Cambrian Explosion Research Articles

Coupling of geographic range and provincialism in Cambrian marine invertebrates

AbstractThe Cambrian saw a dramatic increase in metazoan diversity and abundance. Between-assemblage diversity (beta diversity) soared in the first three Cambrian stages, suggesting a rapid increase in the geodisparity of marine animals during the Cambrian radiation. However, it remains unclear how these changes scale up to first-order biogeographic patterns. Here we outline time-traceable provinces for marine invertebrates across the Cambrian period using a compositional network based on species-level fossil occurrence data. Results confirm an increase in regional differences of faunal composition and a decrease in by-species geographic distribution during the first three stages. We also show that general biogeography tends to be reshaped after global extinction pulses. We suggest that the abrupt biogeographic differentiation during the Cambrian radiation was controlled by a combination of tectonics, paleoclimate, and dispersal capacity changes.

Increases in reef size, habitat and metacommunity complexity associated with Cambrian radiation oxygenation pulses

Oxygenation during the Cambrian Radiation progressed via a series of short-lived pulses. However, the metazoan biotic response to this episodic oxygenation has not been quantified, nor have the causal evolutionary processes been constrained. Here we present ecological analyses of Cambrian archaeocyath sponge reef communities on the Siberian Platform (525–514 Ma). During the oxic pulse at ~521–519 Ma, we quantify reef habitat expansion coupled to an increase in reef size and metacommunity complexity, from individual within-community reactions to their local environment, to ecologically complex synchronous community-wide response, accompanied by an increase in rates of origination. Subsequently, reef and archaeocyath body size are reduced in association with increased rates of extinction due to inferred expanded marine anoxia (~519–516.5 Ma). A later oxic pulse at ~515 Ma shows further reef habitat expansion, increased archaeocyath body size and diversity, but weaker community-wide environmental responses. These metrics confirm that oxygenation events created temporary pulses of evolutionary diversification and enhanced ecosystem complexity, potentially via the expansion of habitable space, and increased archaeocyath individual and reef longevity in turn leading to niche differentiation. Most notably, we show that progression towards increasing biodiversity and ecosystem complexity was episodic and discontinuous, rather than linear, during the Cambrian Radiation.

The record of sea water chemistry evolution during the Ediacaran–Cambrian from early marine cements

AbstractThe Ediacaran–Cambrian Radiation marks the widespread appearance of metazoans and calcareous biomineralised hard parts. These innovations occurred during an interval of dynamic changes in marine redox and sea water chemistry. Here, changing carbonate mineralogy, Mg/Ca ratios and rare earth element concentrations including the relative abundance of cerium (Ce anomaly: Ceanom) are documented to track sea water oxygen levels, in well‐preserved early marine cements from shallow marine reefs from Cambrian Stages 2–4 (ca 525–512 Ma). First, integrating the mineralogical data with published records, several shifts in dominant carbonate mineralogy are inferred: ‘dolomite‐aragonite seas’ in the late Ediacaran; ‘aragonite/high‐Mg calcite seas’ in Cambrian Stage 2; a temporary shift to a ‘calcite sea’ during early Cambrian Stage 3; an ‘aragonite sea’ between late Cambrian Stage 3 and late Cambrian Stage 4, then a gradual shift from mixed ‘aragonite–calcite seas’ during the middle and upper Cambrian towards a ‘calcite sea’ by the early Ordovician. Second, based on measured mMg/Ca in early marine cements, calculated sea water mMg/Ca at 15 and 35°C ranges from 1.2 to 0.8 in Cambrian Stage 2, 0.7–0.4 in Stage 3 and 1.4–0.9 in Stage 4 respectively. Finally, analysed Ceanom data combined with existing Ceanom data suggest potentially three phases of global oxic expansion. First, a long‐lived phase of progressive oxygenation during the late Ediacaran to Fortunian (ca 550–540 Ma; average Ceanom from 0.99 to 0.41), and possibly two shorter phases during early Cambrian Stage 3 (ca 519 Ma; average Ceanom from 0.91 to 0.40) and Stage 4 (ca 512 Ma; average Ceanom from 1.02 to 0.49), bounded by intervals of more dominant anoxia. Summarising, these data demonstrate that early marine cements offer an underused and high‐resolution archive of shallow marine redox and sea water chemistry through this critical transition in Earth's evolution.

Ediacaran Corumbella has a cataphract calcareous skeleton with controlled biomineralization.

Corumbella is a terminal Ediacaran tubular, benthic fossil of debated morphology, composition, and biological affinity. Here, we show that Corumbella had a biomineralized skeleton, with a bilayered construction of imbricated calcareous plates and rings (sclerites) yielding a cataphract organization, that enhanced flexibility. Each sclerite likely possessed a laminar microfabric with consistent crystallographic orientation, within an organic matrix. Original aragonitic mineralogy is supported by relict aragonite and elevated Sr (mean= ca. 11,800 ppm in central parts of sclerites). In sum, the presence of a polarisation axis, sclerites with a laminar microfabric, and a cataphract skeletal organization reminiscent of early Cambrian taxa, are all consistent with, but not necessarily indicative of, a bilaterian affinity. A cataphract skeleton with an inferred complex microstructure confirms the presence of controlled biomineralization in metazoans by the terminal Ediacaran, and offers insights into the evolution of development and ecology at the root of the 'Cambrian radiation'.

The problematic Cambrian arthropod Tuzoia and the origin of mandibulates revisited.

The origin of mandibulates, the hyperdiverse arthropod group that includes pancrustaceans and myriapods, dates back to the Cambrian. Bivalved arthropod groups such as hymenocarines have been argued to be early mandibulates, but many species are still poorly known, and their affinities remain uncertain. One of the most common and globally distributed Cambrian bivalved arthropods is Tuzoia. Originally described in 1912 from the Burgess Shale based on isolated carapaces, its full anatomy has remained largely unknown. Here, we describe new specimens of Tuzoia from the Canadian Burgess Shale (Wuliuan, Cambrian) showcasing exceptionally preserved soft tissues, allowing for the first comprehensive reconstruction of its anatomy, ecology and evolutionary affinities. The head bears antennae and differentiated cephalic appendages. The body is divided into a cephalothorax, a homonomous trunk bearing ca 10 pairs of legs with heptopodomerous endopods and enlarged basipods, and a tail fan with two pairs of caudal rami. These traits suggest that Tuzoia swam along the seafloor and used its spinose legs for predation or scavenging. Tuzoia is retrieved by a Bayesian phylogenetic analysis as an early mandibulate hymenocarine lineage, exemplifying the rapid diversification of this group in open marine environments during the Cambrian Explosion.

Tracking the Travels

Tracking the Travels

Formation conditions of Ediacaran–Cambrian cherts in South China: Implications for marine redox conditions and paleoecology

The Ediacaran–Cambrian transition period was a critical time interval for the environmental fluctuations and organismal evolution in geological history, popularly known as the “Cambrian Explosion”. The diagenetic mechanism of the widely developed Liuchapo bedded cherts in South China remain unclear and controversial during this critical time interval. To address this, we examined silicon and oxygen isotopes, as well as major and trace elements in the Liuchapo cherts from three wells developed from slope to basin facies in South China. Results reveal that the bedded cherts from the three wells were affected by episodic hydrothermal activity, and the percentage of hydrothermal fluids is highest in the Xc1 well and lowest in the Xb1 well. The paleo-seawater temperature, calculated from chert oxygen isotope values, present a gradual increasing trend from shallow- to deep- areas in South China from 542 to 526 Ma. Redox proxies indicate that the water column of the Xa1 and Xc1 wells were dominated by suboxic condition, while those of the Xb1 well was mainly in oxic conditions. There should exist an oxygen minimum zone (OMZ) during the shelf and slope transition. A suitable temperature and oxic environment are conducive to biological reproduction, which is consistent with higher primary productivity and abundant acritarch fossils in the Xb1 well. Silicon isotope values range from −0.6 ‰ to 1.2 ‰ in the three wells. The diagenetic origin of the Xa1, Xb1, and Xc1 wells was the consequence of the mixture of hydrothermal fluid and seawater, biological activity, and direct hydrothermal precipitation, respectively. Due to the redox condition and seawater temperature variation, silica-secreting organisms should be relatively limited. The episodic hydrothermal fluid, biological activity, and replacement of precursor rock may jointly affect the formation of the Liuchapo bedded cherts during the Ediacaran–Cambrian transition.

Guts, gut contents, and feeding strategies of Ediacaran animals

The oldest animals appear in the fossil record among Ediacara biota communities. They prelude animal-dominated ecosystems of the Phanerozoic and may hold clues to the appearance of modern animal phyla in the Cambrian explosion. However, little is known about the phylogeny of the Ediacaran organisms and even less about their diet and feeding behavior.1,2,3 An exception is mollusc-like Kimberella, for which a fossilized gut, feeding traces, and even potential coprolites have been found.4,5 By contrast, Ediacaran organic-walled tubes, such as Sabellidites and Calyptrina, are thought to belong to tube worms comparable with modern Siboglinidae that have no gut but gain their nutrition from symbiotic bacteria.6,7 Here, we examine the gut contents of Ediacaran animals using biomarker molecules. We show that 558-million-year (Ma)-old tube worm-like Calyptrina and mollusc-like Kimberella possessed a gut and shared a diet of green algae and bacteria. Despite their ancient age, sterol metabolism within the gut of both organisms was already comparable to extant invertebrates.8Dickinsonia, one of the key Ediacaran animals, show no traces of dietary molecules, indicating a different feeding mode and possible external digestion analogous to modern Placozoa. Lipid biomarkers uncover a range of feeding strategies in Ediacaran communities, highlighting true eumetazoan physiology of some Ediacaran animals.

Subduction initiation of the Proto-Tethys Ocean that facilitated climate change and biodiversification

The effect of plate interactions on surface processes, environmental changes of the earth and their contributions to biodiversification during Gondwana assembly has been hotly debated in the last decades. The late Neoproterozoic-early Paleozoic was a key period in the tectonic assembly of Gondwana, global climate change and metazoan evolution. During this interval, the amalgamation of Greater Gondwana was succeeded by the subduction initiation of oceanic slabs in the Proto-Tethys Ocean. Here, we use whole-rock geochemical data, detrital zircon U-Pb and Hf isotopic of Cambrian and Ordovician sedimentary rocks from the Baoshan block in the Southeastern Tibetan Plateau combined with existing data in the literature to constrain the timing of subduction initiation of the Proto-Tethys Ocean at 540-500 Ma. The results show that the Baoshan block, adjacent to the Pinjarra Orogen on the northern margin of India, witnessed the transition from passive to active continental margin in East Gondwana. After the subduction initiation, the source of the sedimentary rocks experienced more intense weathering, causing a higher sediment flux. Contemporaneously, the Cambrian Explosion and stepwise increase in atmospheric oxygen accompanied the Greater Gondwana assembly at the late Neoproterozoic-early Paleozoic boundary. Subduction of the Proto-Tethys Ocean and occurrence of peripheral orogens further increased atmospheric O2 content, transported nutrients to the ocean and reduced the greenhouse effect, which provided major propulsion for biological evolution.

Trace elements and stable isotopic geochemistry of two sedimentary sections in the lower Cambrian strata from the Tarim Basin, northwest China: Implications for silicification and biological evolution

Early animal innovations were coupled with profound atmospheric oxidation and oceanic disturbances during the Cambrian explosion; however, the relationship between evolutionary breakthroughs and ocean oxygenation remains controversial. In addition to the sufficiently investigated South China Craton, detailed research on other regions is needed to expand our understanding of ocean oxygenation dynamics and their relationships with animal radiation. Paleoenvironment reconstruction in the Tarim Basin is expected to yield additional contributions to the relevant literature. In this study, rare earth elements, redox-sensitive proxies, organic and inorganic carbon isotopic compositions (δ13CCarb and δ13CKer) of samples from lower Cambrian in the Shiairike and Yaerdangshan Sections of the Tarim Basin were analyzed. Our results showed that the cherts of the studied strata were not directly deposited from hydrothermal fluids but formed in a rather anoxic diagenetic environment where pH drawdown by organic matter degradation facilitated the precipitation of dissolved silica. Yuertusi and Xishanbulake cherts may have derived predominantly from direct seawater precipitation, among which cherts with negative Eu anomalies may have formed near the sulfate–methane transition zone, whereas cherts without Eu anomalies may have formed near the sediment-water interface. The negative δ13CCarb excursion (from 2.47‰ to −11.11‰) detected in the Yuertusi chert unit showed covariations with the enrichment factor of trace elements and other geochemical redox proxies. Therefore, the isotope excursion might be constrained by the anoxic diagenetic pore water and cannot be appropriately utilized for stratigraphic correlation. Multiple geochemical analyses suggest that the Tarim Basin is characterized by a redox-stratified ocean comprising a dynamic oxygenated continental shelf and a persistent anoxic deep basin, which is consistent with the ocean redox structure in South China. Phosphogenesis, enhanced biological pump efficiency and the development of photosynthesis in benthic algae might have comprehensively contributed to the dynamic oxygenation of the marine continental shelf and accelerated the diversification of small shelly fauna in the early Cambrian.

Was There a Cambrian Explosion on Land? The Case of Arthropod Terrestrialization.

Simple SummaryThe transition of life from the aquatic realm onto land represented one of the fundamental episodes in the evolution of the Earth that laid down the foundations for modern ecosystems as we know them today. This key transition in the history of life is poorly known, owing to the scarcity of ancient terrestrial fossil deposits; complex terrestrial ecosystems with plants and animals appear in the fossil record during the Silurian and Devonian. However, recent molecular clock studies and new lines of palaeontological evidence point to a possibly much earlier origin of life on land, dating back as far as the Cambrian. Here, we review this controversy, using the arthropods as a case study of the possible cryptic Cambrian explosion on land. In particular, we highlight approaches for reconciling the disagreement between molecular clock estimates and the fossil record for the arthropod colonization of land.Arthropods, the most diverse form of macroscopic life in the history of the Earth, originated in the sea. Since the early Cambrian, at least ~518 million years ago, these animals have dominated the oceans of the world. By the Silurian–Devonian, the fossil record attests to arthropods becoming the first animals to colonize land, However, a growing body of molecular dating and palaeontological evidence suggests that the three major terrestrial arthropod groups (myriapods, hexapods, and arachnids), as well as vascular plants, may have invaded land as early as the Cambrian–Ordovician. These dates precede the oldest fossil evidence of those groups and suggest an unrecorded continental “Cambrian explosion” a hundred million years prior to the formation of early complex terrestrial ecosystems in the Silurian–Devonian. We review the palaeontological, phylogenomic, and molecular clock evidence pertaining to the proposed Cambrian terrestrialization of the arthropods. We argue that despite the challenges posed by incomplete preservation and the scarcity of early Palaeozoic terrestrial deposits, the discrepancy between molecular clock estimates and the fossil record is narrower than is often claimed. We discuss strategies for closing the gap between molecular clock estimates and fossil data in the evolution of early ecosystems on land

The origin and distribution of the main oxygen sensing mechanism across metazoans.

Oxygen sensing mechanisms are essential for metazoans, their origin and evolution in the context of oxygen in Earth history are of interest. To trace the evolution of a main oxygen sensing mechanism among metazoans, the hypoxia induced factor, HIF, we investigated the phylogenetic distribution and phylogeny of 11 of its components across 566 eukaryote genomes. The HIF based oxygen sensing machinery in eukaryotes can be traced as far back as 800million years (Ma) ago, likely to the last metazoan common ancestor (LMCA), and arose at a time when the atmospheric oxygen content corresponded roughly to the Pasteur point, or roughly 1% of present atmospheric level (PAL). By the time of the Cambrian explosion (541-485Ma) as oxygen levels started to approach those of the modern atmosphere, the HIF system with its key components HIF1α, HIF1β, PHD1, PHD4, FIH and VHL was well established across metazoan lineages. HIF1α is more widely distributed and therefore may have evolved earlier than HIF2α and HIF3α, and HIF1β and is more widely distributed than HIF2β in invertebrates. PHD1, PHD4, FIH, and VHL appear in all 13 metazoan phyla. The O2 consuming enzymes of the pathway, PHDs and FIH, have a lower substrate affinity, Km, for O2 than terminal oxidases in the mitochondrial respiratory chain, in line with their function as an environmental signal to switch to anaerobic energy metabolic pathways. The ancient HIF system has been conserved and widespread during the period when metazoans evolved and diversified together with O2 during Earth history.

A Cambrian tommotiid preserving soft tissues reveals the metameric ancestry of lophophorates

Among extant animals, Lophotrochozoa accounts for the majority of phyla.1 This bilaterian clade radiated rapidly during the Cambrian explosion, obfuscating its phylogenetic relationships and rendering many aspects of its early evolution uncertain. Many early lophotrochozoans are known only from isolated skeletal microfossils, "small shelly fossils," often derived from larger animals with complex multi-element skeletons.2 The discovery of articulated fossils has revealed surprising insights into the animals from which these skeletal pieces were derived, such as paired shells in the mollusc Halkieria.3 Tommotiids are a key group of phosphatic early skeletal fossils that first appear in the late early Cambrian.4,5 Although their affinities were previously obscure, discoveries of partial scleritomes and investigations of growth and microstructure6 provide links with Brachiopoda7,8 and Phoronida,9 two of the lophophorate phyla. By contrast, the body plan of camenellan tommotiids remains a palaeontological mystery, with hypothetical reconstructions representing motile, benthic, dorsally armored worms.4,10 Here, we describe an articulated camenellan (Wufengella bengtsoni gen. et sp. nov.) from the Cambrian Chengjiang Biota, China, revealing the morphology of the scleritome and the first soft tissues from an adult tommotiid. Wufengella carries two dorsal rows of sclerites in a highly asymmetric arrangement, flanked by smaller, cap-shaped sclerites. The scleritome was fringed by iterated fascicles of chaetae and two layers of flattened lobes. Phylogenetic analysis confirms that camenellans occupy a deep branch in lophophorate phylogeny, prior to the acquisition of a sessile lifestyle. Wufengella reveals direct evidence for a metameric body plan reminiscent of annelids early in the evolutionary history of lophophorates.11,12.

Breathless through Time: Oxygen and Animals across Earth's History.

AbstractOxygen levels in the atmosphere and ocean have changed dramatically over Earth history, with major impacts on marine life. Because the early part of Earth's history lacked both atmospheric oxygen and animals, a persistent co-evolutionary narrative has developed linking oxygen change with changes in animal diversity. Although it was long believed that oxygen rose to essentially modern levels around the Cambrian period, a more muted increase is now believed likely. Thus, if oxygen increase facilitated the Cambrian explosion, it did so by crossing critical ecological thresholds at low O2. Atmospheric oxygen likely remained at low or moderate levels through the early Paleozoic era, and this likely contributed to high metazoan extinction rates until oxygen finally rose to modern levels in the later Paleozoic. After this point, ocean deoxygenation (and marine mass extinctions) is increasingly linked to large igneous province eruptions-massive volcanic carbon inputs to the Earth system that caused global warming, ocean acidification, and oxygen loss. Although the timescales of these ancient events limit their utility as exact analogs for modern anthropogenic global change, the clear message from the geologic record is that large and rapid CO2 injections into the Earth system consistently cause the same deadly trio of stressors that are observed today. The next frontier in understanding the impact of oxygen changes (or, more broadly, temperature-dependent hypoxia) in deep time requires approaches from ecophysiology that will help conservation biologists better calibrate the response of the biosphere at large taxonomic, spatial, and temporal scales.

A stem group Codium alga from the latest Ediacaran of South China provides taxonomic insight into the early diversification of the plant kingdom

BackgroundIn recent years, Precambrian lifeforms have generated an ever-increasing interest because they revealed a rich eukaryotic diversity prior to the Cambrian explosion of modern animals. Among them, macroalgae are known to be a conspicuous component of Neoproterozoic ecosystems, and chlorophytes in particular are already documented in the Tonian, when they were so far expected to originate. However, like for other major eukaryotic lineages, and despite predictions of molecular clock analyses placing roots of these lineages well into the Neoproterozoic, a taxonomic constraint on Precambrian green algae has remained difficult.ResultsHere, we present an exceptionally preserved spherical, coenocytic unicellular alga from the latest Ediacaran Dengying Formation of South China (> ca. 541 Ma), known from both external and internal morphology, fully tridimensional and in great detail. Tomographic X-ray and electronic microscopy revealed a characteristic medulla made of intertwined siphons and tightly packed peripheral utricles, suggesting these fossils belong to the Bryopsidales genus Codium. However, its distinctly smaller size compared to extant species leads us to create Protocodium sinense gen. et sp. nov. and a phylomorphospace investigation points to a possible stem group affinity.ConclusionsOur finding has several important implications. First, Protocodium allows for a more precise calibration of Archaeplastida and directly confirms that a group as derived as Ulvophyceae was already well diversified in various ecosystems prior to the Cambrian explosion. Details of tridimensional morphology also invite a reassessment of the identification of other Ediacaran algae, such as Chuaria, to better discriminate mono-versus multicellularity, and suggest unicellular Codium-like morphotypes could be much older and widespread. More broadly, Protocodium provides insights into the early diversification of the plant kingdom, the composition of Precambrian ecosystems, and the extreme longevity of certain eukaryotic plans of organization.

Nitrogen cycling during the peak Cambrian explosion

Previous studies on early-Cambrian nitrogen (N) cycle suggested a modern-type nitrogen reservoir with elevated nitrate (NO3−) availability in a globally oxygenated ocean for the Cambrian explosion. However, recent marine redox studies on the Chengjiang Biota revealed a stratified ocean with anoxic deeper waters for the peak Cambrian explosion, asking for further works on the N cycling during this critical period. Chengjiang and Qingjiang biotas are the two top-rank Burgess Shale-type fossil Lagerstätten that recorded peaked metazoan diversification of the Cambrian explosion at different paleoenvironmental regimes during the Cambrian Age 3. In this study, we conducted a detailed geochemical investigation of the early-Cambrian Shuijingtuo Formation from two drill cores containing the Qingjiang Biota in Hubei Province, South China. The results suggest progressive oxygenation of the depositional environment from euxinic to fluctuating, suboxic conditions with a low chemocline in the water column, accompanied by an increase of δ15N values from <−4‰ to between −2‰ and +1‰. Especially, both event and background beds of the Qingjiang Biota are characterized by δ15N values ranging from −1‰ to +1‰. Comparative analysis reveals consistent variations of δ15N signatures and marine redox states at the Qingjiang and Chengjiang areas during the Cambrian Age 3, with the latter exhibiting systematically higher δ15N values ranging mainly from +1‰ to +2‰ at the biota interval. Taken together, the δ15N signatures of the Qingjiang and Chengjiang biotas can be most parsimoniously explained by prevailed aerobic marine nitrogen cycling featured by active biological N2 fixation and a limited nitrate reservoir in the shallow ocean. The nitrogen isotope gradient between the Qingjiang and Chengjiang areas may be reflective of varied NO3− availability in seawater. These findings contribute to a clearer biogeochemical context that supported prosperous marine ecosystems during the peak Cambrian explosion, and provide new insights on spatiotemporal evolution of the marine nitrogen cycle during this critical period.

Complexity and the Evolution of Consciousness

This article introduces and defends the “pathological complexity thesis” as a hypothesis about the evolutionary origins of minimal consciousness, or sentience, that connects the study of animal consciousness closely with work in behavioral ecology and evolutionary biology. I argue that consciousness is an adaptive solution to a design problem that led to the extinction of complex multicellular animal life following the Avalon explosion and that was subsequently solved during the Cambrian explosion. This is the economic trade-off problem of having to deal with a complex body with high degrees of freedom, what I call “pathological complexity.” By modeling the explosion of this computational complexity using the resources of state-based behavioral and life history theory we will be able to provide an evolutionary bottom-up framework to make sense of subjective experience and its function in nature by paying close attention to the ecological lifestyles of different animals.

Strontium isotope and element constraints on the paleoenvironment of the latest Ediacaran in the Sichuan Basin, southeastern Tibetan Plateau

The Ediacaran–Cambrian period witnessed episodic extinctions, oxygenation of seawaters, Cambrian explosions, and tectonic events. However, compared with the various high-resolution geochemical records of the early–middle Ediacaran and Cambrian, the available geochemical record of the latest Ediacaran (551–542 Ma) is scarce (especially the strontium isotope and elements), which leads to the ambiguous interpretation of the paleoenvironment of the latest Ediacaran. Therefore, we conducted measurements of strontium isotopes and elemental content of a continuous series of carbonate samples from the Dengying Formation of Well PT1, located in the Sichuan Basin, southeastern Tibetan Plateau, in order to constrain the paleoenvironment of the latest Ediacaran. Strict sample screening was used to ensure that the isotopes and elements were not affected by diagenesis. Our analyses show that the environment and geochemical records of the seawater were controlled by tectonic activities, especially the Gondwana assembly. The global strontium isotope correlation indicates that the Sichuan Basin was a restricted basin (high 87Sr/86Sr values, ∼0.7090), which can be attributed to the existence of a submarine high. Under the background of oxic environment, there were two episodes of anoxic expansion. During the initial stage, the stable terrigenous detrital input and oxic environment provided the prerequisite for the emergence of aerobic organisms in the restricted platform. Then, the decreasing sea level and intense tectonic activities improved the terrigenous detrital input with higher 87Sr/86Sr values (∼0.7095), which stimulated the emergence of aerobic organisms, further resulting in the first episode of anoxic environment. Lastly, a global transgressive resulted in a high sea level, and thus, the Sichuan Basin changed to an open platform. The exchange with extensive oceans led to the increased paleoproductivity, which consumed oxygen and nutrients, further resulting in the second episode of anoxic environment. Thus, the restriction degree, eustatic variations, and the terrigenous detrital input affected the biological evolution and redox conditions.

Branching archaeocyaths as ecosystem engineers during the Cambrian radiation.

The rapid origination and diversification of major animal body plans during the early Cambrian coincide with the rise of Earth's first animal-built framework reefs. Given the importance of scleractinian coral reefs as ecological facilitators in modern oceans, we investigate the impact of archaeocyathan (Class Archaeocyatha) reefs as engineered ecosystems during the Cambrian radiation. In this study, we present the first high-resolution, three-dimensional (3D) reconstructions of branching archaeocyathide (Order Archaeocyathida) individuals from three localities on the Laurentian paleocontinent. Because branched forms in sponges and corals display phenotypic plasticity that preserve the characteristics of the surrounding growth environment, we compare morphological measurements from our fossil specimens to those of modern corals to infer the surface conditions of Earth's first reefs. These data demonstrate that archaeocyaths could withstand and influence the flow of water, accommodate photosymbionts, and build topographically complex and stable structures much like corals today. We also recognize a stepwise increase in the roughness of reef environments in the lower Cambrian, which would have laid a foundation for more abundant and diverse coevolving fauna.

Probability-based preservational variations within the early Cambrian Chengjiang biota (China).

The Chengjiang biota (Yunnan Province, China) is a treasure trove of soft-bodied animal fossils from the earliest stages of the Cambrian explosion. The mechanisms contributing to its unique preservation, known as the Burgess Shale-type preservation, are well understood. However, little is known about the preservation differences between various animal groups within this biota. This study compares tissue-occurrence data of 11 major animal groups in the Chengjiang biota using a probabilistic methodology. The fossil-based data from this study is compared to previous decay experiments. This shows that all groups are not equally preserved with some higher taxa more likely to preserve soft tissues than others. These differences in fossil preservation between taxa can be explained by the interaction of biological and environmental characteristics. A bias also results from differential taxonomic recognition, as some taxa are easily recognized from even poorly preserved fragments while other specimens are difficult to assign to higher taxa even with exquisite preservation.