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.

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Evidence Ediacaran Biota Members Survived Into the Cambrian

Transitional Ediacaran–Cambrian small skeletal fossil assemblages from South China and Kazakhstan: Implications for chronostratigraphy and metazoan evolution

Authors:

Yang et al

Abstract:

The Ediacaran–Cambrian transition records distinct evolutionary changes of metazoans. The Ediacaran fossils (i.e., Ediacara-type biota and tubular fossils) are contrasting with the diverse small skeletal fossils (SSFs) from the early Cambrian. The apparent dissimilarities hindered studies deciphering their evolutionary relationships. This also led to a popular assumption that there exists a mass extinction of the Ediacara biota and cloudinids at the Precambrian–Cambrian (Pc–C) boundary. Here we report for the first time a transitional fauna which consists of typical elements of Ediacaran, i.e. cloudinids and related tubicolous organisms, together with Cambrian SSFs including protoconodonts, anabaritids and siphogonuchitids from South China and Maly Karatau (Kazakhstan). The sediments yielding the transitional fauna are characterized by siliceous rocks in both regions and were previously considered to be unfossiliferous. Their chronostratigraphic assignment in South China has been debated for years. Based on the new fossil assemblage, the siliceous strata of the Daibu Member (Northeast Yunnan, South China) and the basal Kuanchuanpu Formation (South Shaanxi, South China) can be assigned to the earliest Cambrian SSF biozone (Anabarites trisulcatus–Protohertzina anabarica Assemblage Zone) and thus can be considered of early Cambrian in age. A new subzone of the earliest SSF zone in eastern Yunnan (South China) is proposed herein defined as Ganloudina symmetrica–Rugatotheca typica Interval Subzone. The new fauna demonstrates that the cloudinids, originally confined to the late Ediacaran, persisted into the Cambrian Fortunian, and no major extinction event occurred at the Pc–C transition.

Evidence of Multiple Increases in Paleoocean Oxygen Levels Across the Ediacaran/Cambrian Boundary

REE and trace element patterns from organic-rich rocks of the Ediacaran-Cambrian transitional interval

Authors:

Guo et al

Abstract:

It has been established that important changes in the marine environment and the biosphere occurred during the Cambrian. However the relationships between the so-called “ Cambrian Explosion” and the concomitant environmental changes are not yet fully understood. This study presents new geochemical data from the black shale successions from different facies belts of the Yangtze Platform in South China. Variations in the concentrations of REE and trace elements (varying Ce/Ce*, Th/U, V/Sc and V/Cr ratios) in kerogen as well as in bulk rocks from different depositional environments along a transect from platform to basin indicate two oxidation events, which led to the oxygenation of the water column in shallow-marine environments and euxinic conditions (weak correlation between TOC, V, U and Mo) in the deeper sea. During the first oxidation event in the late Terreneuvian, anoxic conditions in bottom waters rapidly changed to euxinic conditions. Subsequently, the second oxidation event during the early Epoch 2 of the Cambrian led to oxic-suboxic conditions in deeper seawater.

First Report of Fuxianospira from Cambrian North America

Reexamination of Yuknessia from the Cambrian of China and first report of Fuxianospira from North America

Authors:

LoDuca et al

Abstract:

Yuknessia Walcott, 1919 recently was transferred from the green algae to the Phylum Hemichordata on the basis of new details observed for the type species, Y. simplex, from the Burgess Shale Formation (Cambrian Stage 5) of British Columbia. This has prompted reexamination of material attributed to Yuknessia from various Cambrian localities in South China. Findings preclude both a Yuknessia and a hemichordate affinity for all of the Chinese study material, and most of this material is formally transferred to Fuxianospira Chen and Zhou, 1997, a taxon common in the Chengjiang biota. Comparable material from the Cambrian Marjum, Wheeler, and Burgess Shale formations of North America is also assigned to Fuxianospira, and this reassignment expands both the paleogeographic and stratigraphic range of this taxon. All aspects of the study specimens, including details obtained from scanning electron microscopy, are consistent with an algal affinity, as proposed in the original descriptions of the Chinese material.

Oceanic Redox Conditions Across the Ediacaran/Cambrian Boundary

Redox variations and organic matter accumulation on the Yangtze carbonate platform during Late Ediacaran–Early Cambrian: Constraints from petrology and geochemistry

Authors:

Gao et al

Abstract:

In order to understand redox variations and organic matter accumulation on the Yangtze carbonate platform during the Late Ediacaran–Early Cambrian, petrological and geochemical studies of several wells were carried out in this work. Our data suggest that depositional environments were dominated by oxidizing bottom water conditions during the late Ediacaran, and evolved to anoxic conditions, triggered by blooms of microbial organisms in surface waters during the earliest Cambrian. Subsequently, massive release of H2S derived from both anaerobic recycling of organic matter and, probably, hydrothermal venting promoted a sulfidic ocean. The discovery of a Ni-Mo sulfide ore layer in the basal Cambrian implies that such a sulfidic condition spread onto the Yangtze carbonate platform interior during the late Cambrian Stage 2. Further, transgressive flooding led to widespread black shale deposition and persistently anoxic conditions, as indicated by geochemical proxies. During the late Ediacaran to earliest Cambrian, local and widespread phosphogenesis indicates that organic matter accumulation was intimately associated with microbial (especially cyanobacterial) blooms driven by phosphorus cycling. The organic matter accumulations in early Cambrian black shales, however, were in connection with anoxic bottom water conditions and intermittent replenishment of recycled organic phosphorus to surface waters.

The Earliest Known Pelagic Jellyfish Found in Stage 3 Cambrian China

The earliest pelagic jellyfish with rhopalia from Cambrian chengjiang Lagerstätte

Authors:

Han et al

Abstract:

Modern cnidarian medusae generally show a triphasic life cycle with a succession of a larva, a sessile polyp and a pelagic medusa stage. The debate around the metagenesis of sessile polyps into pelagic medusae has lasted for more than 100 years. When pelagic forms originated is not clear. Hitherto the earliest crown-group medusae have been found at Cambrian Stage 5 (traditional Middle Cambrian, 509 Ma) in Utah, while diverse stem-group medusozoans were found in the basal Cambrian Fortunian Stage. No reliable medusae have been found from Cambrian Series 2 Stage 3 (ca. 521 Ma), although the marine benthic community teemed with many phyla of bilaterians, sponges, and ctenophores. Here we reinterpret Yunnanoascus haikouensis Hu et al., 2007, originally described as a ctenophore, as a pelagic, predatory, crown-group medusozoan, based on the presence of rhopalia, possible radial canals, and marginal tentacles. The medusae were a predatory member of the pelagic food web at the middle level of the ocean at Cambrian Stage 3.

This is how a Cambrian Trilobite Hunted

The Cambrian Period, which occurred between 541 million and 485 million years ago, is an important point in evolutionary history where most of the major groups of animals first appear in the fossil record. Often called the "Cambrian explosion," fossils from this time provide glimpses into evolutionary history as the world's ecosystems were rapidly diversifying. Most fossils preserve the physical remains of organisms and their structure; however, geologists and paleobiologists at the University of Missouri recently collaborated to study fossils that reveal the behaviors of predators preserved as traces in ancient sediments. Thus, fossils from southeast Missouri are helping scientists unlock clues about the behaviors of these predators and their interactions with their prey. Evidence shows that these ancient organisms were behaviorally sophisticated, tailoring their attacks for effectiveness.

Trilobites are a fossil group of extinct marine invertebrate animals with external skeletons (see photo). As predators and scavengers, they flourished in the Cambrian period and were very prominent in the oceans that once were located in Missouri.

"The Saint Francois Mountains in southeastern Missouri have been the focus of geological research for decades and were once islands in the Cambrian ocean," said Kevin Shelton, professor of geological sciences in the MU College of Arts and Science. "I've worked as a geologist studying ore deposits in the area for more than 30 years. In that time, I've run across thousands of fossilized trilobite burrows. It is rare that we get to study the activities of 500 million-year-old organisms, yet the fossils in this locality are helping us determine how these organisms behaved."

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Cambrian Burgess Shale Ecologies Were Stable (!) Temporally and Geographically

Paleocommunity Analysis of the Burgess Shale Tulip Beds, Mount Stephen, British Columbia: Comparison with the Walcott Quarry and Implications for Community Variation in the Burgess Shale

Authors:

O'Brien et al

Abstract:

The Tulip Beds locality on Mount Stephen (Yoho National Park, British Columbia) yields one of the most abundant and diverse (~10,000 specimens in 110 taxa) Burgess Shale fossil assemblages in the Canadian Rockies. Detailed semi quantitative and quantitative analyses of this assemblage suggest strong similarities with the Walcott Quarry on Fossil Ridge. Both assemblages are dominated by epibenthic, sessile, and suspension feeding taxa, mostly represented by arthropods and sponges and have comparable diversity patterns, despite sharing only about half the genera. However, the Tulip Beds has a higher relative abundance of suspension feeders and taxa of unknown affinity compared to the Walcott Quarry. These biotic variations are probably largely attributable to ecological and evolutionary differences between the two temporally distinct communities that adapted to similar, but not identical, environmental settings. For instance, the Tulip Beds is farther away from the Cathedral Escarpment than the Walcott Quarry. The Tulip Beds and Walcott Quarry assemblages are more similar to each other than either one is to the assemblages of the Chengjiang biota, although the relative diversity of major taxonomic groups and ecological patterns are similar in all assemblages. The conserved diversity patterns and ecological structures among sites suggest that the ecological composition of Cambrian Burgess Shale-type communities was relatively stable across wide geographic and temporal scales.

Paleo-marine Environments of the Early Cambrian Yangtze Platform

Paleo-marine environments of the Early Cambrian Yangtze Platform

Authors:

Zhang et al

Abstract:

Previous studies have generated conflicting interpretations of the nature of oceanographic conditions on the Early Cambrian Yangtze Platform. We investigated paleo-environmental conditions there through an analysis of major elements and redox-sensitive trace elements in the lower Cambrian Niutitang Formation from two new drillcores (RY1 and RY2) in Guizhou Province. High TOC and excess Ba concentrations record elevated productivity in the study area. Redox variation was documented based on enrichment factors (EFs) for redox-sensitive trace elements (Mo, U, V, Cr, Ni, and Co) and Corg:P ratios. These proxies indicate that the Lower Unit of the Niutitang Formation was deposited under fluctuating but mostly anoxic conditions, whereas the Upper Unit was deposited under predominantly suboxic conditions. Mo/TOC ratios suggest persistently moderate to strong basinal restriction, although Mo–U covariation patterns show no evidence for operation of a particulate shuttle despite the semi-restricted, anoxic nature of the deep watermass. Episodic deep water oxygenation events occurred within the Yangtze Platform sea, and longer-term secular variation may record eustatic influences. Additionally, patterns of trace-metal enrichment record evidence of submarine hydrothermal inputs. We propose an integrated paleoenvironmental model to account for the observed productivity, redox, and hydrographic patterns in the Early Cambrian Yangtze Platform. On the basis of paleo-marine environmental analysis, we ascribe the scarcity of trace fossils in the bottom to oxygen deficiency, and deny considering the Songlin Biota as the onset of the intense biogenic evolution after global atmospheric and oceanic oxygenation.

Mud Dragons in the Cambrian

A team of Virginia Tech researchers have discovered fossils of kinorhynch worms - commonly known as mud dragons - dating back more than 530 million years.

The historic find - made in South China - fills a huge gap in the known fossil record of kinorhynchs, small invertebrate animals that are related to arthropods, featuring exoskeletons and segmented bodies, but not jointed legs.

The first specimen was unearthed in rocks in Nanjiang, China, in 2013 and more fossils were found later that year and in 2014.

Helping lead the international team of scientists and biomedical engineers who unearthed, studied, and imaged the ancient, armored, worm-like creature is Shuhai Xiao, a professor of geobiology in the Department of Geosciences, part of the College of Science at Virginia Tech.

Dubbed Eokinorhynchus rarus - or rare ancient mud dragon, the newly discovered animal dates back from the Cambrian period and contains five pairs of large bilaterally placed spines on its trunk. It is believed to be related to modern kinorhynchs.

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Sino-Korean Block was Located at the Northern Margin of East Gondwana Across the Ediacaran/Cambrian Boundary

Detrital zircon geochronology and Nd isotope geochemistry of the basal succession of the Taebaeksan Basin, South Korea: Implications for the Gondwana linkage of the Sino-Korean (North China) block during the Neoproterozoic–early Cambrian

Authors:

Lee et al

Abstract:

The paleogeographic configuration of continental blocks around East Gondwana during the Neoproterozoic–early Cambrian is controversial. This study reports the U–Pb ages of detrital zircons and Nd isotopic composition of the Neoproterozoic–early Cambrian succession developed on the Precambrian basement in South Korea, which formed the southeastern portion of the Sino-Korean block (SKB) in its present configuration. Three stratigraphic units are addressed in this study: the Neoproterozoic Jangsan, early Cambrian Myeonsan, and early–middle Cambrian Myobong Formations. Both the Jangsan (white to pink quartz sandstone) and Myeonsan (dark gray ilmenite-rich sandstone/shale) formations are barren and are unconformably and conformably overlain, respectively, by the dark gray, fossiliferous fine-grained Myobong Formations. The Jangsan and Myeonsan Formations contain zircons with Archean–Paleoproterozoic ages, indicative of detritus derived from the local Precambrian basement. In contrast, the Myobong Formation is dominated by Mesoproterozoic to Neoproterozoic zircons, which are not represented in the local Precambrian basement. The Sm–Nd model ages of the Myobong Formation are younger than those of the underlying strata, indicative of significant changes in provenance during the deposition of this formation. Comparison with coeval sediment having Gondwana signatures in the southern margin of the SKB and the Tethyan Himalayan terrane strongly suggests that the Myobong Formation was derived from orogens in East Gondwana. The results of this study reveal that the timing of the Myobong Formation deposition marks the onset of a sedimentation episode on the southeastern margin of the SKB, which was related to the emergence of a vast source province in East Gondwana, possibly aided by the Cambrian transgression onto the SKB. In comparison with the published literature, we argue for the paleogeographic continuity of the SKB with the northern margin of East Gondwana, possibly between northwestern Australia and northeastern India during the Neoproterozoic–early Cambrian.

Cambrian Arthropod Waptia From Burgess Shale Brooded its Eggs

Long before kangaroos carried their joeys in their pouches and honey bees nurtured their young in hives, there was the 508-million-year-old Waptia. Little is known about the shrimp-like creature first discovered in the renowned Canadian Burgess Shale fossil deposit a century ago, but recent analysis by scientists from the University of Toronto, Royal Ontario Museum, and Centre national de la recherche scientifique has uncovered eggs with embryos preserved within the body of the animal. It is the oldest example of brood care in the fossil record.

"As the oldest direct evidence of a creature caring for its offspring, the discovery adds another piece to our understanding of brood care practices during the Cambrian Explosion, a period of rapid evolutionary development when most major animal groups appear in the fossil record," said Jean-Bernard Caron, curator of invertebrate palaeontology at the Royal Ontario Museum and associate professor in the Departments of Earth Sciences and Ecology & Evolutionary Biology at the University of Toronto.

Caron, along with Jean Vannier at the Centre national de la recherche scientifique in Lyon, France, describe the findings in a study published December 17 in Current Biology.

Waptia fieldensis is an early arthropod, belonging to a group of animals that includes lobsters and crayfish. It had a two-part structure covering the front segment of its body near the head, known as a bivalved carapace. Caron and Vannier believe the carapace played a fundamental role in how the creature practised brood care.

"Clusters of egg-shaped objects are evident in five of the many specimens we observed, all located on the underside of the carapace and alongside the anterior third of the body," said Caron.

The clusters are grouped in a single layer on each side of the body with no or limited overlapping among the eggs. In some specimens, eggs are equidistant from each other, while in others, some are are closer together, probably reflecting variations in the angle of burial and movement during burial. The maximum number of eggs preserved per per individuals probably reached 24.

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Australian Kalkarindji Continental Flood Basalt Probably did NOT Cause Cambrian Mass Extinction

The Giant Lavas of Kalkarindji: rubbly pāhoehoe lava in an ancient continental flood basalt province

Authors:

Marshall et al

Abstract:

The Kalkarindji continental flood basalt province of northern Australia erupted in the mid Cambrian (c. 511-505 Ma). It now consists of scattered basaltic lava fields, the most extensive being the Antrim Plateau Volcanics (APV) – a semi-continuous outcrop (c. 50,000 km2) reaching a maximum thickness of 1.1 km. Cropping out predominately in the SW of the APV, close to the top of the basalt succession, lies the Blackfella Rockhole Member (BRM). Originally described as ‘basaltic agglomerate’ the BRM has, in recent years, been assumed to be explosive tephra of phreatomagmatic origin, thus providing a potent vehicle for volatile release to the upper atmosphere. Our detailed field investigations reveal that this basaltic agglomerate is, in reality, giant rubble collections (15–20 m thick) forming the upper crusts of rubbly pāhoehoe lava units 25–40 m thick; covering 18,000–72,000 km2 and an estimated volume of 1,500–19,200 km3. These flows, rheologically but not chemically, distinct from the majority of Kalkarindji lavas, indicate a fundamental change in eruption dynamics. A low volatile content, induced high amounts of pre-eruptive degassing causing super-cooling and an increase in crystal nucleation and viscosity. A more viscous lava and a consistently faster rate of effusion (analogous to that of Laki, Iceland) created the flow dynamics necessary to disturb the lava crust to the extent seen in the BRM. Volatile release is estimated at 1.65 × 104–2.11 × 105 Tg total CO2 at a rate of 867 Tg a- 1 and 9.07 × 103–1.16 × 105 Tg SO2 at 476.50 Tg a- 1. These masses accounted for 0.5% of Cambrian atmospheric conditions whilst limiting factors reduced the effect of volatile delivery to the atmosphere, thus any potential global impact caused by these flows alone was minimal.

The Emu Bay Shale Cambrian Konservat-Lagerstätte

The Emu Bay Shale Konservat-Lagerstätte: a view of Cambrian life from East Gondwana

Authors:

Paterson et al

Abstract:

Recent fossil discoveries from the lower Cambrian Emu Bay Shale (EBS) on Kangaroo Island, South Australia, have provided critical insights into the tempo of the Cambrian explosion of animals, such as the origin and seemingly rapid evolution of arthropod compound eyes, as well as extending the geographical ranges of several groups to the East Gondwa-nan margin, supporting close faunal affinities with South China. The EBS also holds great potential for broadening knowl-edge on taphonomic pathways involved in the exceptional preservation of fossils in Cambrian Konservat-Lagerstätten. EBS fossils display a range of taphonomic modes for a variety of soft tissues, especially phosphatization and pyritization, in some cases recording a level of anatomical detail that is absent from most Cambrian Konservat-Lagerstätten.

Continuous Neoproterozoic to Ordovician Sedimentation in Victoria Land, Antarctica

Continuous Neoproterozoic to Ordovician sedimentation at the East Gondwana margin – implications from detrital zircons of the Ross Orogen in northern Victoria Land, Antarctica

Authors:

Estrada et al

Abstract:

The Ross-Delamerian orogenic belt was formed along the eastern side of the Australian-East Antarctic continent during west-directed subduction of the Palaeo-Pacific Ocean in the early Palaeozoic. Northern Victoria Land (NVL) in Antarctica was located at a central position of the Ross-Delamerian system. Its metamorphic basement is formed by three lithotectonic units formerly interpreted as terranes: the Wilson, Bowers and Robertson Bay terranes (from west to east). Dating of detrital zircons from 14 metasedimentary samples of these terranes combined with petrographical and whole-rock geochemical studies give new insights into the stratigraphic and tectonic evolution of NVL. All samples show very similar zircon age spectra with two main intervals, a Ross/Pan-African-age interval (470–700 Ma) and a Grenville-age interval (900–1300 Ma), as well as subordinate craton-related ages dispersed over the range of ca. 1600–3500 Ma. The Ross/Pan-African-age zircon population tends to get more dominant from the Priestley Formation of the Wilson Terrane to the Molar Formation of the Bowers Terrane, and finally to the Robertson Bay Group, whereas the number of craton-related ages diminishes in this direction. A common East Antarctic source area is indicated for all analyzed samples. The Priestley Formation was deposited on the Palaeo-Pacific passive continental margin of East Gondwana in the late Neoproterozoic after Rodinia breakup. The sequence was subsequently metamorphosed and intruded by the Granite Harbour Intrusives during the Ross Orogeny. The Molar Formation of the Bowers Terrane is interpreted as a turbiditic sequence deposited in an accretionary setting on the active continental margin in the Late Cambrian during and after accretion of the Glasgow island arc allochthon. The thick, homogeneous sequence of the Robertson Bay Group resulted from continuous turbiditic sedimentation in an accretionary wedge in front of the Ross Orogen after docking and imbrication of the Glasgow island arc in the Early Ordovician.

The Implications of the Cambrian Brachiopod Fauna From the Three Gorges area of China

The Cambrian brachiopod fauna from the first-trilobite age Shuijingtuo Formation in the Three Gorges area of China

Authors:

Zhang et al

Abstract:

The Yangtze platform of South China offers evidence within its Ediacaran–Cambrian geological record of the Cambrian explosion and diversification events in metazoan history. To understand the explosive radiation of animals and the environments in which it took place, the basal Cambrian fauna succession of the Aijiahe section in the Three Gorges area, western Hubei Province, has been studied, revealing the earliest brachiopod fauna (Tsunyidiscus trilobite Zone) in this region, which was dominated numerically by Acrotretoids. This is accompanied by abundant skeletal fossils including minute well-preserved phosphatized archaeocyath cups and an assortment of abundant sponge spicules, chancelloriids, mollusks, hyoliths, and bradoriids, retrieved by acid-etching limestone interbeds in the black shale-dominated Shuijingtuo Formation (Series 2). The brachiopods comprise two species of acrotretoids, two types of botsfordiids (Botsfordiidae gen. et sp. indet. A and B), and four species of linguloids. Of the latter, Spinobolus popovi n. gen. n. sp. is strikingly distinctive and typified by spine-like ornamentation seen for the first time in the Lower Cambrian; the remaining three linguloid genera, Palaeobolus, Eoobolus, and Lingulellotreta, have a trans-paleocontinental distribution. The Three Gorges Shuijingtuo brachiopod assemblage differs from that of the upper Atdabanian Stage (Cambrian Stage 3) in Siberia and South China, but shows great similarities with those discovered in the Tsanglangpuan (equivalent to Botoman or Stage 4) Stage of eastern Yunnan Province, Siberia, and South Australia, suggesting a much more prolonged sedimentary hiatus in basalmost Shuijingtuo Formation of the Three Gorges area than previously expected. The presence of such unconformities provides a caveat to stable isotope-based correlations that involve a number of discussions of global ocean geochemical changes across the time interval that witnessed Cambrian explosion of metazoans.

The BRAAAAAAAAAAAINNNNNSSSSS of Cambrian Euarhropod Fuxianhuia protensa

Preservational Pathways of Corresponding Brains of a Cambrian Euarthropod

Authors:

Ma et al

Abstract:

The record of arthropod body fossils is traceable back to the “Cambrian explosion,” marked by the appearance of most major animal phyla. Exceptional preservation provides crucial evidence for panarthropod early radiation. However, due to limited representation in the fossil record of internal anatomy, particularly the CNS, studies usually rely on exoskeletal and appendicular morphology. Recent studies [ 1–3 ] show that despite extreme morphological disparities, euarthropod CNS evolution appears to have been remarkably conservative. This conclusion is supported by descriptions from Cambrian panarthropods of neural structures that contribute to understanding early evolution of nervous systems and resolving controversies about segmental homologies [ 4–12 ]. However, the rarity of fossilized CNSs, even when exoskeletons and appendages show high levels of integrity, brought into question data reproducibility because all but one of the aforementioned studies were based on single specimens [ 13 ]. Foremost among objections is the lack of taphonomic explanation for exceptional preservation of a tissue that some see as too prone to decay to be fossilized. Here we describe newly discovered specimens of the Chengjiang euarthropod Fuxianhuia protensa with fossilized brains revealing matching profiles, allowing rigorous testing of the reproducibility of cerebral structures. Their geochemical analyses provide crucial insights of taphonomic pathways for brain preservation, ranging from uniform carbon compressions to complete pyritization, revealing that neural tissue was initially preserved as carbonaceous film and subsequently pyritized. This mode of preservation is consistent with the taphonomic pathways of gross anatomy, indicating that no special mode is required for fossilization of labile neural tissue.

Burgess Shale-type Organisms Found in Middle Cambrian UTah

New records of Burgess Shale-type taxa from the middle Cambrian of Utah

Authors:

Conway Morris et al

Abstract:

Cambrian strata of the Laurentian craton contain numerous examples of Burgess Shale–type faunas. Although displaying a more or less concentric distribution around the cratonal margin, most faunal occurrences are in present-day western North America, extending from the Northwest Territories to California. Nevertheless, the soft-bodied and lightly skeletalized fossils in most of these Lagerstätten are highly sporadic. Here, we extend knowledge of such Middle Cambrian occurrences in Utah with reports of four taxa. An arthropod from the Marjum Formation, Dytikosicula desmatae gen. et sp. nov., is a putative megacheiran. It is most similar to Dicranocaris guntherorum, best known from the younger Wheeler Formation, but differs primarily in the arrangement of pleurae and overall size. Along with a specimen of ?Yohoia sp, a new species of Yohoia, Y. utahana sp. nov., is described. It differs from the type and only known species, Y. tenuis, principally in its larger size and shorter exopods; it is the first description of this genus from outside the Burgess Shale. A new species of a stem-group lophotrochozoan from the Spence Shale, Wiwaxia herka sp. nov., possesses a palisade of dorso-lateral spines that are more robust and numerous than the type species of Wiwaxia, W. corrugata. Another notable taxon is Eldonia ludwigi from the Marjum Formation, which is interpreted as a primitive ambulacrarian (assigned to the cambroernids) and a new specimen of the ?cnidarian Cambrorhytium from the Wheeler Shale is illustrated.

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.

No Sharp Delineation of Oxic/Anoxic Regions Across Cambrian/Ordovician Boundary PaleoOceans

Redox conditions across the Cambrian–Ordovician boundary: Elemental and isotopic signatures retained in the GSSP carbonates

Authors:

Azmy et al

Abstract:

Lime mudstone samples (rhythmites) were collected at high resolution from outcrops of the Cambrian–Ordovician GSSP boundary section at Green Point (western Newfoundland, Canada). The sequence (~ 45 m-thick) consists of slope carbonates with alternating shale and siltstone interbeds, and it spans the boundary located between the Martin Point and Broom Point members of the Green Point Formation (Cow Head Group). Samples were extracted from micritic rhythmites by microdrilling and subsequently screened using petrographic and geochemical criteria to evaluate their degree of preservation. Although the δ13Corg profile (− 29.7 to − 25.6‰ VPDB) shows insignificant variations, the TOC values (0.1 to 4.1%) exhibit a generally upward decreasing trend. A negative δ13Ccarb excursion, reflecting a sealevel rise, marks a geochemical anomaly that correlates with an increase in the N contents (0 to 2.9%) of organic matter and the δ15Norg values (− 0.6 to + 6.0‰), which suggests a change to more reducing oceanic conditions. The U contents vary from 0.1 to 3.0 ppm and the δ238U values (− 0.97 to − 0.18‰) generally decrease with the negative δ13Ccarb excursion. The U isotopic variations suggest a widespread increase in reducing conditions associated with sealevel rise during this interval. The investigated sedimentary rocks were slope carbonates where dysoxic conditions likely dominated throughout the entire section. Therefore, the changes in the TOC, N, δ15Norg, and δ238U profiles across the boundary are not as sharp as would be expected by a local change from oxic shallow-water to dysoxic/anoxic deep-water settings.