Last month I commented on a paper in Science which claimed to find Precambrian bilaterian animal fossils (see here, here, here, and here). In my opinion a decent case can be made for one of their fossils being a bilaterian animal, but the rest are dubious for various reasons. It turns out that an independent paleontologist and Honorary Research Fellow at the National Museum Wales, Joseph Botting, also is quite skeptical. He has posted a keen analysis where he too expresses various degrees of skepticism of bilaterian assignments for these fossils, and in some cases doubts that they are even animals. I had not seen his analysis prior to posting mine, and I was encouraged that in some ways he came to parallel conclusions — and in fact sometimes he expresses more skepticism than I did.
First, Botting is actually very skeptical of the paper’s purported example of an Ediacaran ctenophore, and he believes it is in fact a cnidarian (which are long known from the Precambrian). Botting argues that the transverse structures in the fossil are not diagnostic of a ctenophore and could represent a type of cnidarian called a conulariid — which are also well known from the Ediacaran. He says, “I do not see any reason you could call that a ctenophore” and even calls it the “least convincing.” This was not a fossil assignment that I had previously expressed much skepticism toward, but if he’s right then the number of examples of Cambrian animal phyla that this Science paper newly establishes as present in the Ediacaran would drop from one to zero.
Purported Bilaterian Fossils
Second, Botting also has some strong criticisms of the purported bilaterian fossils. For example:
He observes that the purported bilaterian organism labelled A to E in Figure 3 of the paper — one they found many specimens of — would be extremely strange as a worm-like organism with a gut that is attached to a discoidal holdfast. He points out that the purported “gut” interpretation is confounded by the fact that the black lines interpreted as guts are often found in multiples. Either there are multiple guts or something else is going on. He opts for “something else,” because he feels this fossil is highly similar to another previously known Ediacaran fossil that has been interpreted as a “weird algae.” In the end he says “I think it’s not even an animal” — which means he was even more skeptical than I was in my analysis which would have viewed it as some kind of an animal with non-bilaterian symmetry.
He is also skeptical that the potential wormlike organism labelled “F to H” and “M to P” actually has a gut, in this case because the preservation is not good enough to establish what it is. I agree. He doesn’t think you cannot rule out the possibility that it also represents algae and is skeptical that it’s “definitely a bilaterian.”
As for the tubelike organism labelled “I to L,” he says that it “looks” like Margaretia, but raises the possibility that the similarity with Margaretia could be convergent — which is exactly what I proposed. Tubelike organisms with perforations are not unusual, and it’s hard to say what this enigmatic fossil really is.
As for the one I thought was the best case for a bilaterian, labelled “M to P” (+ “Q and R”), which was compared to cambroernids, he too agrees it’s their best case, but he again expresses even more skepticism than I do. He doesn’t think that all the specimens of this purported organism all belong to the same organism, so some may have been misidentified. He notes that the presence of tentacles would be the best diagnostic feature of a cambroernid, however the so-called tentacles in these Ediacaran fossils look very different from the long, highly branched tentacles of true cambroernids known from the Cambrian (see for example here). He even notes that the Science paper admits this fossil shows “unbranched to minimally branched tentacles.” Botting calls them “very short tapered tentacles.” Another issue he raises is that the stalk on this organism is also branched, which raises the possibility that it might be a colonial organism — but cambroernids weren’t colonial, so this might count against it being a cambroernid. He notes strange aspects of this fossil which suggest the possibility of “incomplete preservation” which could mean that it’s something completely different. He concludes that “we need more information” before we can definitely say this is a stem group ambulacrarian, as the Science paper claims it is. Interestingly, he points out that in other cases there are far better-preserved fossils have faced doubts about their assignment as cambroernids, so he believes that the fossils described here deserve more scrutiny than they’re being given.
Lastly, Botting also points out that the stratigraphy of this Ediacaran fossil locality could allow that it sits just below the base of the Cambrian and is not buried deep in the Ediacaran. The paper assigns a likely age of 554 to 546 Myr for the fossils, but he says it could also be “much closer to the base of the Cambrian,” which in this area has an age of 539 Myr. So it could be very close to the Cambrian explosion, geologically speaking. He concludes — much like I did — that “some of it has been interpreted more towards the animals than it should be, but others, yeah, consider me intrigued.” And another point of agreement: he calls this a “wonderful, interesting assemblage of fossils.” Indeed!
Confusing Bacteria or Algae for Worms
Now, another new paper, this one in the journal Gondwana Research, further reminds us why we should be very cautious when interpreting supposed Precambrian animal fossils. The paper is titled “Proposed Ediacaran meiofaunal burrows from Brazil are pyritized algal/microbial consortia,” and as the title suggests, it reinterprets what were once called trace fossils of animals from the Ediacaran as algae or bacteria — not animals. An article at Science Daily explains what they found:
Scientists revisiting mysterious 540-million-year-old microfossils from Brazil have overturned a major idea about early animal life. What were once thought to be trails left behind by tiny worm-like creatures are now believed to be fossilized communities of bacteria and algae — some with remarkably preserved cells and organic material still intact.
The subtitle of the Science Daily article puts it bluntly: “A stunning fossil reanalysis suggests some of Earth’s ‘earliest animals’ were actually ancient bacteria and algae hiding in plain sight.” The technical article elaborates:
Meiofaunal trace fossils potentially provide an alternative line of evidence for the presence of early branching bilaterian lineages, but their interpretation is complicated by their similarity with a range of different geological and biological structures. … The multimodal size distribution, with at least three distinct size classes, and their partial in-situ preservation further suggest that they represent different species, possibly living in a microbial consortium. While their exact biological affinities remain uncertain, the largest populations share similarities with benthic red or green algae, while the smallest populations could be algal, cyanobacterial or large sulfur-oxidizing bacteria. The preservation of these remains suggests that bioturbation, macrofaunal or meiofaunal, was limited or absent where and when these fossils formed.
[…]
Our study investigated the morphology and taphonomy nature of the putative oldest meiofaunal traces in the fossil record. The evidence indicates that the pyritized structures from the Tamengo Formation represent three-dimensional preservation of microbial consortia composed of filamentous algae and bacteria, rather than true meiofaunal burrows.
In other words, these fossils aren’t the trace fossils of bilaterian worms. They’re a type of bacteria or algae. It’s just another reminder to be very careful about jumping on the bandwagon of the latest evidence for Precambrian animal fossils.
Two Interesting Comments
The technical paper makes two other interesting comments:
First, it notes that “While the metazoan affinities of many late Ediacaran macrofossils are well established, their precise placement in the phylogenetic tree remains debated.” We saw this in my previous series (especially here) where the best case for an Ediacaran bilaterian animal fossil was stymied by the fact that the messy deuterostome animal tree meant it was virtually impossible to clearly connect this fossil as an ancestor to any of the Cambrian animal phyla.
Second, it says the following about the Cambrian explosion: “one of the major evolutionary events in the history of life, the geologically rapid appearance of most animal phyla during the Cambrian Explosion.” Note that this comment came right after the previous quote which claimed to find “metazoan affinities of many late Ediacaran macrofossils.” In other words, just because you find fossils that might be Ediacaran animals, that doesn’t mean you have explained away the Cambrian explosion. I covered this in my final post of the previous series noting that Günter Bechly had rightly predicted “Even if some relatives of sponges, cnidarians, stem (eu)metazoans, and maybe even stem bilaterians should be present in the Ediacaran, this would do absolutely nothing to explain the sudden appearance of the many different bilaterian animal body plans in the Cambrian explosion.” Thankfully, these authors seem to tacitly appreciate this important point.