Recent research on developemental processes in Ediacaran metazoans

[erik simpson]

Developmental processes in Ediacara macrofossils
(Scott D. Evans1,†, Mary L. Droser2and Douglas H. Erwin)

https://royalsocietypublishing.org/doi/pdf/10.1098/rspb.2020.3055

Abstract:

The Ediacara Biota preserves the oldest fossil evidence of abundant, complex metazoans. Despite their significance, assigning individual taxa to specific phylogenetic groups has proved problematic. To better understand these forms, we identify developmentally controlled characters in representative taxa from the Ediacaran White Sea assemblage and compare them with the regulatory tools underlying similar traits in modern organisms. This analysis demonstrates that the genetic pathways for multicellularity, axial polarity, musculature, and a nervous system were likely present in some of these early animals. Equally meaningful is the absence of evidence for major differentiation of macroscopic body units, including distinct organs, localized sensory machinery or appendages. Together these traits help to better constrain the phylogenetic position of several key Ediacara taxa and inform our views of early metazoan evolution. An apparent lack of heads with concentrated sensory machinery or ventral nerve cords in such taxa supports the hypothesis that these evolved independently in disparate bilaterian clades.

A nice account of recent results in Ediacaran organisms and their relation to later (and much better understood) metazoa.

[John Harshman]

The idea that Dickinsonia is a bilaterian is going to be controversial.

[erik simpson]

It already is. Droser has found fossils that are unquestionably bilaterally symmetric (among others that display the "glide" symmetry),
but obviously that doesn't mean that couldn't be independent of *true* bilaterians. There's also controverial affinities between
placozoa and Dickensonia. Including placozoa as bilaterians would also raise hackles.

[Oxyaena]

Placozoans have some tantalizing traits hinting towards ancestry from a
more complex organism, so the idea isn't as far-fetched as it seems.

[erik simpson]

I don't think it's far-fetched. Actually a lot of the ideas about the nature of Ediacaran critters (if they actually critters) might
be called far-fetched. I like the idea that placozoa (now containing three genera, ain't sequencing great?) are devolved from
more complex animals (Dickensonia?). There's evidence that sponges weren't always as simple either. That we don't know
so much about them is what makes the Ediacarans so fascinating.

[John Harshman]

Molecular studies all (as far as I know) agree that placozoans aren't
bilaterians.

[erik simpson]

At least one DNA study

https://royalsocietypublishing.org/doi/10.1098/rspb.2019.0831

raises the possibility that Placozoa might be placed as high as sister group to
Cnidaria (their Fig 5d). That's about as close to BIlateria as they seem to get.

[John Harshman]

Yes, many studies put them in various places within or sister to
Eumetazoa, but never within Bilateria. So if they're anything to do with
Dickinsonia, that's strong evidence that Dickinsonia isn't bilaterian.

[erik simpson]

Definitely. The only things that I know of that might relate plaozoans to Dickensonia is
that they're both flat and apparently had similar feeding strategies. Only with the
recently discovered sterane traces associated with D. have they been unambiguously
identified as metazoan animals.

[Glenn]

Talked yourself right into that, eh.

"Previously, hydrocarbon biomarkers for steranes had even been reported in 2.7-billion-year-old rocks (Brocks et al. 1999), but were later rejected as contaminations (French et al. 2015),"
...
" Yet, coprostanes are absent in much younger, exceptionally preserved animal fossils, where the dominant steranes are 5α(H)- cholestanes (14). The association of unusual steroids associated with Dickinsonia suggests that it may have had a distinct metabolic physiology. "

'In short: the found cholesteroid should neither have been preserved for more than a half billion years, nor should it be expected to be found in invertebrate animals at all, including an Ediacaran stem group representative."

https://evolutionnews.org/2018/09/why-dickinsonia-was-most-probably-not-an-ediacaran-animal/

[Glenn]

"Finally, cholesterol even occurs in plants (Behrman & Gopalan 2005), of which some (e.g., canola) can have more than 70 percent of cholesterol in the sterol fraction of their surface."

The OP title assumed the existence of animals (metazoan) in the Ediacaran, and the claim stands out like a sore thumb in all the posts here, in addition to most all scientific literature on the subject.

Ironically, Wiki, known for bias in such controversies, with several articles that define some Ediacaran fossils as "anamalia', is much clearer than most here:

"Determining where Ediacaran organisms fit in the tree of life has proven challenging; it is not even established that they were animals, with suggestions that they were lichens (fungus-alga symbionts), algae, protists known as foraminifera, fungi or microbial colonies, or hypothetical intermediates between plants and animals."

https://en.wikipedia.org/wiki/Ediacaran_biota

IMO, the biota of the Ediacaran should be regarded as more likely to be plants.

[erik simpson]

Concerning Ediacaran biomarkers, I know:

https://groups.google.com/g/sci.bio.paleontology/c/hv6rDAZFzfM/m/mDI5UCjNAwAJ

[Glenn]

You know what? The definition of "vendobionts"?

"vendobionts such as Dickinsonia were at temperate to subtropical
latitudes."

https://www.sciencedirect.com/science/article/abs/pii/S1342937X20303038#:~:text=The%20discovery%20of%20Dickinsonia%20in,reconstructions%20for%20the%20late%20Ediacaran.&text=Cloudina%20and%20other%20small%20shelly,at%20temperate%20to%20subtropical%20latitudes

"Other vendobionts resemble nothing so much as the leaves of present-day land plants "

https://www.accessscience.com/content/vendobionts-precambrian-evolution/YB030430#YB030430FG0010

[Glenn]

Consistent with "The boron content of Ediacaran vendobionts is indistinguishable from that of fossil plants and paleosols,and significantly lower than that of known marine fossils, including Ediacaran stromatolites and algae."

https://cpb-us-e1.wpmucdn.com/blogs.uoregon.edu/dist/d/3735/files/2020/01/Retallack-2019-boron-paleosalinity-1.pdf

[erik simpson]

Thanks for alerting me to these references. They are old, and I am familiar with them. There are many more references if you want to chase them down,
but I doubt you'll find any I haven't seen unless they are very recent (2020 or 2021).

[Glenn]

So you say. But "they" are not old. Only one is dated, and you make no comment on that one. You could be entirely ignorant of the subject and still make the same claim. Perhaps you should answer the question of what you "know".

[erik simpson]

The "they" refers to your last three references. Two to papers by Retallack, one to a paper by Seilacher. The two by Retalleck have been is dicussed, probably to death, in another thread
recently. I have read both. The reference to Seilacher is dated 2003 in the reference you give, but I think it's a secondary reference to an older date. I am very familiar with Seilacher's
ideas, less so with Retallack's. The Ediacaran-Cambrian period has been as especial interest of mine for many years, and I have read most of the references (excepting stuff
by nutters) I could find. I'm an amateur paleontolgist, I talk to paleontologist, I "know" quite a bit about it, but I wouldn't call myself an expert. If you think Dickensonia "looks
like a leaf", you haven't examined either leaves or Dickensonia well enough.

[Glenn]

Well enough, eh. You're about a half step away from insulting me. Should I now say something like if you don't think it looks like a leaf, you're a nutter?

Charnia is another that looks like a leaf. Have you "examined" that one "well enough" in order to make such a claim as I haven't examined leaves or Dickensonia 'well enough'?

[erik simpson]

Yes. You haven't looked at it very well either. BTW, Seilacher didn't think it looked like a leaf. It might, to a naive glance, but he was neither naive nor careless in
his observations. I have no interest in insulting you, but I'm not wasting any more time replying to your posts if this last one is characteristic.

[Glenn]

Ask me if I care. You are insulting. And I am neither naive or careless. Dickensonia looks like a leaf, and that should be obvious to anyone who has looked at a leaf. But I don't expect any evolutionist to make such a comparison, since the fossil is the best so far at pinning the label "animal" on. And that should be obvious to most anyone with even a passing interest in the subject, or to anyone who has read the references I provided.

[Oxyaena]

It's Glenn, do you honestly expect better from him?

[erik simpson]

Obviously not.

[Glenn]

You said it. You're obviously not honest, nor honorable.

[Glenn]

So that's it, Sponge Bob?