Tribrachidium

Abstract: A new interpretation of fossilized soils (palaeosols) suggests that at least some Ediacaran (625–542 million years ago) organisms lived on land; thus these Ediacaran fossils were not animals, but a fungus-dominated terrestrial biota that predated vascular plants by about 100 million years. Ediacaran fossils —542 to 635 million years old — occur worldwide in a variety of sedimentary deposits, generally interpreted as shallow to deep marine origin. They have been regarded as early animal ancestors of the Cambrian evolutionary explosion of marine invertebrate phyla, as giant marine protists and as lichenized fungi. Here, Gregory Retallack raises doubts over the assumption that Ediacaran organisms were marine: a new interpretation of fossilized soils ('palaeosols') from South Australia suggests that at least some lived on land. Retallack's suggestion that some Ediacaran fossils were large sessile organisms of cool, dry soils is compatible with observations that Ediacaran fossils are similar in appearance and preservation to lichens and other microbial colonies of biological soil crusts, rather than marine animals or protists. Ediacaran (635–542 million years ago) fossils have been regarded as early animal ancestors of the Cambrian evolutionary explosion of marine invertebrate phyla1, as giant marine protists2 and as lichenized fungi3. Recent documentation of palaeosols in the Ediacara Member of the Rawnsley Quartzite of South Australia4 confirms past interpretations of lagoonal–aeolian deposition based on synsedimentary ferruginization and loessic texture5,6. Further evidence for palaeosols comes from non-marine facies, dilation cracks, soil nodules, sand crystals, stable isotopic data and mass balance geochemistry4. Here I show that the uppermost surfaces of the palaeosols have a variety of fossils in growth position, including Charniodiscus, Dickinsonia, Hallidaya, Parvancorina, Phyllozoon, Praecambridium, Rugoconites, Tribrachidium and ‘old-elephant skin’ (ichnogenus Rivularites7). These fossils were preserved as ferruginous impressions, like plant fossils8, and biological soil crusts9,10 of Phanerozoic eon sandy palaeosols. Sand crystals after gypsum11 and nodules of carbonate12 are shallow within the palaeosols4, even after correcting for burial compaction13. Periglacial involutions and modest geochemical differentiation of the palaeosols are evidence of a dry, cold temperate Ediacaran palaeoclimate in South Australia4. This new interpretation of some Ediacaran fossils as large sessile organisms of cool, dry soils, is compatible with observations that Ediacaran fossils were similar in appearance and preservation to lichens and other microbial colonies of biological soil crusts3, rather than marine animals1, or protists2.