to successfully design and build an autonomous, electric agricultural tractor that can autonomously perform recurring tasks in open-field and greenhouse applications. This project is fully part of the new industrial and agronomic revolution, known as Factory 4.0 and Agriculture 4.0. The predetermined functional requirements for the vehicle are its lightweight, accessible price, the easy availability of its spare parts, and its simple, ordinary maintenance. In this first study, the preliminary phases of sizing and conceptual design of the rover are reported before subsequently proceeding to the dynamical analysis. To optimize the design of the various versions of the automated vehicle, it is decided that a standard chassis would be built based on a robot operating inside a greenhouse on soft and flat terrains. The SimScape multi-body environment is used to model the kinematics of the non-back-drivable screw jack mechanism for the hitch-lifting arms. The control unit for the force exerted is designed and analyzed by means of an inverse dynamics simulation to evaluate the force and electric power consumed by the actuators. The results obtained from the analysis are essential for the final design of the autonomous electric tractor.

Assessing the expansion of the Cambrian Agronomic Revolution into fan-delta environments

The intensity, extent, and ecosystem-level impact of bioturbation (i.e. Agronomic Revolution) at the dawn of the Phanerozoic is a hotly debated issue. Middle Cambrian fan-delta deposits in southwestern Saskatchewan provide insights into the paleoenvironmental extent of the Agronomic Revolution into marginal-marine environments. The studied deposits reveal that several environmental stressors had direct impact on trace-fossil distribution and bioturbation intensities in Cambrian fan deltas. Basal and proximal subaerial deposits are characterized by very coarse grain size and absence of bioturbation. Mid-fan and fan-toe deposits were formed under subaqueous conditions and are characterized by rapid bioturbation events in between sedimentation episodes when environmental stressors were ameliorated, providing evidence of a significant landward expansion of the Agronomic Revolution. Transgressive marine deposits accumulated after the abandonment of the fan-delta system display high levels of bioturbation intensity, reflecting stable environmental conditions that favored endobenthic colonization. The presence of intense bioturbation in both subaqueous fan delta and transgressive deposits provides further support to the view that Cambrian levels of biogenic mixing were high, provided that stable environmental conditions were reached. Our study underscores the importance of evaluating sedimentary facies changes to assess the impact of environmental factors prior to making evolutionary inferences.

The first edrioasteroid echinoderm from the lower Cambrian Chengjiang biota of Yunnan Province, China

AbstractUntil now, the earliest fossil echinoderms have been predominantly known from the early Cambrian of Laurentia and West Gondwana. Here, we describe Sprinkleoglobus extenuatus gen. et sp. nov. from the Chengjiang biota (Cambrian Series 2, Stage 3) of Yunnan Province, China. The overall profile of the theca and the morphology of the ambulacra clearly establish it as an edrioasteroid echinoderm. This represents the first discovery of an unequivocal echinoderm from the early part of the Cambrian of South China and is among the oldest records of echinoderms globally. Our material shows that edrioasteroids were diverse and widely distributed from their first occurrence in the fossil record, suggesting an earlier origin for the group. We infer that Sprinkleoglobus and other early echinoderms were adapted for attaching to hard substrates, implying that the so‐called Cambrian substrate revolution was not a strong control on the early evolution of attachment in echinoderms.

Paleontology and ichnology of the late Ediacaran Nasep–Huns transition (Nama Group, southern Namibia)

AbstractThe Nasep and Huns members of the Urusis Formation (Nama Group), southern Namibia, preserve some of the most diverse trace-fossil assemblages known from the latest Ediacaran worldwide, including potentially the world's oldest “complex” vertical sediment-penetrating burrows. These sediments record relatively diverse communities of bilaterian metazoans existing before the base of the Cambrian and an increase in the intensity of metazoan ecosystem engineering behaviors that could eventually produce profound changes in the character of the Phanerozoic sedimentary record (the “agronomic revolution”). Despite this, relatively little about this trace-fossil assemblage is known. We explore the Nasep–Huns transition at two localities in the Witputs sub-basin and describe the trace- and body-fossil diversity present in these horizons alongside a paleoenvironmental reconstruction. We document eight unique ichnotaxa from these localities, including well-preserved “probes” potentially left by priapulids. We also report the first occurrence ofCorumbellafrom Namibia, helping to establish a biostratigraphic link between Namibia, Brazil, Paraguay, Iran, and the southwestern United States. Last, we find that several ichnotaxa, in particular small treptichnids, appear to be preferentially preserved on the bases of gutter casts, hinting at the potential existence of an unusual late Ediacaran preservational window with possible implications for timing the first appearance of key bilaterian behaviors.

The interplay of environmental constraints and bioturbation on matground development along the marine depositional profile during the Ordovician Radiation.

This study documents the distribution of matgrounds in a wide variety of environments recorded in the Ordovician Lashkerak and Ghelli Formations in the Alborz Mountains of northern Iran in order to evaluate controls on their distribution along the marine depositional profile. Detailed facies analysis allowed differentiating three groups of facies associations in the Lower to Upper Ordovician deposits of the Lashkerak formation: (i) estuarine system; (ii) wave-dominated shoreface-offshore complex; and (iii) mixed river- and wave-influenced deltaic system. The Middle to Upper Ordovician deposits of the Ghelli formation are divided into two groups of facies associations: (i) tide-influenced deltaic succession and (ii) deep-water fan system. Microbially induced sedimentary structures (MISS) are present in deposits formed in the central estuarine basin (Lashkerak formation) and in proximal lobes and lobe fringes of deep-water turbidite fans (Ghelli formation). On the contrary, MISS are absent in deposits from the wave-dominated shoreface-offshore complex, river- and tide-dominated deltas, and various subenvironments of the incised wave-dominated estuary (i.e., bayhead delta and estuary mouth) and the deep-marine turbidite fan system (i.e., turbidite channel, slope, and outer lobe). The lack of evidence of mat-building microorganisms in the deltaic systems may have resulted from two factors: (1) high physico-chemical stressors caused by river-induced processes, and (2) increase in degree of sediment disturbance, biodiffusion, and bioirrigation by burrowing organisms. Formation of microbial mats in the wave-dominated shoreface-offshore complex was inhibited by the activity of an abundant and diverse infauna capable of reworking the sediment. Our analysis shows that the spatial distribution of microbial mats was controlled by an interplay of environmental factors and innovations in animal-substrate interactions, mostly expressed by secular changes in bioturbation. This study supports the notion that the agronomic revolution was diachronic, with marginal-marine and deep-sea ecosystems lagging behind shallow-marine settings.

CrowdedTrichophycusichnofabrics in the early Ordovician successions of central Iran: insight into the Ordovician radiation

The Tremadocian-to-Floian siliciclastic deposits of the Shirgesht Formation in the Kalmard Block of central Iran show abundant occurrences of the mid-tier Trichophycus venosus, a common ichnotaxon in the archetypical Cruziana ichnofacies. This trace fossil records a considerable increase in exploitation of offshore infaunal ecospace in comparison with older formations. Here, Trichophycus is relatively long and wide, with numerous and deep successively stacked causative burrows, which form dense burrow systems building the Crowded Trichophycus ichnofabric (CTI). Such development of CTI and the increase in depth of bioturbation had a negative effect on preservation of shallow-tier traces, for example arthropod burrows and trackways. This resulted in decreasing abundance of trilobite-produced trace fossils towards the top of the succession. The palaeobiological and palaeoecological interpretation of CTI points to stable habitats in muddy substrates above the storm wave base with high content of food and oxygen in pore waters, low-energy hydrodynamic regimes and a long colonization window. Moreover, the architecture and the morphological features of Trichophycus suggest a combined dwelling-feeding activity of the resident fair-weather producer showing the K-selected/climax strategy. The ethology of the producer, palaeoecological interpretations and taphonomic signatures revealed that it preferred dewatered, compacted muddy substrates (firmgrounds) in offshore settings, which might be related to minor hiatuses or short-lived discontinuities. Although the appearance of Trichophycus is concomitant to the earliest Cambrian agronomic revolution, it was uncommon until the early Ordovician, when locally it records infaunalization related to the Great Ordovician Biodiversification Event.

Petrographic evidence for ediacaran microbial mat-targeted behaviors from the great basin, United States

The lower Member of the Wood Canyon Formation contains a variety of simple late Precambrian–early Cambrian dwelling and grazing traces, typical of early bioturbators. The traces, dominated by Planolites and Palaeophycus, are heavily abundant throughout sections comprised of higher energy siliciclastics. Two of which, Chicago Pass and Boundary Canyon (California, USA), also contain evidence for targeted microbial mat penetration by undermat mining organisms below and into the PC–C boundary. The co-occurence of penetrative burrows within microbial microlaminae, surficial trails, and dwelling structures is characteristic of the complex Ediacaran trace fossil, Lamonte trevallis, as described by Meyer et al., (2014) from the Dengying Formation of South China. The specimens from the Wood Canyon Formation present similar structures resulting from altering behaviors of epibenthic locomotion, undermat mining, and dwelling (Meyer et al., 2014). Principally, grain size analyses confirmed the presence of microbial mat-targeted mining, seemingly resistant to overprinting by co-occurring horizontal traces. Therefore, we have classified the traces present in the California sections as Lamonte trevallis, broadening the geographic extent of this particular mining behavior in the Precambrian and the instances of complex bioturbating behaviors preceding the Cambrian substrate revolution.

Early Palaeozoic diversifications and extinctions in the marine biosphere: a continuum of change

AbstractA review of biodiversity curves of marine organisms indicates that, despite fluctuations in amplitude (some large), a large-scale, long-term radiation of life took place during the early Palaeozoic Era; it was aggregated by a succession of more discrete and regionalized radiations across geographies and within phylogenies. This major biodiversification within the marine biosphere started during late Precambrian time and was only finally interrupted in the Devonian Period. It includes both the Cambrian Explosion and the Great Ordovician Biodiversification Event. The establishment of modern marine ecosystems took place during a continuous chronology of the successive establishment of organisms and their ecological communities, developed during the ‘Cambrian substrate revolution’, the ‘Ordovician plankton revolution’, the ‘Ordovician substrate revolution’, the ‘Ordovician bioerosion revolution’ and the ‘Devonian nekton revolution’. At smaller scales, different regional but important radiations can be recognized geographically and some of them have been identified and named (e.g. those associated with the ‘Richmondian Invasion’ during Late Ordovician time in Laurentia and the contemporaneous ‘Boda event’ in parts of Europe and North Africa), in particular from areas that were in or moved towards lower latitudes, allowing high levels of speciation on epicontintental seas during these intervals. The datasets remain incomplete for many other geographical areas, but also for particular time intervals (e.g. during the late Cambrian ‘Furongian Gap’). The early Palaeozoic biodiversification therefore appears to be a long-term process, modulated by bursts in significant diversity and intervals of inadequate data, where its progressive character will become increasingly clearer with the availability of more complete datasets, with better global coverage and more advanced analytical techniques.

Penetrative trace fossils from the late Ediacaran of Mongolia: early onset of the agronomic revolution

The Cambrian radiation of complex animals includes a dramatic increase in the depth and intensity of bioturbation in seafloor sediment known as the ‘agronomic revolution’. This bioturbation transition was coupled with a shift in dominant trace fossil style from horizontal surficial traces in the late Precambrian to vertically penetrative trace fossils in the Cambrian. Here we show the existence of the first vertically penetrative trace fossils from the latest Ediacaran: dense occurrences of the U-shaped trace fossil Arenicolites from late Precambrian marine carbonates of Western Mongolia. Their Ediacaran age is established through stable carbon isotope chemostratigraphy and their occurrence stratigraphically below the first appearance of the trace fossil Treptichnus pedum. These Arenicolites are large in diameter, penetrate down to at least 4 cm into the sediment, and were presumably formed by the activity of bilaterian animals. They are preserved commonly as paired circular openings on bedding planes with maximum diameters ranging up to almost 1 cm, and as U- and J-shaped tubes in vertical sections of beds. Discovery of these complex penetrative trace fossils demonstrates that the agronomic revolution started earlier than previously considered.

New paleoscolecid worms from the early Cambrian north margin of the Yangtze Platform, South China

AbstractThe Cambrian Yanwangbian assemblage (Series 2, Stage 4) in South Shaanxi, China, is one of the Burgess Shale–type faunas as it represents the only relatively diverse Cambrian biota from the north margin of the Yangzte Platform, South China. The paleoscolecids (Cycloneuralia) illustrated herein are one of the major components of the fauna, although they appear to be much less abundant than skeletonized fossils, according to available collections. A new taxon,Shaanxiscolex xixiangensisnew genus new species, is described based on the scleritome pattern: each annulus has two rows of alternatingHadimopanella-type plates positioned close to the borders, and a mosaic pattern of microplates occurs between the plates and within intersegmental furrows. The occurrence of the new taxon confirms a fairly diversified and widespread distribution of paleoscolecidan worms recognized in the early Cambrian of South China. Moreover, the ecology of paleoscolecids is reappraised based on burial position of theS. xixiangensisand gut contens of Cambrian taxa from South China, hinting that paleoscolecids (at least some taxa) were both deposit feeders and carnivores, as well as active bioturbators in the substrates of the Cambrian sea, which sheds new light on the ‘Cambrian Substrate Revolution.’

Ichnologic evidence of a Cambrian age in the southern Amazon Craton: Implications for the onset of the Western Gondwana history

Colonization of the infaunal ecospace by burrowing bilaterians is one of the most important behavioral innovations during the Ediacaran-Cambrian transition. The establishment of vertical burrows by suspension feeders in high-energy nearshore settings during Cambrian Age 2 is reflected by the appearance of the Skolithos Ichnofacies. For the first time, unquestionable vertical burrows typical of the Skolithos Ichnofacies, such as Skolithos linearis, Diplocraterion parallelum and Arenicolites isp., are recorded from nearshore siliciclastic deposits of the Raizama Formation, southeastern Amazon Craton, Brazil. Integration of ichnologic and sedimentologic datasets suggests that these trace fossils record colonization of high-energy and well-oxygenated nearshore sandy environments. Chronostratigraphically, the presence of these vertical burrows indicates an age not older than early Cambrian for the Raizama Formation, which traditionally has been regarded as Ediacaran. Therefore, the Raizama ichnofauna illustrates the advent of modern Phanerozoic ecology marked by the Agronomic Revolution. The discovery of the Skolithos Ichnofacies in these shallow-marine strata suggests possible connections between some central Western Gondwana basins.

Cryptic Disc Structures Resembling Ediacaran Discoidal Fossils from the Lower Silurian Hellefjord Schist, Arctic Norway.

The Hellefjord Schist, a volcaniclastic psammite-pelite formation in the Caledonides of Arctic Norway contains discoidal impressions and apparent tube casts that share morphological and taphonomic similarities to Neoproterozoic stem-holdfast forms. U-Pb zircon geochronology on the host metasediment indicates it was deposited between 437 ± 2 and 439 ± 3 Ma, but also indicates that an inferred basal conglomerate to this formation must be part of an older stratigraphic element, as it is cross-cut by a 546 ± 4 Ma pegmatite. These results confirm that the Hellefjord Schist is separated from underlying older Proterozoic rocks by a thrust. It has previously been argued that the Cambrian Substrate Revolution destroyed the ecological niches that the Neoproterozoic frond-holdfasts organisms occupied. However, the discovery of these fossils in Silurian rocks demonstrates that the environment and substrate must have been similar enough to Neoproterozoic settings that frond-holdfast bodyplans were still ecologically viable some hundred million years later.

New fossil discoveries in the Chilhowee Group (southern Appalachians, USA): evidence for the Ediacaran-Cambrian transition, 'Cambrian Agronomic Revolution', and earliest trilobites at the southern margin of Laurentia

New fossil discoveries in the Chilhowee Group (southern Appalachians, USA): evidence for the Ediacaran-Cambrian transition, 'Cambrian Agronomic Revolution', and earliest trilobites at the southern margin of Laurentia