ALBERT

Description: Framboidal pyrite has been used as a paleo-redox proxy and a biomarker in ancient sediments, but the interpretation of pyrite framboids can be controversial, especially where later overgrowths have obscured primary textures. Here we show how nano-scale chemical mapping of organic carbon and nitrogen (CN org ) can detect relict framboids within Precambrian pyrite grains and determine their formation mechanism. Pyrite grains associated with an Ediacaran fossil Lagerstätte from Newfoundland (ca. 560 Ma) hold significance for our understanding of taphonomy and redox history of the earliest macrofossil assemblages. They show distinct chemical zoning with respect to CN org . Relict framboids are revealed as spheroidal zones within larger pyrite grains, whereby pure pyrite microcrystals are enclosed by a mesh-like matrix of pyrite possessing elevated CN org , replicating observations from framboids growing within modern biofilms. Subsequent pyrite overgrowths also incorporated CN org from biofilms, with concentric CN org zoning showing that the availability of CN org progressively decreased during later pyrite growth. Multiple framboids are commonly cemented together by these overgrowths to form larger grains, with relict framboids only detectable in CN org maps. In situ sulfur isotope data ( 34 S = ~–24 to –15) show that the source of sulfur for the pyrite was also biologically mediated, most likely via a sulfate-reducing microbial metabolism within the biofilms. Relict framboids have significantly smaller diameters than the pyrite grains that enclose them, suggesting that the use of framboid diameters to infer water column paleo-redox conditions should be approached with caution. This work shows that pyrite framboids have formed within organic biofilms for at least 560 m.y., and provides a novel methodology that could readily be extended to search for such biomarkers in older rocks and potentially on other planets.

Description: The first appearance of animals in the geological record is a matter of continuing debate: how deep were the roots of the Cambrian explosion? Molecular clock estimates indicate that the deepest divergences of the Metazoa had occurred by the Ediacaran Period (635–541 Ma), yet evidence of animal activity from well below the Ediacaran-Cambrian boundary has been rare and often questionable. Meanwhile, the Ediacaran macrobiota has remained enigmatic, as emphasized by recent controversial claims that South Australia Ediacaran forms were not marine animals at all, but land-based lichens and microbial colonies. Here we report evidence for animal-like behavior in a submerged setting in a key Ediacaran form, Aspidella terranovica Billings 1872, a discoidal fossil from the ca. 560 Ma Fermeuse Formation of Newfoundland (Canada). We describe sedimentary fabrics indicating progressive vertical movement of an organism through sediment in response to an aggrading sediment-water interface. Such equilibrium traces are familiar from the Phanerozoic and are observed in partially buried marine animals such as tube anemones today. Furthermore, horizontal trails closely comparable to trails previously described from ~565 m.y. old Mistaken Point (Newfoundland) are now linked to Aspidella . Our findings constitute evidence of both vertical and horizontal movement in a key Ediacaran taxon, consistent with an animal of cnidarian grade. Moreover, because Aspidella is also reported from the Rawnsley Quartzite of South Australia, our evidence conflicts with the proposed radical interpretation of that Ediacaran fossil assemblage. We demonstrate that at least some Ediacaran forms were probably early animals, and that they lived underwater.