Month: November 2022

How did the earliest animals feed?

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Among the strange early animals of the latest Precambrian, known as the Ediacaran fauna, is the slug-like Kimberella. Unlike most of its cohort, which are impressions in sediment or trace fossils,  Kimberella is a body fossil in which can be seen signs of a front and back, i.e. mouth and anus (See also: A lowly worm from the Ediacaran?). In that respect they are the same as us: bilaterians both. Indeed, Kimberella may be one of the oldest of our broad kind that we will ever be able to see. Rare examples have fans of grooves radiating from their ‘front’. It may have grated its food, a bit like a slug does, but drew it in to its mouth. Some enthusiasts have likened the little beasty to a JCB digger, able to rotate and rake stuff into its mouth. In that case, Kimberella would have moved ‘backwards’ while feeding. If it can be likened to any modern animals, it may be a simple mollusc.

A Kimberella fossil, about 10 centimetres long, and a speculative reconstruction showing its feeding apparatus.

Other Ediacaran animals show no such mouth-gut-anus symmetry. Some have tops and bases, but most show no symmetry at all, being flaccid bag-like creatures. Palaeontologists provisionally suggest that they are primitive sponges, ctenophores, placozoans and cnidarians. Such animals excrete through pores on their surfaces and draw food in either through a simple mouth or their skins. The early bilaterians probably ‘grazed’ on bacterial or algal mats, but until now that has been conjectural. Ilya Bobrovskiy of the Australian National University and colleagues from Russia and Australia have managed to extract and analyse biomarker chemicals contained in well-preserved specimens of three Ediacaran animals from strata on the White Sea coast of Russia (Bobrovskiy, I. et al. 2022. Guts, gut contents, and feeding strategies of Ediacaran animals. Current Biology, v. 32,   ; DOI: 10.1016/j.cub.2022.10.051). Biomarkers are molecules, such as fatty acids, phospholipids, triglycerides, hopanes and steranes, that definitively indicate metabolic processes of once living organisms, sometimes referred to as ‘molecular fossils’. Their varying proportions relative to one another are key to recognising the presence of different groups of organisms.

Specifically, hopane molecules are the best indicators of the former metabolism of bacteria whereas steranes (based on linked chains of carbon atoms bonded in rings) are typical products of degradation of sterols in eukaryotes. One sterane group involving 27 carbon atoms (C27 steranes) are typically formed when and animal dies and decays.   C28 and C29 steranes likely form when algae decay, as when they are digested in the gut of a herbivore. Specimens of one of the Ediacaran animals analysed by the team – Dickinsonia – contained far more C27 steranes than C28 and C29, a sign of biomarkers associated with its decay. It probably absorbed food, weirdly, through its skin. Kimberella and a worm-like animal – Calyptrina – had sterane proportions which suggested that they digested algae or bacteria in a gut, as befits bilaterians. Simple as they may appear, these are among the earliest ancestors of modern animals, including us: of course!

See also: Lu, D. 2022. The real paleo diet: researchers find traces of world’s oldest meal in 550m-year-old fossil. The Guardian, 22 November 2022.;  World’s oldest meal helps unravel mystery of our earliest animal ancestors. scimex, 23 November 2022

Early land plants and oceanic extinctions

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In September 2022 Earth-logs highlighted how greening of the continents affected the composition of the continental crust. It now seems that was not the only profound change that the first land plants wrought on the Earth system. Beginning in the Silurian, the spread of vegetation swept across the continents during the Devonian Period. From a height of less than 30 cm among the earliest species by the Late Devonian the stature of plants went through a large increase with extensive forests of primitive tree-sized conifers, cycads, horsetails and sporiferous lycopods up to 10 m tall. Their rapid evolution and spread was not hampered by any herbivores. It was during the Devonian that tetrapod amphibians emerged from the seas, probably feeding on burgeoning terrestrial invertebrates. The Late Devonian was marked by five distinct episodes of extinction, two of which comprise the Devonian mass extinction: one of the ‘Big Five’. This affected both marine and terrestrial organisms. Neither flood volcanism nor extraterrestrial impact can be linked to the extinction episodes. Rather they marked a long drawn-out period of repeated environmental stress.

Phytoplankton bloom off the east coast of Scotland ‘fertilised’ by effluents carried by the Tay and Forth estuaries.

One possibility is that a side effect of the greening of the land was the release of massive amounts of nutrients to the seas that would have resulted in large-scale blooms of phytoplankton whose death and decay depleted oxygen levels in the water column. That is a process seen today where large amounts of commercial fertilisers end up in water bodies to result in their eutrophication. Matthew Smart and others from Indiana University-Purdue University, USA and the University of Southampton, UK, geochemically analysed Devonian lake deposits from Greenland and Scotland to test this hypothesis (Smart, M.S. et al. 2022. Enhanced terrestrial nutrient release during the Devonian emergence and expansion of forests: Evidence from lacustrine phosphorus and geochemical records. Geological Society of America Bulletin, v. 134, early release article;  DOI: 10.1130/B36384.1).

Smart et al. show that in the Middle and Late Devonian the lacustrine strata show cycles in their abundance of phosphorus (P an important plant nutrient) that parallel evidence for wet and dry cycles in the lacustrine basins. The cycles show that the same phosphorus abundance patterns occurred at roughly the same times at five separate sites. This may suggest a climatic control forced by changes in Earth’s orbital behaviour, similar to the Milankovich Effect on the Pleistocene climate and at other times in Phanerozoic history. The wet and dry intervals show up in the changing ratio between strontium and copper abundances (Sr/Cu): high values signify wet conditions, low suggesting dry. The wet periods show high ratios of rubidium to strontium (Rb/Sr) that suggest enhanced weathering, while dry periods show the reverse – decreased weathering.

When conditions were dry and weathering low, P built up in the lake sediments, whereas during wet conditions P decreases; i.e. it was exported from the lakes, presumably to the oceans. The authors interpret the changes in relation to the fate of plants under the different conditions. Dry periods would result in widespread death of plants and their rotting, which would release their P content to the shallowing, more stagnant lakes. When conditions were wetter root growth would have increased weathering and more rainfall would flush P from the now deeper and more active lake basins. The ultimate repository of the sediments and freshwater, the oceans, would therefore be subject to boom and bust (wet and dry) as regards nutrition and phytoplankton blooms. Dead phytoplankton, in turn, would use up dissolved oxygen during their decay. That would lead to oceanic anoxia, which also occurred in pulses during the Devonian, that may have contributed to animal extinction.

See also: Linking mass extinctions to the expansion and radiation of land plants, EurekaAlert 10 November 2022; Mass Extinctions May Have Been Driven by the Evolution of Tree Roots, SciTechDaily, 14 November 2022.