The Ediacaran Avalon biota: New insights from old fossils - Alex G. Liu

The Ediacaran Avalon biota: New insights from old fossils
Alex G. Liu
University of Cambridge
In recent years, substantial progress has been made in constraining the phylogenetic affinities of fossils of the Ediacaran macrobiota, with both metazoan and non-metazoan clades now recognised amongst latest Ediacaran global fossil assemblages. However, the oldest palaeocommunities of the Ediacaran macrobiota - those of the ~574-557Ma Avalon assemblage - have proven particularly challenging to interpret. Here I will summarise ongoing palaeontological, taphonomic, palaeoecological and palaeoenvironmental research from Newfoundland, Canada, showcasing the record of macroscopic life in the oceans prior to the appearance of the 'classic' Ediacara biota taxa of South Australia and the White Sea. Recent findings permit identification of trends in biodiversity and morphology through the frond-dominated, largely deep-water Avalonian palaeocommunities, and provide key lines of evidence for tracing the earliest stages of eumetazoan evolution. The implications of recent radiometric dates for our understanding of the links between Newfoundland's Ediacaran biota and broader regional and global environmental events, including the Gaskiers glaciation and the Shuram anomaly, will also be discussed.

[@catherinehubbard1167]

This was a spectacular presentation presenting new findings and re-examining older findings about worldwide Ediacaran macroscopic life forms. I was spellbound for the entire time. Thank you! I am eager to hear what happens next.

[@quantumcat7673]

Very interesting lecture.

[@SamuelVarela-ej2zp]

Do you what was Charnia's life span? Was it due to viruses and/or pathogens? Thanks! Samuel Varela.

[@wcdeich4]

Great presentation!!!!!!!! And Dr Frankie Dunn's phylogenetic at 22:00 is really brilliant!!!! But are we restricting our comparisons too much by only comparing Charnia to animal groups? Could it be possible Charnia & the rangeomorphs are a stem group off the fungi instead of the animals?

[@alexliu2201]

Thank you! Sorry for the delay in replying. This is a good question, but there are enough characters now for rangeomorphs that are specific to animals rather than fungi that lead us to be reasonably confident the rangeomorphs are more closely related to animals. This is summarised most succinctly in Frankie's 2021 paper: "C. masoni maintains differentiation of elements with concurrent axially delineated inflation, exhibits evidence for transitions in the primary developmental mode, and is compatible with indeterminate growth [the largest described specimens of C. masoni are >65 cm in length, reviewed in (11)], and the form of the organism is regular and predictable. This combination of characters is only otherwise seen within the Metazoa. Algae do not display a conserved form (25, 26), and fungal fruiting bodies do not display the maintained differentiation of new elements (27, 28) [reviewed in (6)]. Therefore, using these data in tandem with a large, multicellular organization, we conclude that there is no justification for considering an affinity for Charnia outside the animal total group." If you're interested to read more, it can be found here: www.science.org/doi/10.1126/sciadv.abe0291

[@wcdeich4]

Thanks :)

[@liamredmill9134]

I have found a few specimens of charnia in cherty flint from the thames foreshore in london,maybe they would be interesting to you.all english flint is Cambrian or is that now rong?

[@wcdeich4]

If we ever find biomarkers from any Rangeomorph I will be very interested!

[@alexliu2201]

So will I! We just need to keep our eyes open in the field and hope to find a suitable bedding plane containing a rangeomorph specimen.

[@wcdeich4]

@@alexliu2201 Do the fossils we do have tell us if Rageomorphs were hollow on the inside like Dr Adolf Seilacher concept of the "Vendo bionta"? Or were the Rangeomorphs paper thin & only some of the Vendobionta / Ediacarian biota had hollow space or jelly on the inside?

[@alexliu2201]

@@wcdeich4 There are beautifully preserved specimens of Charnia from the White Sea coastline of Russia that are preserved with the rangeomorph branching cast in three dimensions by sand particles. This tells us that there was a three-dimensionality to the rangeomorphs, but whether they were hollow 'bags' that could fill with seawater (see Butterfield 2020), or jelly-filled, or composed of internal cells, remains to be determined.

[@wcdeich4]

@@alexliu2201 Thanks!!!!!!!!!!!!!!!!!!!!!!!!!!!

[@emawerna]

Why do you dismiss the idea that they could be giant single cell algae like Caulerpa? Yes, there would have been no sunlight at those depths. However, you've already resigned yourself to say that Rangeomorphs fed like no living animal or plant alive today via direct osmosis. It does not follow that they must be metazoans. Why couldn't they be algae that fed like no living animal or plant alive today? Here's a suggestion: I think you should reach out to your local fish club and ask if any of them can try to grow Caulerpa in an entirely dark tank using only the water they collect during water changes from other tanks for nitrogen plus added minerals, sugar, and protein.

[@alexliu2201]

Thank you for the suggestion. We actually have studied Caulerpa and kept some in our marine tanks, so we know they specifically are unable to survive in a dark room for longer than a few days. All green algae require sunlight to photosynthesise, and it is assumed that this trait is something that has been consistant throughout their evolutionary history. This is why we don't think the rangeomorphs, which are inferred from sedimentological evidence to have lived at depths below the photic zone, were not algae. Caulerpa specifically also grows in a very different way to rangeomorphs, which have quite complex branching structures. There are other Ediacaran taxa that are likely to be green algal fronds, but these are typically simpler in morphology, and are found in shallower palaeoenvironments. This paper is a good starting point to learn more about them: onlinelibrary.wiley.com/doi/full/10.1111/pala.12485

[@emawerna]

@@alexliu2201 Maybe the ocean water was clearer back then because the sediment was locked up in a pre-worm crust at the bottom of the ocean and there was less "stuff" in the water because there were fewer or no animals to poop or die. Land hadn't developed soil yet, so there was no fertilizing runoff. With improved water clarity, light would have penetrated deeper.