Saccorhytus coronarius: a Basal Deuterostome From the Ediacaran/Cambrian Boundary

Researchers have identified traces of what they believe is the earliest known prehistoric ancestor of humans -- a microscopic, bag-like sea creature, which lived about 540 million years ago.

Named Saccorhytus, after the sack-like features created by its elliptical body and large mouth, the species is new to science and was identified from microfossils found in China. It is thought to be the most primitive example of a so-called "deuterostome" -- a broad biological category that encompasses a number of sub-groups, including the vertebrates.

If the conclusions of the study, published in the journal Nature, are correct, then Saccorhytus was the common ancestor of a huge range of species, and the earliest step yet discovered on the evolutionary path that eventually led to humans, hundreds of millions of years later.

link.

A Comb Jelly With a Tripartite Gut

The Presence of a Functionally Tripartite Through-Gut in Ctenophora Has Implications for Metazoan Character Trait Evolution

Authors:

Presnell et al

Abstract:

The current paradigm of gut evolution assumes that non-bilaterian metazoan lineages either lack a gut (Porifera and Placozoa) or have a sac-like gut (Ctenophora and Cnidaria) and that a through-gut originated within Bilateria [ 1–8 ]. An important group for understanding early metazoan evolution is Ctenophora (comb jellies), which diverged very early from the animal stem lineage [ 9–13 ]. The perception that ctenophores possess a sac-like blind gut with only one major opening remains a commonly held misconception [ 4, 5, 7, 14, 15 ]. Despite descriptions of the ctenophore digestive system dating to Agassiz [ 16 ] that identify two openings of the digestive system opposite of the mouth—called “excretory pores” by Chun [ 17 ], referred to as an “anus” by Main [ 18 ], and coined “anal pores” by Hyman [ 19 ]—contradictory reports, particularly prominent in recent literature, posit that waste products are primarily expelled via the mouth [ 4, 5, 7, 14, 19–23 ]. Here we demonstrate that ctenophores possess a unidirectional, functionally tripartite through-gut and provide an updated interpretation for the evolution of the metazoan through-gut. Our results resolve lingering questions regarding the functional anatomy of the ctenophore gut and long-standing misconceptions about waste removal in ctenophores. Moreover, our results present an intriguing evolutionary quandary that stands in stark contrast to the current paradigm of gut evolution: either (1) the through-gut has its origins very early in the metazoan stem lineage or (2) the ctenophore lineage has converged on an arrangement of organs functionally similar to the bilaterian through-gut.

Ediacaran Doushantuo Embryo-like Fossils are Metazoans

Meroblastic cleavage identifies some Ediacaran Doushantuo (China) embryo-like fossils as metazoans

Authors:

Yin et al

Abstract:

Ediacaran Doushantuo (Formation) embryo-like fossils (EDEFs, ca. 600 Ma) from South China display cellular and sub-cellular structures and provide a unique window on the early evolution of multicellular eukaryotes. But there have been widely disparate interpretations of these fossils. Here we report new fossil embryo-like forms from the Doushantuo phosphorite that exhibit a meroblastic cleavage pattern. Our results from high-resolution propagation phase contrast–synchrotron radiation X-ray microtomography (PPC-SRμCT) demonstrate that these fossils preserve features directly comparable to those of modern meroblastic animal embryos that utilize discoidal cleavage. Given that discoidal-type meroblastic cleavage occurs only in metazoans, the phylogenetic positions of these fossils probably fall into the animal branch of the holozoan tree. Meroblastic as well as holoblastic cleavage forms were thus present by ca. 600 Ma, substantiating the conclusion derived from molecular clock estimates that a variety of metazoan lineages had evolved by the mid-Ediacaran after the termination of the Marinoan glaciation, if not earlier.

Earth’s oxygen cycle and the evolution of animal life

Earth’s oxygen cycle and the evolution of animal life

Authors:

Reinhard et al

Abstract:

The emergence and expansion of complex eukaryotic life on Earth is linked at a basic level to the secular evolution of surface oxygen levels. However, the role that planetary redox evolution has played in controlling the timing of metazoan (animal) emergence and diversification, if any, has been intensely debated. Discussion has gravitated toward threshold levels of environmental free oxygen (O2) necessary for early evolving animals to survive under controlled conditions. However, defining such thresholds in practice is not straightforward, and environmental O2 levels can potentially constrain animal life in ways distinct from threshold O2 tolerance. Herein, we quantitatively explore one aspect of the evolutionary coupling between animal life and Earth’s oxygen cycle—the influence of spatial and temporal variability in surface ocean O2 levels on the ecology of early metazoan organisms. Through the application of a series of quantitative biogeochemical models, we find that large spatiotemporal variations in surface ocean O2 levels and pervasive benthic anoxia are expected in a world with much lower atmospheric pO2 than at present, resulting in severe ecological constraints and a challenging evolutionary landscape for early metazoan life. We argue that these effects, when considered in the light of synergistic interactions with other environmental parameters and variable O2 demand throughout an organism’s life history, would have resulted in long-term evolutionary and ecological inhibition of animal life on Earth for much of Middle Proterozoic time (∼1.8–0.8 billion years ago).

Soft Bodied Metazoans (Animals) Found in From Ediacaran NeoProterozoic Namibia

A mixed Ediacaran-metazoan assemblage from the Zaris Sub-basin, Namibia

Authors:

Darroch et al

Abstract:

It has been proposed that the terminal Neoproterozoic Ediacara biota were driven to extinction by the evolution of metazoan groups capable of engineering their environments (the ‘biotic replacement’ model). However, evidence for an overlapping ecological association between metazoans and soft-bodied Ediacaran organisms is limited. Here, we describe new fossil localities from southern Namibia that preserve soft-bodied Ediacara biota, enigmatic tubular organisms thought to represent metazoans, and vertically-oriented metazoan trace fossils. Although the precise identity of the tracemakers remains elusive, the structures bear several striking similarities with the Cambrian-Recent ichnogenus Conichnus. These new data support inference of stratigraphic and ecological overlap between two very different eukaryotic clades, and indicate the existence of unusual ecosystems comprising both Ediacara biota and metazoans immediately prior to the Cambrian explosion.

Academic Bun Fight Over an Extraordinary Claim: do Tardigrades Have Extensive Horizontal Gene Transfer Like Bacteria?!

Original Paper:

Evidence for extensive horizontal gene transfer from the draft genome of a tardigrade

Authors:

Boothby et al

Abstract:

Horizontal gene transfer (HGT), or the transfer of genes between species, has been recognized recently as more pervasive than previously suspected. Here, we report evidence for an unprecedented degree of HGT into an animal genome, based on a draft genome of a tardigrade, Hypsibius dujardini. Tardigrades are microscopic eight-legged animals that are famous for their ability to survive extreme conditions. Genome sequencing, direct confirmation of physical linkage, and phylogenetic analysis revealed that a large fraction of the H. dujardini genome is derived from diverse bacteria as well as plants, fungi, and Archaea. We estimate that approximately one-sixth of tardigrade genes entered by HGT, nearly double the fraction found in the most extreme cases of HGT into animals known to date. Foreign genes have supplemented, expanded, and even replaced some metazoan gene families within the tardigrade genome. Our results demonstrate that an unexpectedly large fraction of an animal genome can be derived from foreign sources. We speculate that animals that can survive extremes may be particularly prone to acquiring foreign genes.

No!  They don't!

Genome of a tardigrade: Horizontal gene transfer or bacterial contamination?

Authors:

Bemm et al

Extract:

We have read the article “Evidence for extensive horizontal gene transfer from the draft genome of a tardigrade” (1) with interest and were astonished by the high number of genes horizontally transferred into the tardigrade genome. Still, we were surprised by the reported genome size of greate than 200 Mbp, which is in stark contrast to a previously published size of ∼78 Mbp determined by the same group (2).

Seriously!  You guys screwed up!

No evidence for extensive horizontal gene transfer from the draft genome of a tardigrade

Author:

Arakawa

Extract:

Through a draft genome sequencing of a tardigrade Hypsibius dujardini, Boothby et al. (1) report that “Approximately one-sixth of the genes in the tardigrade genome were found to have been acquired through horizontal transfer, a proportion nearly double the proportion of previous known cases of extreme horizontal gene transfer (HGT) in animals.” However, the authors also state that the “cultures are not axenic,” which means that they are highly prone to contamination.

Damnit! yes! It Does!

Reply to Bemm et al. and Arakawa: Identifying foreign genes in independent Hypsibius dujardini genome assemblies

Authors:

Boothyby et al

Extract:

Our report (1) describing the discovery of extensive horizontal gene transfer in a tardigrade genome has raised questions from other groups who were sequencing the Hypsibius dujardini genome in parallel or who have done new experiments and analyses since our report (2⇓⇓–5). Bemm et al. (2) now report filtering our data for likely contaminants, resulting in a new, prefiltered genome assembly. Arakawa (3) has sequenced genomes of starved, washed, individual animals that had been treated with antibiotics for 48 h, and used this genomic sequence and RNA-Seq data to identify likely bona fide tardigrade contigs. Two other reports have contributed data and analysis: Delmont and Eren (4) used a newly published analysis and visualization platform, Anvi'o (6), to identify likely contaminants in our genome assembly, and Koutsovoulos et al. (5) applied useful taxon-annotated GC coverage plots (Blobplots) (7) to our data and reported an independent genome assembly.

Bdelloid rotifers Exchange DNA Like Bacteria do, a First for Metazoans (animals)

Bdelloid rotifers were believed to have persisted and diversified in the absence of sex. Two papers now show they exchange genes with each other, via horizontal gene transfers as known in bacteria and/or via other forms of non-canonical sex.

link.

Metazoans (Animals) Really do Need Oxygen

n 2010, a research team garnered attention when it published evidence of finding the first animals living in permanently anoxic conditions at the bottom of the sea. But a new study, led by scientists at the Woods Hole Oceanographic Institution (WHOI), raises doubts.

One alternative scenario is that cadavers of multicellular organisms were inhabited by bacteria capable of living in anoxic conditions, and these "bodysnatchers" made it seem that the dead animals were living, said Joan Bernhard, a geobiologist with WHOI and the lead author of the new study published in the December 2015 issue of the scientific journal BMC Biology.

Bernhard and Virginia Edgcomb, her colleague at WHOI, led an expedition in 2011 that returned to the site of the initial findings: a deep hypersaline anoxic basin (or DHAB) two miles deep in the Mediterranean Sea. DHABs are curious phenomena. They exist in depressions on the seafloor, where long-buried salt deposits become exposed to seawater and dissolve into the sea. The hypersaline water is extremely dense and remains separated, like oil and water, from surrounding normal seawater. It forms "lakes" on the seafloor, tens to hundreds of meters deep, that are extremely salty and devoid of oxygen.

"We have known for a long time that some metazoans inhabit extreme anoxic habitats on a periodic or even semi-permanent basis," Bernhard said. "But scientists have thought that metazoan's high-energy activities, such as reproduction, would require oxygen. If these loriciferans spend their whole lives and reproduce in a zero-oxygen environment, we would have to reconsider our concepts of animal metabolism. It was important to revisit the DHABs to confirm and understand those previous remarkable findings."

In the 2010 study published in the same journal, researchers from Polytechnic University of Marche and the Natural History Museum of Denmark, led by Roberto Danovaro, analyzed samples collected from a Mediterranean DHAB called L'Atalante. They reported finding multicellular animals (or metazoans), including previously unknown species of a type of tiny animals called loriciferans.

The contrast between conditions at the seafloor and at the surface makes it nearly impossible to recover live specimens, and so in the past, metazoan specimens collected from DHABs have been "interpreted as the result of a rain of cadavers that sunk to the anoxic zone from adjacent oxygenated areas," according to the Danovaro study.

But the scientists conducted experiments with fluorescent tags, taken up only by metabolically active organisms, which gave indications that the loriciferans had been alive. In addition, a few loriciferans appeared to have reproductive structures called oocytes (or eggs), indicating that the organisms were reproducing.

Intrigued by these findings, Bernhard and Edgcomb returned to L'Atalante and other nearby DHABs in 2011 to further investigate aboard the research vessel Atlantis. They collected sediment and water from the edges of three brine pools with different chemical compositions, using WHOI's remotely operated vehicle Jason to visually guide carefully targeted push-core samples. Samples were taken from points in the upper, middle, and lower levels of the layer of water immediately overlying the brine lake. This so-called "interface zone" is where normal seawater at the top transitions to the brine at the bottom, becoming more concentrated and anoxic the closer to the brine. The highly dense, saline, chemical-laden and oxygen-depleted water in all three pools was too dense for Jason to fully penetrate. Control samples from nearby sediment and water of normal oxygen and salinity were also collected.

link.

Sadly, this seems to be not true.

Sufficient Oxygen in Paleoatmosphere at the Calymmian/Ectasian MesoProterozoic Boundary for Animals to Breathe

Sufficient oxygen for animal respiration 1,400 million years ago

Authors:

Zhang et al

Abstract:

The Mesoproterozoic Eon [1,600–1,000 million years ago (Ma)] is emerging as a key interval in Earth history, with a unique geochemical history that might have influenced the course of biological evolution on Earth. Indeed, although this time interval is rather poorly understood, recent chromium isotope results suggest that atmospheric oxygen levels were less than 0.1% of present levels, sufficiently low to have inhibited the evolution of animal life. In contrast, using a different approach, we explore the distribution and enrichments of redox-sensitive trace metals in the 1,400 Ma sediments of Unit 3 of the Xiamaling Formation, North China Block. Patterns of trace metal enrichments reveal oxygenated bottom waters during deposition of the sediments, and biomarker results demonstrate the presence of green sulfur bacteria in the water column. Thus, we document an ancient oxygen minimum zone. We develop a simple, yet comprehensive, model of marine carbon−oxygen cycle dynamics to show that our geochemical results are consistent with atmospheric oxygen levels greater than 4% of present-day levels. Therefore, in contrast to previous suggestions, we show that there was sufficient oxygen to fuel animal respiration long before the evolution of animals themselves.

pop sci write up here and here.

Uncertainty in the Timing of Animal (Metazoan) Origins Strongly Suggest Molecular Clock Dating is Premature for use

Uncertainty in the Timing of Origin of Animals and the Limits of Precision in Molecular Timescales

Authors:

dos Reis et al

Abstract:

The timing of divergences among metazoan lineages is integral to understanding the processes of animal evolution, placing the biological events of species divergences into the correct geological timeframe. Recent fossil discoveries and molecular clock dating studies have suggested a divergence of bilaterian phyla greater than 100 million years before the Cambrian, when the first definite crown-bilaterian fossils occur. Most previous molecular clock dating studies, however, have suffered from limited data and biases in methodologies, and virtually all have failed to acknowledge the large uncertainties associated with the fossil record of early animals, leading to inconsistent estimates among studies. Here we use an unprecedented amount of molecular data, combined with four fossil calibration strategies (reflecting disparate and controversial interpretations of the metazoan fossil record) to obtain Bayesian estimates of metazoan divergence times. Our results indicate that the uncertain nature of ancient fossils and violations of the molecular clock impose a limit on the precision that can be achieved in estimates of ancient molecular timescales. For example, although we can assert that crown Metazoa originated during the Cryogenian (with most crown-bilaterian phyla diversifying during the Ediacaran), it is not possible with current data to pinpoint the divergence events with sufficient accuracy to test for correlations between geological and biological events in the history of animals. Although a Cryogenian origin of crown Metazoa agrees with current geological interpretations, the divergence dates of the bilaterians remain controversial. Thus, attempts to build evolutionary narratives of early animal evolution based on molecular clock timescales appear to be premature.

Did Suspension Feeding Animals in the Ediacaran Cause an Ecological Revolution in Autotrophs?

Proterozoic photosynthesis – a critical review

Author:

Butterfield

Abstract:

Chlorophyll-based photosynthesis has fuelled the biosphere since at least the early Archean, but it was the ecological takeover of oxygenic cyanobacteria in the early Palaeoproterozoic, and of photosynthetic eukaryotes in the late Neoproterozoic, that gave rise to a recognizably modern ocean–atmosphere system. The fossil record offers a unique view of photosynthesis in deep time, but is deeply compromised by differential preservation and non-diagnostic morphologies. The pervasively polyphyletic expression of modern cyanobacterial phenotypes means that few Proterozoic fossils are likely to be members of extant clades; rather than billion-year stasis, their similarity to modern counterparts is better interpreted as a combination of serial convergence and extinction, facilitated by high levels of horizontal gene transfer. There are few grounds for identifying cyanobacterial akinetes or crown-group Nostocales in the Proterozoic record. Such recognition undermines the results of various ancestral state reconstruction analyses, as well as molecular clock estimates calibrated against demonstrably problematic Proterozoic fossils. Eukaryotic organisms are likely to have acquired their (stem-group nostocalean) photoendosymbionts/plastids by at least the Palaeoproterozoic, but remained ecologically marginalized by incumbent cyanobacteria until the late Neoproterozoic appearance of suspension-feeding animals.

Marine Animals Became More and More Diverse From the Mesozoic Through the Present

Sustained Mesozoic–Cenozoic diversification of marine Metazoa: A consistent signal from the fossil record

Authors:

Bush et al

Abstract:

Paleobiological data provide a key historical record of global biodiversity dynamics, but their interpretation is controversial due to geological and sampling biases. Raw data suggest that marine metazoans diversified dramatically during the late Mesozoic and Cenozoic, whereas bias-corrected analyses based on occurrence-level data in the Paleobiology Database (PBDB) have indicated much less Cenozoic diversification. These standardized analyses are cited as evidence that biases strongly conceal underlying patterns in the global fossil record. However, we show that marine diversity did increase substantially and continuously from the Jurassic to the Neogene, even after correcting for biases in PBDB data. Previous standardized analyses did not capture this diversification in full because they were based on incomplete data. In the Cenozoic, observed richness rose to twice the Paleozoic average, which is within the range of values seen in analyses of raw data, suggesting that even the raw global marine fossil record preserves first-order signals of diversity history.

The Curious Case of Tribrachidium "Boneyards" From Ediacaran NeoProterozoic South Africa

Paleoecology of the enigmatic Tribrachidium: New data from the Ediacaran of South Australia

Authors:

Hall et al

Abstract:

Tribrachidium is a monospecific genus of the Ediacara biota found globally. In the Ediacara Member of the Rawnsley Quartzite, Flinders Ranges of South Australia, the spatial distribution of Tribrachidium across the seafloor is best described as patchy. Although Tribrachidium is the dominant fossil on two of the twenty-six beds currently excavated and is present in large numbers on another, the genus most commonly occurs as no more than a handful of specimens on a bed. Tribrachidium size frequency distributions of each of the three beds with more than 5 specimens are all statistically distinguishable from one another. Additionally, the size range on any given bed is smaller than the overall size range observed for the genus. These patterns suggest that these organisms lived in populations composed of single generations. The beds with numerous Tribrachidium come from different facies and are characterized by the presence of dissimilar mixes of taxa and textured organic surfaces, indicating that Tribrachidium was a generalist, able to adapt to a variety environments. Uniquely, the base or internal structure of Tribrachidium is also found preserved in both positive and negative relief as a sequence of concentric ridges on beds where Tribrachidium is the dominant genus. The most parsimonious explanation for the presence of these concentric ridge fossils is that they are the fossilized form of a Tribrachidium preserved when the organism was buried upside-down, flipped over, or dead and partially decayed prior to burial.

Podolimirus mirus: a Bilateral Animal From Ediacaran NeoProterozoic Ukraine

Taphonomy of the Ediacaran Podolimirus and associated dipleurozoans from the Vendian of Ukraine

Authors:

Dzik et al

Abstract:

Fossiliferous strata of the late Ediacaran Lomoziv Member of the Mohyliv Formation, once known from now flooded outcrops along the Dniester River, have been recently exposed in a large quarry near the Novodnistrovs’k electric plant dam in Podolia, Ukraine. Finely bedded arkosic sandstone with claystone/siltstone intercalations is locally rich in ‘elephant skin’ surfaces indicating original presence of microbial mats. Imprints of soft-bodied organisms frequently occur on the sole surfaces of the sandstones. There are no signs of early diagenetic cementation with iron sulfides and the fossils are usually strongly compacted, with low relief. Specimens of bilaterally symmetrical latest Precambrian animals newly collected at the quarry offer additional evidence that their bodies had a complex internal anatomy. This relates especially to Podolimirus mirus, previously known from only fragmentary specimens representing only marginal parts of its chambered organ (‘quilt’). It appears that there is a large region of the body in front of the ‘quilt’, with a lobate organ resembling bifurcating anterior structures earlier thought to represent intestinal diverticula in other dickinsoniids. The ‘quilt’ (presumably a dorsal muscular organ) of Podolimirus has a deep medial sinus that may have hosted a cylindrical axial organ analogous to that reported in the dickinsoniids from northern Russia and Australia.

No Real Increase in Oxygen Levels at the end of the NeoProterozoic

Statistical analysis of iron geochemical data suggests limited late Proterozoic oxygenation

Authors:

Sperling et al

Abstract:

Sedimentary rocks deposited across the Proterozoic–Phanerozoic transition record extreme climate fluctuations, a potential rise in atmospheric oxygen or re-organization of the seafloor redox landscape, and the initial diversification of animals. It is widely assumed that the inferred redox change facilitated the observed trends in biodiversity. Establishing this palaeoenvironmental context, however, requires that changes in marine redox structure be tracked by means of geochemical proxies and translated into estimates of atmospheric oxygen. Iron-based proxies are among the most effective tools for tracking the redox chemistry of ancient oceans. These proxies are inherently local, but have global implications when analysed collectively and statistically. Here we analyse about 4,700 iron-speciation measurements from shales 2,300 to 360 million years old. Our statistical analyses suggest that subsurface water masses in mid-Proterozoic oceans were predominantly anoxic and ferruginous (depleted in dissolved oxygen and iron-bearing), but with a tendency towards euxinia (sulfide-bearing) that is not observed in the Neoproterozoic era. Analyses further indicate that early animals did not experience appreciable benthic sulfide stress. Finally, unlike proxies based on redox-sensitive trace-metal abundances, iron geochemical data do not show a statistically significant change in oxygen content through the Ediacaran and Cambrian periods, sharply constraining the magnitude of the end-Proterozoic oxygen increase. Indeed, this re-analysis of trace-metal data is consistent with oxygenation continuing well into the Palaeozoic era. Therefore, if changing redox conditions facilitated animal diversification, it did so through a limited rise in oxygen past critical functional and ecological thresholds, as is seen in modern oxygen minimum zone benthic animal communities.

Are the new Ediacaran Doushantuo Megasphaera-like Acritarchs Really Animals? Or Something Else?

Are the new Ediacaran Doushantuo Megasphaera-like acritarchs early metazoans?

Author:

Tang

Abstract:

A recent report by Chen et al. (Chen, L., Xiao, S., Pang, K., Zhou, C.M., Yuan, X.L., 2014. Cell differentiation and germ-soma separation in Ediacaran animal embryo-like fossils. Nature 516, 238-241) describes the new Ediacaran Doushantuo acritarchs with fascinating features. Nested within Megaclonophycus-like fossils that exhibit palintomic cell division are multicellular spheroid structures termed matryoshkas, which are interpreted to be germ cells. Such distinctive features of cellular differentiation and germ-soma separation suggest affinities of these with early stem group metazoans. In this commentary, I suggest some alternative interpretations to those elaborated by Chen et al. (2014).

Royal Tyrrell Museum Lecture Series: On the Origin of Chordates in the Cambrian Explosion

Corumbella: A Controversial, but Common Skeletonized Ediacaran NeoProterozoic Scyphozoan Cnidarian

Insights into the Skeletonization, Lifestyle, and Affinity of the Unusual Ediacaran Fossil Corumbella

Authors:

Forancelli Pacheco et al

Abstract:

The Ediacaran fossil Corumbella is important because it is hypothesized to be a scyphozoan cnidarian, and thus might be one of the rare examples of bona fide Neoproterozoic animals. Unfortunately, its mode of life, style of skeletonization, and taxonomic affinity have been very controversial. Here, we use X-ray micro-CT, SEM, and taphonomic analysis to compare preservational modes of Corumbella, in order to better understand the symmetry, mode of construction, preservational style, and taxonomy of this group. Results suggest that articulated and disarticulated specimens of Corumbella from the Ediacaran of Brazil, Paraguay, and the United States, although sometimes preserved very differently, represent the same taxon—Corumbella werneri. Corumbellids had a thick but flexible theca and probably lived with their basalmost part anchored in the sediment, much like Conotubus. When considered together, these results suggest that Corumbella was one of the first animals to build a skeleton, employing a lamellar microfabric similar to conulariids.

Cambrian Exoplosion Substrate Adaptations of Sessile Benthic Animals

Substrate adaptations of sessile benthic metazoans during the cambrian radiation

Authors:

Kloss et al

Abstract:

Many marine benthic metazoans must stabilize themselves upon the seafloor for survival, and as a result their morphologies are controlled in part by local substrate conditions. The Agronomic Revolution (AR), spurred by increasing vertical bioturbation during the Ediacaran—Cambrian transition, permanently altered the nature of shallow marine substrate conditions and led to a major shift in adaptive strategies among benthic metazoans. These ecological and evolutionary changes, known as the Cambrian Substrate Revolution (CSR), are generally understood from observations of benthic metazoan fossils across the Ediacaran/Cambrian boundary, but the timing and geographic extent of this transition are less well known. This analysis attempts to constrain the temporal and spatial pattern of the AR and CSR by performing a global-scale paleoecological analysis of the adaptive strategies of benthic fauna living during the Cambrian. This analysis focused on Burgess Shale-type (BST) faunas because of their exceptional preservation, and was conducted through direct observation of fossil specimens, analysis of data compiled from the Paleobiology Database, and literature review. From these analyses, faunal groups are assigned a metric, the Substrate Adaptability Index (SAI), that relates the overall affinity the fauna demonstrates toward either Proterozoic-style (SAI=0) or Phanerozoic-style (SAI= 1) substrate conditions. The results of this analysis demonstrate that most early and middle Cambrian faunas were mixtures of Phanerozoic- and Proterozoic-style adaptive strategists, suggesting that Proterozoic-style substrates were still influential in controlling adaptive strategies in marine environments until at least that time. This is further supported by ichnofabric analysis of many of these localities, where overall bioturbation levels are exceedingly low, indicating a lack of mixed-layer development and the prevalence of firm Proterozoic-style substrates well into the Cambrian.

Did Ediacaran NeoProterozoic Kotlinian Mass Extinction Pave way for the Cambrian Explosion?

Ediacaran biota in the aftermath of the Kotlinian Crisis: Asha Group of the South Urals

Authors:

Kolesnikov et al

Abstract:

The Asha Group of the South Urals is dominated by laminated shales and thin siltstone–sandstone alternations interpreted as a prograding low-energy inner shelf depositional system. The succession includes several thick sharp-based sandstone bodies comprising interstratified fine- to medium-grained fine-laminated, planar-laminated, hummocky-, convolute- and wave-bedded, planar and trough cross-bedded sandstones, occasionally pebble- to boulder conglomerates, regarded as tidal, deltaic and shoreface depositional systems. The sandstone shoreface bodies are interpreted as forced-regressive deposits stranded in offshore positions during subsequent transgressions. Although palaeoecological and taphonomic context of the Asha Group is favourable for the Ediacara-type biofacies, the associated fossil assemblages nevertheless are depauperate and consist of frondomorph holdfast structures, palaeopascichnids, microbial colonies, arumberiamorph structures, as well as lithified microbial substrates (shagreen texture, biolaminites). In terms of fidelity and fossil completeness, preservation of Ediacaran fossils in the Asha Group is by no means inferior to that seen in other Ediacaran macrofossil localities. The low biodiversity of Ediacaran macrofossils can be attributed to a relatively young Ediacaran age of the Asha Group, which is constrained by a U–Pb zircon date of 547.6 ± 3.8 Ma from an ash bed in the lower part of the sedimentary succession. The uppermost part of the Asha Group has yielded bilobed burrows with a backfill structure suggesting an affinity with the ichnogenus Didymaulichnus from the Ediacaran–Cambrian boundary strata. The Asha Group therefore offers an important glimpse into the history of the Edacaran biota in the aftermath of the Kotlinian Crisis that caused extinction of dickinsoniomorphs, tribrachiomorphs and bilateralomorphs in wave- and current-agitated shoreface depositional systems ∼550 million years ago, but did not significantly affect frondomorphs and palaeopascichnids.