Microfossils from the late Mesoproterozoic - early Neoproterozoic Atar/EI Mreiti Group, Taoudeni Basin, Mauritania, northwestern Africa

The well-preserved Meso-Neoproterozoic shallow marine succession of the Atar/EI Mreiti Group, in the Taoudeni Basin, Mauritania, offers a unique opportunity to investigate the mid-Proterozoic eukaryotic record in Western Africa. Previous investigations focused on stromatolites, biomarkers, chemostratigraphy and palaeoredox conditions. However, only a very modest diversity of organic-walled microfossils (acritarchs) has been documented. Here, we present a new, exquisitely well-preserved and morphologically diverse assemblage of organic-walled microfossils from three cores drilled through the Atar/El Mreiti Group. A total of 48 distinct entities including 11 unambiguous eukaryotes (ornamented and process-bearing acritarchs), and 37 taxonomically unresolved taxa (including 9 possible eukaryotes, 6 probable prokaryotes, and 22 other prokaryotic or eukaryotic taxa) were observed. Black shales preserve locally abundant fragments of organic-rich laminae interpreted as benthic microbial mats. We also document one of the oldest records of Leiosphaeridia kulgunica, a species showing a circular opening interpreted as a sophisticated circular excystment structure (a pylome), and one of the oldest records of Trachyhystrichosphaera aimika and T. botula, two distinctive process-bearing acritarchs present in well dated 1.1 Ga formations at the base of the succession. The general assemblage composition and the presence of three possible index fossils (A. tetragonala, S. segmentata and T. aimika) support a late Mesoproterozoic to early Neoproterozoic (Tonian) age for the Atar/El Mreiti Group, consistent with published lithostratigraphy, chemostratigraphy and geochronology. This study provides the first evidence for a moderately diverse eukaryotic life, at least 1.1 billion years ago in Western Africa. Comparison with coeval worldwide assemblages indicates that a broadly similar microbial biosphere inhabited (generally redox-stratified) oceans, placing better time constraints on early eukaryote palaeogeography and biostratigraphy

Foraminifera in elevated Bermudian caves provide further evidence for +21 m eustatic sea level during Marine Isotope Stage 11

Two hypotheses have been proposed to explain the origin of marine isotope stage (MIS) 11 deposits in small Bermudian caves at +21 m above modern sea level: (1) a +21 m MIS 11 eustatic sea-level highstand, and (2) a MIS 11 mega-tsunami event. Importantly, the foraminifera reported in these caves have yet to be critically evaluated within a framework of coastal cave environments. After statistically comparing foraminifera in modern Bermudian littoral caves and the MIS 11 Calonectris Pocket A (+21 m cave) to the largest available database of Bermudian coastal foraminifera, the assemblages found in modern littoral caves – and Calonectris Pocket A – cannot be statistically differentiated from lagoons. This observation is expected considering littoral caves are simply sheltered extensions of a lagoon environment in the littoral zone, where typical coastal processes (waves, storms) homogenize and rework lagoonal, reefal, and occasional planktic taxa. Fossil protoconchs of the Bermudian cave stygobite Caecum caverna were also associated with the foraminifera. These results indicate that the MIS 11 Bermudian caves are fossil littoral caves (breached flank margin caves), where the total MIS 11 microfossil assemblage is preserving a signature of coeval sea level at +21 m. Brackish foraminifera (Polysaccammina, Pseudothurammina) and anchialine gastropods (w95%, >300 individuals) indicate a brackish anchialine habitat developed in the elevated caves after the prolonged littoral environmental phase. The onset of sea-level regression following the +21 m highstand would first lower the ancient brackish Ghyben-Herzberg lens (<0.5 m) and flood the cave with brackish water, followed by drainage of the cave to create a permanent vadose environment. These interpretations of the MIS 11 microfossils (considering both taphonomy and paleoecology) are congruent with the micropaleontological, hydrogeological and physical mechanisms influencing modern Bermudian coastal cave environments. In conclusion, we reject the mega-tsunami hypothesis, concur with the +21 m MIS 11 eustatic sea-level hypothesis, and reiterate the need to resolve the disparity between global marine isotopic records and the physical geologic evidence for sea level during MIS 11

Eukaryotic organisms in Proterozoic oceans

The geological record of protists begins well before the Ediacaran and Cambrian diversification of animals, but the antiquity of that history, its reliability as a chronicle of evolution and the causal inferences that can be drawn from it remain subjects of debate. Well-preserved protists are known from a relatively small number of Proterozoic formations, but taphonomic considerations suggest that they capture at least broad aspects of early eukaryotic evolution. A modest diversity of problematic, possibly stem group protists occurs in ca 1800–1300 Myr old rocks. 1300–720 Myr fossils document the divergence of major eukaryotic clades, but only with the Ediacaran–Cambrian radiation of animals did diversity increase within most clades with fossilizable members. While taxonomic placement of many Proterozoic eukaryotes may be arguable, the presence of characters used for that placement is not. Focus on character evolution permits inferences about the innovations in cell biology and development that underpin the taxonomic and morphological diversification of eukaryotic organisms

Onset of plate tectonics on Earth and implications for habitability

International audienc

1.1-billion-year-old porphyrins establish a marine ecosystem dominated by bacterial primary producers

The average cell size of marine phytoplankton is critical for the flow of energy and nutrients from the base of the food web to higher trophic levels. Thus, the evolutionary succession of primary producers through Earth's history is important for our understanding of the radiation of modern protists similar to 800 million years ago and the emergence of eumetazoan animals similar to 200 million years later. Currently, it is difficult to establish connections between primary production and the proliferation of large and complex organisms because the mid-Proterozoic (similar to 1,800-800 million years ago) rock record is nearly devoid of recognizable phytoplankton fossils. We report the discovery of intact porphyrins, the molecular fossils of chlorophylls, from 1,100-million-year-old marine black shales of the Taoudeni Basin (Mauritania), 600 million years older than previous findings. The porphyrin nitrogen isotopes (delta N-15(por) = 5.6-10.2 parts per thousand) are heavier than in younger sedimentary sequences, and the isotopic offset between sedimentary bulk nitrogen and porphyrins (epsilon(por) = -5.1 to -0.5 parts per thousand) points to cyanobacteria as dominant primary producers. Based on fossil carotenoids, anoxygenic green (Chlorobiacea) and purple sulfur bacteria (Chromatiaceae) also contributed to photosynthate. The low epsilon(por) values, in combination with a lack of diagnostic eukaryotic steranes in the time interval of 1,600-1,000 million years ago, demonstrate that algae played an insignificant role in mid-Proterozoic oceans. The paucity of algae and the small cell size of bacterial phytoplankton may have curtailed the flow of energy to higher trophic levels, potentially contributing to a diminished evolutionary pace toward complex eukaryotic ecosystems and large and active organisms