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.

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.

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.

China is Using CRISPR to Rapidly Developing Genetically Customized Animals

China’s western Shaanxi Province is known for rugged windswept terrain and its coal and wool, but not necessarily its science. Yet at the Shaanxi Provincial Engineering and Technology Research Center for Shaanbei Cashmere Goats, scientists have just created a new kind of goat, with bigger muscles and longer hair than normal. The goats were made not by breeding but by directly manipulating animal DNA—a sign of how rapidly China has embraced a global gene-changing revolution.

Geneticist Lei Qu wants to increase goatherd incomes by boosting how much meat and wool each animal produces. For years research projects at his lab in Yulin, a former garrison town along the Great Wall, stumbled along, Qu’s colleagues say. “The results were not so obvious, although we had worked so many years,” his research assistant, Haijing Zhu, wrote in an e-mail.

That changed when the researched adopted the new gene-customizing technology called CRISPR–Cas9, a technique developed in the U.S. about three years ago. CRISPR uses enzymes to precisely locate and snip out segments of DNA, much like a word-processor finding and deleting a given phrase—a process known as “gene-editing.” Although it is not the first tool scientists have used to tweak DNA, it is by far more precise and cheaper than past technologies. The apparent ease of this powerful method now raises both tantalizing possibilities and pressing ethical questions.

link.

Animal Populations do NOT Respond Uniformly to Climate Change

Geographical variation in species' population responses to changes in temperature and precipitation

Authors:

Pearce-Higgins et al

Abstract:

Despite increasing concerns about the vulnerability of species' populations to climate change, there has been little overall synthesis of how individual population responses to variation in climate differ between taxa, with trophic level or geographically. To address this, we extracted data from 132 long-term (greater than or equal to 20 years) studies of population responses to temperature and precipitation covering 236 animal and plant species across terrestrial and freshwater habitats. Our results identify likely geographical differences in the effects of climate change on populations and communities in line with macroecological theory. Temperature tended to have a greater overall impact on populations than precipitation, although the effects of increased precipitation varied strongly with latitude, being most positive at low latitudes. Population responses to increased temperature were generally positive, but did not vary significantly with latitude. Studies reporting significant climatic trends through time tended to show more negative effects of temperature and more positive effects of precipitation upon populations than other studies, indicating climate change has already impacted many populations. Most studies of climate change impacts on biodiversity have focused on temperature and are from middle to high northern latitudes. Our results suggest their findings may be less applicable to low latitudes.

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.

What's Weirder Than Weird? Turtles Grooming a Warthog (not kidding)

link.

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.

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.

Viewing Some Ediacaran NeoProterozoic Fossils as Animals Without the Constraints of Phylogenetics

The advent of animals: The view from the Ediacaran

Authors:

Droser et al

Abstract:

Patterns of origination and evolution of early complex life on this planet are largely interpreted from the fossils of the Precambrian soft-bodied Ediacara Biota. These fossils occur globally and represent a diverse suite of organisms living in marine environments. Although these exceptionally preserved fossil assemblages are typically difficult to reconcile with modern phyla, examination of the morphology, ecology, and taphonomy of these taxa provides keys to their relationships with modern taxa. Within the more than 30 million y range of the Ediacara Biota, fossils of these multicellular organisms demonstrate the advent of mobility, heterotrophy by multicellular animals, skeletonization, sexual reproduction, and the assembly of complex ecosystems, all of which are attributes of modern animals. This approach to these fossils, without the constraint of attempting phylogenetic reconstructions, provides a mechanism for comparing these taxa with both living and extinct animals.

Demosponges Diverged From Other Animals During the Cryogenian Sturtian Glaciations (Snowball Earth?)

Early divergence dates of demosponges based on mitogenomics and evaluated fossil calibrations

Authors:

Ma et al

Abstract:

Demosponges are among the most primitive biomineralized metazoans to appear first in the fossil record with hard skeletons; their confirmed earliest fossils are from the lower Cambrian rocks about 520 Ma, with putative demosponge biomarkers reported from 713∼635 Ma sediments. In this study, we use mitogenomic data to approach the early divergence timescale of demosponges using relaxed molecular clock techniques and likelihood-evaluated fossil calibration strategies. We found that among various molecular dating models, the correlated rate model yielded time estimates of demosponges in this analysis which is most congruent with the fossil appearance dates of demosponges. Our dating analyses show that crown groups of Demospongiae appeared at about 704 (674∼741) Ma, and the silicification in demosponges (divergence of spicular sponges) began about 633 (616∼648) Ma indicating a gap of over 100 million years between the origin of silicification and their first unequivocal appearance of siliceous spicules in the fossil record (520∼525 Ma); demosponges with tetraxon-type spicules (Tetractinellida) are dated here at about 514 (498∼530) Ma, an estimate comparable with the earliest tetraxial megasclere fossil records (510∼520 Ma, Ordian Age, middle Cambrian).

Cities are Having an Immediate, Significant Evolutionary Impact

That humans and the cities we build affect the ecosystem and even drive some evolutionary change in species' traits is already known. The signs are small but striking: Spiders in cities are getting bigger and salmon in rivers smaller; birds in urban areas are growing tamer and bolder, outcompeting their country cousins.

What's new is that these evolutionary changes are happening much more quickly than previously thought, and have potential impacts on ecosystem function on a contemporary scale. Not in the distant future, that is -- but now.

A new paper by Marina Alberti of the University of Washington College of Built Environments' Urban Ecology Research Lab published this month in the journal Trends in Ecology & Evolution suggests that if human-driven evolutionary change affects the functioning of ecosystems -- as evidence is showing -- it "may have significant implications for ecological and human well-being."

Alberti, a professor of urban design and planning, said that until recently it was assumed that evolutionary change would take too long to affect ecological processes quite so immediately. Such thinking has prevented evidence from coming together "in a way that can only emerge through a cross-disciplinary lens," she said, observing the interactions between humans and natural processes.

"We now have evidence that there is rapid evolution. These changes may affect the state of the environment now. This is what's called eco-evolutionary feedback.

"Cities are not simply affecting biodiversity by reducing the number and variety of species that live in urban habitats," Alberti said. Humans in cities are causing organisms to undergo accelerated evolutionary changes "that have effects on ecosystem functions such as biodiversity, nutrient cycling, seed dispersal, detoxification, food production and ultimately on human health and well-being."

link.

Trackway From Ediacaran NeoProterozoic Newfoundland Confirmed as Metazoan (Animal)

CONFIRMING THE METAZOAN CHARACTER OF A 565 MA TRACE-FOSSIL ASSEMBLAGE FROM MISTAKEN POINT, NEWFOUNDLAND

Authors:

Liu et al

Abstract:

Surface locomotory trace fossils from the Mistaken Point Formation of Newfoundland, dated at ∼ 565 Ma, suggest that organisms capable of controlled locomotion and possessing muscular tissue may have existed among Avalonian Ediacaran macrofossil assemblages. Here we describe the Mistaken Point trace-fossil assemblage in full, discuss its stratigraphic context within the Mistaken Point Formation, and explore the competing hypotheses for the formation of the traces. We find that the trace fossils, preserved within a turbidite succession in a deep-marine depositional environment, are not attributable to abiogenic structures, to Ediacaran tubular or filamentous body fossils, to rangeomorph stems, or to a host of late Ediacaran and early Phanerozoic ichnofossils. Specimens within the assemblage show some similarities to the ichnogenera Helminthoidichnites and Archaeonassa, but discrepancies in certain aspects of their structure mean that we do not formally attribute them to these ichnotaxa at this time. The Mistaken Point ichnofossils possess morphological characteristics indicative of formation by an organism with a round base. Comparison with traces formed by modern organisms of such character appears to rule out formation by protistan, echinoderm, or annelid styles of movement, but is consistent with organisms moving via muscular controlled locomotion in a similar way to some modern mollusks and actinian cnidarians. We suggest therefore that the Mistaken Point trace-fossil assemblage reveals the presence of muscular metazoans in late Ediacaran deep-marine ecosystems. Such organisms cannot yet be attributed to specific phyla, but their inferred locomotory mechanisms share closest similarities with those utilized by extant actinians.

Evidence of Sponges in Cryogenian NeoProterozoic Reefs?

Enigmatic chambered structures in Cryogenian reefs: The oldest sponge-grade organisms?

Authors:

Wallace et al

Abstract:

Previously undescribed chambered structures are common and widespread in the Cryogenian (post-Sturtian glacial) carbonates of the Oodnaminta Reef Complex (Adelaide Geosyncline, South Australia), the Rasthof-Berg Aukas Formation (Namibia) and the Gauss Formation (Namibia). These carbonate structures have millimetre to centimetre-scale chambers separated by well-defined and generally thin micritic walls. Chamber walls now consist of dolomite, but were probably originally aragonitic. The chambers may have a lobate, polygonal or dendritic morphology and are often further divided into smaller chambers. Chambered structures occur as reefal growth frameworks; as cavity-fillings in neptunean dykes and growth cavities; and as intercolumnar material within stromatolite frameworks. In the Oodnaminta Reef Complex, these structures are only present in the sub-photic deep water framework.

These structures probably represent the calcified remains of an organism or community of organisms that was globally distributed and widespread for a significant time period following the Sturtian glaciation. No precisely analogous structures have been previously described from modern or ancient settings, but the complexity and degree of organization suggests a significant evolutionary advance over older Proterozoic fossils. The closest morphological analogues for the structures are: (a) some types of reef-dwelling sponges; and (b) some complex microbialites from Archaean and Paleoproterozoic carbonates. The structures lack spicules and ostia found in sponges, ruling out a true Poriferan origin. However, it is plausible that they are proto-sponges, sponge-grade organisms, or complex microbial precursors to sponge-grade organisms. Whatever their affinity, we suggest these structures record a significant evolutionary event on the path towards organic complexity.

Evidence Ediacaran NeoProterozoic Doushantuo Embryo Fossils may be Stem-Group Animals

Cell differentiation and germ–soma separation in Ediacaran animal embryo-like fossils

Authors:

Chen et al

Abstract:

Phosphorites of the Ediacaran Doushantuo Formation (~600 million years old) yield spheroidal microfossils with a palintomic cell cleavage pattern. These fossils have been variously interpreted as sulphur-oxidizing bacteria, unicellular protists4, mesomycetozoean-like holozoans green algae akin to Volvox and blastula embryos of early metazoans or bilaterian animals. However, their complete life cycle is unknown and it is uncertain whether they had a cellularly differentiated ontogenetic stage, making it difficult to test their various phylogenetic interpretations. Here we describe new spheroidal fossils from black phosphorites of the Doushantuo Formation that have been overlooked in previous studies. These fossils represent later developmental stages of previously published blastula-like fossils, and they show evidence for cell differentiation, germ–soma separation, and programmed cell death. Their complex multicellularity is inconsistent with a phylogenetic affinity with bacteria, unicellular protists, or mesomycetozoean-like holozoans. Available evidence also indicates that the Doushantuo fossils are unlikely crown-group animals or volvocine green algae. We conclude that an affinity with cellularly differentiated multicellular eukaryotes, including stem-group animals or algae, is likely but more data are needed to constrain further the exact phylogenetic affinity of the Doushantuo fossils.