Up-to-date review of animals before the Cambrian ‘Explosion’

Artist’s impression of the Ediacaran Fauna (credit: Science)

Since I began this blog in 2000 one of my most regular topics concerns the animals of the latest Precambrian: the Ediacaran fauna. If you want to browse through the items use ‘Ediacaran’ in the Search Earth-logs box. New material and ideas about those precursors to modern life forms (and some that are still puzzling) appear on a regular basis. Science journalist Traci Watson has just summarised the latest developments in an essay for Nature. It is a nicely written and copiously illustrated piece with lots of links. Rather than precis her article, I suggest that you go straight to it, if the topic piques your interest.

(Watson, T. 2020. The bizarre species that are rewriting animal evolution. Nature, v. 586, p. 662-665; DOI: 10.1038/d41586-020-02985-z)

Geochemistry and the Ediacaran animals

Hopefully, readers will be fairly familiar with the sudden appearance of the Ediacaran fauna – the earliest abundant, large animals – at the start of the eponymous Period of the Neoproterozoic around 635 Ma. If not, use the Search Earth-logs box in the side bar to find extensive coverage since the start of the 21st century. A June 2019 Earth-logs review of the general geochemical background to the Ediacaran Period can be found here. Ten years ago I covered the possible role of the element phosphorus (P) – the main topic here – in the appearance of metazoans (see: Phosphorus, Snowball Earth and origin of metazoans – November 2010).

One of the major changes in marine sedimentation seen during the Ediacaran was a rapid increase in the deposition on the ocean floor of large bodies of P-rich rock (phosphorite), on which a recent paper focuses (Laakso, T.A. et al. 2020. Ediacaran reorganization of the marine phosphorus cycle. Proceedings of the National Academy of Sciences, v. 117, p. 11961-11967; DOI: 10.1073/pnas.1916738117). It has been estimated that on million-year time scales phosphorites remove only a tiny amount of the phosphorus carried into the oceans by rivers. So, conversely, an increase in deposition of marine P-rich sediment would have little effect on the overall availability of this essential nutrient from the oceans. The Ediacaran boost in phosphorites suggests a connection between them and the arrival of totally new ecosystems: the global P-cycle must somehow have changed. This isn’t the only change in Neoproterozoic biogeochemistry. Thomas Laakso and colleagues note signs of slightly increased ocean oxygenation from changes in sediment trace-element concentrations, a major increase in shallow-water evaporites dominated by calcium sulfate (gypsum) and changes in the relative proportions of different isotopes of sulfur.

Because all marine cycles, both geochemical and those involving life, are interwoven, the authors suggest that changes in the fate of dead organic matter may have created the phosphorus paradox. Phosphorus is the fifth most abundant element in all organisms after carbon, hydrogen, nitrogen and oxygen, followed by sulfur (CHNOPS), P being a major nutrient that limits the sheer bulk of marine life. Perhaps changes to dead organic matter beneath the ocean floor released its phosphorus content, roughly in the manner that composting garden waste releases nutrients back to the soil. Two chemical mechanisms can do this in the deep ocean: a greater supply of sinking organic matter – essentially electron donors – and of oxidants that are electron acceptors. In ocean-floor sediments organic matter can be altered to release phosphorus bonded in organic molecules into pore water and then to the body of the oceans to rise in upwellings to the near surface where photosynthesis operates to create the base of the ecological food chain.

Caption The Gondwana supercontinent that accumulated during the Neoproterozoic to dominate the Earth at the time of the Ediacaran (credit: Fama Clamosa, at Wikimedia Commons)

There is little sign of much increase in deep-ocean oxygen until hundreds of million years after the Ediacaran. It is likely, therefore, that increased availability of oxidant sulfate ions (SO42-) in ocean water and their reduction to sulfides in deep sediment chemically reconstituted the accumulating dead organic matter to release P far more rapidly than before. This is supported by the increase in CaSO4 evaporites in the Ediacaran shallows. So, where did the sulfate come from? Compressional tectonics during the Neoproterozoic Era were at a maximum, particularly in Africa, South America, Australia and Antarctica, as drifting continental fragments derived from the break-up of the earlier Rodinia supercontinent began to collide. This culminated during the Ediacaran around 550 Ma ago with assembly of the Gondwana supercontinent. Huge tracts of it were new mountain belts whose rapid erosion and chemical weathering would have released plenty of sulfate from the breakdown of common sulfide minerals.

So the biological revolution and a more productive biosphere that are reflected in the Ediacaran fauna ultimately may have stemmed from inorganic tectonic changes on a global scale

Geochemical background to the Ediacaran explosion

The first clear and abundant signs of multicelled organisms appear in the geological record during the 635 to 541 Ma Ediacaran Period of the Neoproterozoic, named from the Ediacara Hills of South Australia where they were first discovered in the late 19th century. But it wasn’t until 1956, when schoolchildren fossicking in Charnwood Forest north of Leicester in Britain found similar body impressions in rocks that were clearly Precambrian age that it was realised the organism predated the Cambrian Explosion of life. Subsequently they have turned-up on all continents that preserve rocks of that age (see: Larging the Ediacaran, March 2011). The oldest of them, in the form of small discs, date back to about 610 Ma, while suspected embryos of multicelled eukaryotes are as old as the very start of the Edicaran (see; Precambrian bonanza for palaeoembryologists, August 2006).

Artist’s impression of the Ediacaran Fauna (credit: Science)

The Ediacaran fauna appeared soon after the Marinoan Snowball Earth glaciogenic sediments that lies at the top of the preceding Cryogenian Period (650-635 Ma), which began with far longer Sturtian glaciation (715-680 Ma). A lesser climatic event – the 580 Ma old Gaskiers glaciation – just preceded the full blooming of the Ediacaran fauna. Geologists have to go back 400 million years to find an earlier glacial epoch at the outset of the Palaeoproterozoic. Each of those Snowball Earth events was broadly associated with increased availability of molecular oxygen in seawater and the atmosphere. Of course, eukaryote life depends on oxygen. So, is there a connection between prolonged, severe climatic events and leaps in the history of life? It does look that way, but begs the question of how Snowball Earth events were themselves triggered. Continue reading “Geochemical background to the Ediacaran explosion”