How did the earliest animals feed?

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

A lowly worm from the Ediacaran?

Humans are more or less symmetrical, our left and right sides closely resembling each other. That is not so comprehensive for our innards, except for testes and ovaries, kidneys, lungs, arteries and veins, lymph and nervous systems. We have front- and rear ends, top and bottom, input and output orifices. All that we share with almost all other animals from mammals to worms, particularly at the earliest, embryonic stage of development. We are bilaterians, whereas sponges, ctenophores, placozoans and cnidarians are not – having either no symmetry at all, or just a bottom and a top – and are in a minority.  Fossil collections from Cambrian times also reveal bilaterians in the majority, at least insofar as preservation allows us to tell. Before 541 Ma ago, in the Precambrian, there are few signs of such symmetry and faunas are dominated by the flaccid, bag like creatures that form much of the Ediacaran Fauna, although there are traces of creatures that could move and graze, and had a rudimentary sense of direction (see: Burrowers: knowing front from back, July 2012 and Something large moved 2 billion years ago). Unsurprisingly, palaeobiologists would like to know when ‘our lot’ arose. One route is via comparative genetics among living animals, using DNA differences and the ‘molecular clock’ approach to estimate the age of evolutionary separation between ‘us’ and ‘them’. But the spread of estimated ages is so broad as to render them almost meaningless. And the better constrained ages of very old trace fossils rely on accepting an assumption that they were, indeed, formed by bilaterians. Yet ingenuity may have revealed an actual early bilaterian from such traces.lowly worm

Palaeobiologists from the US and Australia have scoured the famous Ediacara Hills of South Australia for traces of burrowing and signs of the animal that did it (Evans, S.D. et al. 2020. Discovery of the oldest bilaterian from the Ediacaran of South Australia. Proceedings of the National Academy of Sciences, v. 117, online; DOI: 10.1073/pnas.2001045117). One Ediacaran trace fossil, known as Helminthoidichnites is preserved as horizontal trails on the tops and bottoms of thin, discontinuous sand bodies. Luckily, these are sometimes accompanied by elongate ovoids, like large grains of rice. From numerous laser scans of these suspected burrowers, and the traces that they left the authors have reconstructed them as stubby, possibly segmented, worm-like animals that they have called Ikaria wariootia, which may have grazed on algal mats. This name is derived from the local Adnyamathanha people’s word (Ikara  or ‘meeting place’) for the locality, a prominent landmark, near Warioota Creek. The age of the sedimentary sequence is between 551 to 560 Ma, and perhaps a little earlier. They could be the earliest-known bilaterians, but the sandy nature of the rocks in which they occur precludes preservation of the necessary detail to be absolutely sure: that would require silt- or. clay-sized granularity

See also: Fossil worm shows us our evolutionary beginnings (BBC, Science and Environment)