Systematic extinctions can leave major morphological gaps between living crown‐group clades. Such morphological gaps would be perceived, from a neontological point of view, as major evolutionary ...transitions. In order to fill these morphological gaps and to map the evolutionary steps toward major evolutionary transitions, we need to integrate extinct stem‐group taxa in phylogenetic studies. However, the recognition of stem group has not been widely adopted in the study of early animal fossils, despite that all fossils are stem groups at one level or another. Part of the difficulty is that stem groups may not have all features that collectively diagnose the respective crown group, and they can have unique (autapomorphic) features, making them tantalizingly similar to and frustratingly different from the crown group (e.g., stem‐group eukaryotes can be prokaryotic and stem‐group animals can be protistan). The need to embrace stem groups and to implement the PhyloCode, in order to achieve phylogenetic clarity and to offer key paleontological insights into the origin and early animal evolution, is illustrated in debates on several controversial Ediacaran and Cambrian fossils.
Ediacara fossils document an important evolutionary episode just before the Cambrian explosion and hold critical information about the early evolution of macroscopic and complex multicellular life. ...They also represent an enduring controversy in paleontology. How are the Ediacara fossils related to living animals? How did they live? Do they share any evolutionary patterns with other life forms? Recent developments indicate that Ediacara fossils epitomize a phylogenetically diverse biosphere, probably including animals, protists, algae, fungi and others. Their simple ecology is dominated by epibenthic osmotrophs, deposit feeders and grazers, but few if any predators. Their evolution started with an early morphospace expansion followed by taxonomic diversification within confined morphospace, and concluded by extinction of many taxa at the Ediacaran–Cambrian boundary.
The colonization of land by fungi had a significant impact on the terrestrial ecosystem and biogeochemical cycles on Earth surface systems. Although fungi may have diverged ~1500-900 million years ...ago (Ma) or even as early as 2400 Ma, it is uncertain when fungi first colonized the land. Here we report pyritized fungus-like microfossils preserved in the basal Ediacaran Doushantuo Formation (~635 Ma) in South China. These micro-organisms colonized and were preserved in cryptic karstic cavities formed via meteoric water dissolution related to deglacial isostatic rebound after the terminal Cryogenian snowball Earth event. They are interpreted as eukaryotes and probable fungi, thus providing direct fossil evidence for the colonization of land by fungi and offering a key constraint on fungal terrestrialization.
The deep-water Avalon biota (about 579 to 565 million years old) is often regarded as the earliest-known fossil assemblage with macroscopic and morphologically complex life forms. It has been ...proposed that the rise of the Avalon biota was triggered by the oxygenation of mid-Ediacaran deep oceans. Here we report a diverse assemblage of morphologically differentiated benthic macrofossils that were preserved largely in situ as carbonaceous compressions in black shales of the Ediacaran Lantian Formation (southern Anhui Province, South China). The Lantian biota, probably older than and taxonomically distinct from the Avalon biota, suggests that morphological diversification of macroscopic eukaryotes may have occurred in the early Ediacaran Period, perhaps shortly after the Marinoan glaciation, and that the redox history of Ediacaran oceans was more complex than previously thought.
Chlorophytes (representing a clade within the Viridiplantae and a sister group of the Streptophyta) probably dominated marine export bioproductivity and played a key role in facilitating ecosystem ...complexity before the Mesozoic diversification of phototrophic eukaryotes such as diatoms, coccolithophorans and dinoflagellates. Molecular clock and biomarker data indicate that chlorophytes diverged in the Mesoproterozoic or early Neoproterozoic, followed by their subsequent phylogenetic diversification, multicellular evolution and ecological expansion in the late Neoproterozoic and Palaeozoic. This model, however, has not been rigorously tested with palaeontological data because of the scarcity of Proterozoic chlorophyte fossils. Here we report abundant millimetre-sized, multicellular and morphologically differentiated macrofossils from rocks approximately 1,000 million years ago. These fossils are described as Proterocladus antiquus new species and are interpreted as benthic siphonocladalean chlorophytes, suggesting that chlorophytes acquired macroscopic size, multicellularity and cellular differentiation nearly a billion years ago, much earlier than previously thought.
The origin of motility in bilaterian animals represents an evolutionary innovation that transformed the Earth system. This innovation probably occurred in the late Ediacaran period-as evidenced by an ...abundance of trace fossils (ichnofossils) dating to this time, which include trails, trackways and burrows
. However, with few exceptions
, the producers of most of the late Ediacaran ichnofossils are unknown, which has resulted in a disconnection between the body- and trace-fossil records. Here we describe the fossil of a bilaterian of the terminal Ediacaran period (dating to 551-539 million years ago), which we name Yilingia spiciformis (gen. et sp. nov). This body fossil is preserved along with the trail that the animal produced during a death march. Yilingia is an elongate and segmented bilaterian with repetitive and trilobate body units, each of which consists of a central lobe and two posteriorly pointing lateral lobes, indicating body and segment polarity. Yilingia is possibly related to panarthropods or annelids, and sheds light on the origin of segmentation in bilaterians. As one of the few Ediacaran animals demonstrated to have produced long and continuous trails, Yilingia provides insights into the identity of the animals that were responsible for Ediacaran trace fossils.
The Ediacaran Period is characterized by the most profound negative carbon isotope (δ13C) excursion in Earth history, the Shuram Excursion. Various hypotheses – including the massive oxidation of ...dissolved organic carbon (DOC) in the oceans, the weathering of terrestrial organic carbon, or the release and oxidation of methane hydrates and/or expelled petroleum from the subsurface – have been proposed as sources of the 13C-depleted carbon. More recently, it has been suggested that global-scale precipitation of early authigenic carbonates, driven by anaerobic microbial metabolism in unconsolidated sediments, may have caused the Shuram Excursion, but empirical evidence is lacking. Here we present a comprehensive analysis of a Shuram-associated interval from the uppermost Doushantuo Formation in South China. Our study reveals petrographic evidence of methane-derived authigenic calcite (formed as early diagenetic cements and nodules) that are remarkably depleted in 13C – suggesting a buildup of alkalinity in pore fluids through the anaerobic oxidation of methane (AOM) – and systematically depleted in 18O relative to co-occurring dolomite. Early authigenesis of these minerals is likely to be driven by increased microbial sulfate reduction, triggered by enhanced continental weathering in the context of a marked rise in atmospheric oxygen levels. In light of the finding of methane-derived authigenic carbonates at Zhongling, and based on our basin-scale stratigraphic correlation, we hypothesize that the marked 13C and 18O depletion (including their co-variation noted worldwide) in the Shuram Excursion may reflect an episode of authigenesis occurring within a sulfate–methane transition zone (SMTZ). If true, the Shuram Excursion was then a global biogeochemical response to enhanced seawater sulfate concentration in the Ediacaran ocean driven by the Neoproterozoic oxidation of surface environments. This paleo-oceanographic transition may have therefore paved the way for subsequent evolution and diversification of animals. Our study highlights the significance of an integrated approach that combines petrography, mineralogy, and texture-specific micro-drilling geochemistry in chemostratigraphic studies.
•Pulses of methane-derived authigenic carbonates formation•An early diagenetic origin for the Shuram Excursion•Sulfate-methane transition zone near the seafloor•Positive δ13C–δ18O correlation due to early authigenic mineralization
The Ediacaran Doushantuo Formation (ca. 635–551
Ma) in South China contains exceptionally well-preserved fossils of multicellular eukaryotes including early animals, and it is one of the most ...intensively investigated Ediacaran units in the world. Various stratigraphic methods including litho-, chemo-, bio-, and sequence-stratigraphy have been applied to establish a stratigraphic framework for the Doushantuo Formation, but so far regional correlation across the basin relies heavily on two distinctive marker beds, the cap carbonate at the base and the organic-rich black shale at the top of the Doushantuo Formation. The majority of the Doushantuo Formation in the Yangtze platform was deposited on a rimmed carbonate shelf, with a shelf margin shoal complex that restricted the shelf lagoon from the open ocean. Large facies variations are observed in the shallow margins of the shelf lagoon and in the shelf margin-to-slope transition, where depositional environments were near the chemocline of the stratified, anoxic/euxinic shelf lagoon and of the broader Nanhua basin, respectively. Chemocline instability in the shelf lagoon and in the Nanhua basin caused local geochemical cycling, resulting in significant variations in carbon and sulfur isotopes and in redox-sensitive elemental concentrations. Most benthic eukaryotic fossils (including animal fossils) of the Doushantuo Formation have been found from the shallow margins of the shelf lagoon and from the shelf margin–slope transition, but rarely from deep-water environments that may have been below the chemocline for most of the Doushantuo time, implying the sensitivity of eukaryotes to paleogeographically controlled chemocline fluctuations.
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► Environmental restriction during Doushantuo deposition. ► Geochemical variability across the Doushantuo basin. ► Chemocline instability and Ediacaran biotas.
Recent geochemical data from Oman, Newfoundland, and the western United States suggest that long-term oxidation of Ediacaran oceans resulted in progressive depletion of a large dissolved organic ...carbon (DOC) reservoir and potentially triggered the radiation of acanthomorphic acritarchs, algae, macroscopic Ediacara organisms, and, subsequently, motile bilaterian animals. However, the hypothesized coupling between ocean oxidation and evolution is contingent on the reliability of continuous geochemical and paleontological data in individual sections and of intercontinental correlations. Here we report high-resolution geochemical data from the fossil-rich Doushantuo Formation (635-551 Ma) in South China that confirm trends from other broadly equivalent sections and highlight key features that have not been observed in most sections or have received little attention. First, samples from the lower Doushantuo Formation are characterized by remarkably stable $\delta {}^{13}{\rm C}{}_{\text{org}}$ (carbon isotope composition of organic carbon) values but variable $\delta {}^{34}{\rm S}{}_{{\rm CAS}}$ (sulfur isotope composition of carbonate-associated sulfate) values, which are consistent with a large isotopically buffered DOC reservoir and relatively low sulfate concentrations. Second, there are three profound negative $\delta {}^{13}{\rm C}{}_{\text{carb}}$ (carbon isotope composition of carbonate) excursions in the Ediacaran Period. The negative $\delta {}^{13}{\rm C}{}_{\text{carb}}$ excursions in the middle and upper Doushantuo Formation record pulsed oxidation of the deep oceanic DOC reservoir. The oxidation events appear to be coupled with eukaryote diversity in the Doushantuo basin. Comparison with other early Ediacaran basins suggests spatial heterogeneity of eukaryote distribution and redox conditions. We hypothesize that the distribution of early Ediacaran eukaryotes likely tracked redox conditions and that only after ≈551 Ma (when Ediacaran oceans were pervasively oxidized) did evolution of oxygen-requiring taxa reach global distribution.
Phosphatized and silicified microfossils – preserved through replication of organic templates by authigenic calcium phosphate and silica, respectively – form through mechanistically similar ...mineralization processes but under notably different conditions. In order to characterize the environments and taphonomic processes associated with these preservational pathways, we compare phosphatized and silicified fossil assemblages in the Ediacaran Doushantuo Formation (South China) in terms of stratigraphic and paleogeographic distribution, preservation, and biodiversity. Several differences are recognized: (1) phosphatization and silicification both occurred in sub-tidal Doushantuo sediments, but phosphatized fossils generally occur in shallower sub-tidal facies than silicified fossils; (2) although both phosphatized and silicified fossils are preserved three dimensionally, the latter are typically more compressed; and (3) whereas phosphatized assemblages are numerically dominated by multicellular acritarchs (putative metazoan eggs and embryos), silicified assemblages are dominated by prokaryotes. These differences can be explained by environmental and taphonomic distinctions. Phosphatized fossils occur in intraclastic phosphorites and phosphatic carbonates, and were likely fossilized near the suboxic/anoxic boundary in sediments under oxic bottom waters with redox and biogeochemical gradients conducive to P remineralization, phosphate burial via iron-pumping, and calcium phosphate precipitation. Reworking and winnowing of phosphatized fossils preferentially concentrated certain taxa prior to final burial. In contrast, silicified fossils typically occur in chert nodules, which formed deeper in the sediment column – in ferruginous pore waters and perhaps under ferruginous bottom waters – where bacterial sulfate reduction and pyrite formation created low-pH conditions conducive to calcite dissolution and silica precipitation. Once formed, chert nodules mostly remained in situ, but some were reworked with turbidites and olistostromes developed in slope facies. Despite these distinctions, phosphatized and silicified fossil assemblages in the Doushantuo Formation generally overlap in time and space, and representatives of all major groups (and many common genera) of Doushantuo organisms are present in both phosphorites and cherts. Consequently, although both preservational pathways have paleoenvironmental and taphonomic biases, their relative biases on taxonomic presence/absence data are minimal given sufficient sampling efforts, and paleobiologic and biostratigraphic information based on presence/absence (but not abundance) data from the Doushantuo Formation appears to be robust.
•We compare phosphatized and silicified assemblages in the Ediacaran Doushantuo Formation.•Geographic, preservational, and biodiversity distinctions are related to environments.•Phosphatized fossils formed in shallow sediments underlying oxic bottom waters.•Silicified fossils formed deeper in sediments or underneath ferruginous bottom waters.•Taxonomic presence/absence data are robust.
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