A major problem in animal phylogeny seems to have been solved

May 21, 2023 • 9:30 am

As a new article in Nature (title below) notes, there are five major groups of animals that arose early in animal evolution and persist today: ctenophores (comb jellies), sponges (Porifera), placozoans (small, simple multicellular organisms), cnidarians (jellyfish, corals, sea anemones), and bilaterians (all other animals ranging rom molluscs to vertebrates). We have a pretty good notion of when their ancestors branched off from each other in evolution (this is in effect their relatedness, expressed in their phylogeny, or family tree), except for one question:  which group’s ancestors branched off first? That group would be called the “sister group” of all living animals. (It could also be called “the outgroup among all groups of animals”.)

DNA sequencing has shown that it’s either the ctenophores or the sponges (the most common candidate), but it’s been very difficult to decide between the two because there’s been so much time since the ancestors of modern sponges and ctenophores branched off from the other groups—700-800 million years—that too many DNA changes have accumulated to allow a firm DNA-based resolution. (DNA is now the best way to go to resolve these trees.) Every few years then, someone attempts another DNA-based phylogeny of animals, and the outgroup keeps changing between sponges and ctenophores.

Why is this an important question? Not just curiosity alone, for its resolution bears on understanding an important fact: like all other animals except sponges, ctenophores have nerves and muscles. This would seem to show that ctenophores are grouped with the other animals, while sponges branches off early, and then nerves and muscles evolved in the ancestor of all other animals.  This convinced many that sponges were the outgroup. If ctenophores, on the other hand, were the sister outgroup that branched off first, that would leave us with a puzzle: why are sponges the one exception, lacking nerves and muscles, among all other animals?  Here are the two possibilities for the outgroups, with “N&M” showing where nerves and muscles evolved. (I’ve put the dots and “N&M” stuff in myself.)

A.  Ctenophore outgroup: The ancestor of ALL animals did have nerves and muscles, but sponges lost them.


B.  Sponge outgroup: Nerves and muscles evolved after the ancestor of sponges had branched off from the ancestor of all other animals. (No loss of already-evolved characters required.)

The left side shows “A”, with the common ancestor of all animals having nerves and muscles, but then they were lost in the ancestor of living sponges. The right side shows possibility “B,” with the ancestor of all living animals (red dot) lacking nerves and muscles, which appeared later in the common ancestor of all living animals after that ancestor had branched off from the ancestor of sponges.

As you can see, “A” posits two evolutionary events: the evolution of nerves and muscles in the ancestor of all living animals, and then their loss in the sponge lineage, while “B” posits nerves and muscles evolving evolving just once: in the ancestor of all non-sponge animals.

However, if “A” is the case, you could posit another scenario in which ctenophores independently evolved nerves and muscles from other groups of animals, while the ancestor of all animals lacked them.

The diagram below shows a common ancestor of all animals (red dot) lacking nerves and muscles, and then they evolved twice independently: in ctenophores, and then also in the other groups that branched off later from the common ancestor with sponges. Thus, if A is correct and ctenophores are the outgroup, there are still two explanations for the nerve/muscle presence in animals: either they were in the ancestor of all living animals and then lost in the ancestor of sponges, or they didn’t occur in the animal ancestor but then evolved twice independently (N&M shown where the evolution happened). As you can see, if the red-dot ancestor lacked nerves and muscles, but all modern animals save sponges have them, AND ctenophores were the sister group, then nerves and muscles must have evolved twice OR (as you see above), all early animals had them but the ancestor of sponges lost them. So the left side of the diagram above OR the diagram below show the two evolutionary possibilities for where muscles and nerves occur.

This is why resolving the outgroup is important: it leads to different hypotheses about how evolution worked. Again, the alternatives are A with the ctenophore outgroup, in which cases nerves and muscles were either lost in sponges or evolved twice independently; or B, with the sponge outgroup, in which case nerves and muscles evolved just once—in the common ancestor of all other animals.  Because “B” seems more parsimonious to many, that has been the consensus scenario.

But now the consensus seems wrong: new data show pretty convincingly that ctenophores do appear to be the outgroup, and sponges are more closely related to all other living animals than are ctenophores.  You can read about this by clicking on the screenshot below, or going to the pdf here (reference at bottom).


The analysis was very clever. Instead of just looking at large amounts of DNA in the animals, they looked at the order of DNA sequences (genes) on the chromosomes.  Over the last 800 million years, that DNA has been shuffled around as chromosome fuse or bits of chromosomes come loose and stick to other chromosomes (translocations).  In either case, chunks of DNA then get shuffled around among chromosomes and on a given chromosome by inversions.

But this gives us a way to see which groups have undergone unique fusion/translocations and shuffling events, for once this takes place in a common ancestor, it is unlikely to be undone by a reversal of all the processes that lead to genes being ordered as they are now.  Thus, if you see a group of animals that share a common gene order different from that of another lineage, you can be pretty sure that that group is more closely related to each other than to that other lineage.

And that’s what the authors did: they not only sequenced or took sequences from entire genomes of all the animal lineages above (including two species of ctenophores), but ordered genes along chromosomes. (This isn’t hard to do: you get the DNA as a sequence, and the DNA sequence on one chromosome will not run on to the DNA sequence on another chromosome.)  They not only looked at all animal lineages, but also single-celled groups that are less closely related to animals, like amoebas and choanoflagellates (these aren’t considered “animals” but whose ancestors are considered outgroups to all living animals).

The results were pretty unequivocal: they found several chunks of DNA that were shared by the single-celled relatives and ctenophores, but also four ordered chunks of genes that were shared by all living multicellular animals except for ctenophores.  That is, sponges shared gene chunks with vertebrates, cnidarians, and placozoans, but those chunks were in completely different places in the ctenophores.

The conclusion: the chunks found their shared locations in modern animals after they had already branched off from the ancestor of modern ctenophores. Ctenophores are thus the outgroup, and we’re less closely related to them than to sponges. The scenario in A above is the correct one.  (The three groups at the top of the diagram below are single-celled non-animal organisms that are distantly related to animals.) As you see, the ctenophores branched off from all other living animals before any other animal group, making them less closely related to modern animals than are sponges.

Now this analysis may be wrong, but given the irreversibility of moving gene chunks around repeatedly, shared gene chunks on chromosomes almost certainly means shared ancestry. I’m pretty confident, then, that this paper has resolved the long-standing controversy about the “outgroup” of all animals.

But this leaves us, of course, with two questions.  Did the ancestor of all living animals have muscles and nerves, and sponges simply lost them, or did nerves and muscles evolve twice independently?

Each of these comes with another puzzle. The first one is this: why did sponges have complex and highly evolved set of features to sense the environment and move about, but then lost it?  The second one is even more puzzling: how could such complex features evolve twice independently?

UPDATE: I forgot about this but was reminded. Another trait shared by ctenophores and all other animals save sponges is the gut:  a digestive channel formed by “gastrulation”—invagination of the embryo.  Thus we have to account for the disappearance of three features in sponges or the independent evolution of guts, nerves, AND muscles.

While we know that the best information we have is scenario “A” above, we don’t know whether sponges lost their gear or that gear evolved twice independently.  The authors of the paper don’t discuss this, but in a NYT article on the piece by Carl Zimmer, he finds a hint that nerves and muscles may have evolved independently in ctenophores and in all other animals that have them:

Instead, researchers are looking now to comb jellies to see how similar and different their nervous systems are from those of other animals. Recently, Maike Kittelmann, a cell biologist at Oxford Brookes University, and her colleagues froze comb jelly larvae so that they could get a microscopic look at their nervous system. What they saw left them baffled.

Throughout the animal kingdom, neurons are typically separated from one another by tiny gaps called synapses. They can communicate across the gap by releasing chemicals.

But when Dr. Kittelmann and her colleagues started to inspect the comb jelly neurons, they struggled to find a synapse between the neurons. “At that point, we were like, ‘This is curious,’” she said.

In the end, they failed to find any synapses between them. Instead, the comb jelly nervous system forms one continuous web.

When Dr. Kittelmann and her colleagues reported their findings last month, they speculated yet another possibility for the origin of animals. Comb jellies may have evolved their own weird nervous system independently of other animals, using some of the same building blocks.

Dr. Kittelmann and her colleagues are now inspecting other species of comb jellies to see if that idea holds up. But they won’t be surprised to be surprised again. “You have to assume nothing,” she said.

That is, there are differences between the nerves in ctenophores and in all other nerve-bearing animals: the former appear to lack synapses. This suggests that nerves could have evolved independently, and taken two routes, one route lacking a gap (the synapse) between the nerves.  As for the muscles, neither the paper nor Zimmer deals with whether there’s some fundamental differences between how muscles are structured or how they work between ctenophores on the one hand and all other muscle-bearing animals on the other.

As usual, we’ve probably settled one evolutionary question but it’s raised several others. People now will be devoting more attention to nerves and muscles in animals.

As one of my friends, who teaches introductory biology in a major university, said, “Well, I guess I’ll have to revise my lecture notes. For years I’ve been telling students that while the outgroup of all animals isn’t known for sure, it is most likely the sponges.”

Here’s a ctenophore shown on Wikipedia. They are really cool animals, and if you want to see a bunch of them, go to the Monterey Aquarium in California, where they have a mesmerizing display:



The New Yorker writes about the hoatzin, implies that Darwin’s idea of evolutionary trees may be a phantom

July 17, 2022 • 1:00 pm

I have one hour to analyze in detail a new article about a bird. Can I do it? The answer, yes I did!

When the New Yorker publishes a piece about pure science, as it does in this article about a South American bird, I always suspect there’s a hidden agenda. That’s because the magazine isn’t really pro-science. As I quoted a respected colleague in an earlier post:

The New Yorker is fine with science that either serves a literary purpose (doctors’ portraits of interesting patients) or a political purpose (environmental writing with its implicit critique of modern technology and capitalism). But the subtext of most of its coverage (there are exceptions) is that scientists are just a self-interested tribe with their own narrative and no claim to finding the truth, and that science must concede the supremacy of literary culture when it comes to anything human, and never try to submit human affairs to quantification or consilience with biology. Because the magazine is undoubtedly sophisticated in its writing and editing they don’t flaunt their postmodernism or their literary-intellectual proprietariness, but once you notice it you can make sense of a lot of their material.

. . . Obviously there are exceptions – Atul Gawande is consistently superb – but as soon as you notice it, their guild war on behalf of cultural critics and literary intellectuals against scientists, technologists, and analytic scholars becomes apparent.

I think a lot of that is true, so what’s the reason we get a rather confusing article about the evolutionary history of a South American bird? Well, I don’t know the author’s brief or motivation (Ben Crair is a freelance writer), but judging from the title and the content, this is an article in the “Darwin-was-wrong” genre—in this case mentioning Darwin and the tree of life as being somehow overturned by the ancestry of the hoatzin bird. It isn’t, of course, but in trying to make his case the author produces a farrago of confusion that will not only befuddle the layperson, but also confused both Matthew and me. Click to read (it’s free):

Let’s meet the subject, first. The hoatzin (Opisthocomus hoazin) is one of the world’s weirdest birds in appearance and habits. Here’s its South American range:

Below: a video of what it looks like and the very weird climbing behavior of the chicks, who have retained ancestral claws on its wings (they lose them as adults). When predators attack, the clawed chicks, whose nests are built over water, simply drop into the drink, swim to shore, and use their claws to climb back to the nest.

Here’s a video that makes the climbing behavior more obvious:

The front claws, which made people think the bird is primitive, is not the only weird thing about it (and no, it’s not the only species with claws, just one that uses them for such a bizarre reason). This excerpt is from an article by Elizabeth Deatrick on Sketch, a feature of the Audubon Society.

Hoatzins are the only birds in the world that eat nothing but leaves, which, compared to seeds and fruit, aren’t very nutritious, and are hard to digest. So to accommodate this diet, the Hoatzin has evolved a multi-chambered digestive tract with lots of little “stomachs,” where the leaves can sit for a while and be digested by friendly bacteria. During the digestion process, the bacteria release methane that the bird then belches out, producing an olfactory aura that’s landed the Hoatzin a less-than-flattering nicknamed: the stinkbird. So much for fitting in.

It’s also called the “skunkbird.”

So we have some interesting facts. But what intrigues Crair is that when you try to place the hoatzin on the family tree of birds, it’s very hard. Its structure and morphology aren’t useful, because it diverged from other birds so long ago (it may occupy a long, old branch of its own). So people turn to DNA, which, if you use enough of it, should, via inspecting similarities and differences, tell you what species the hoatzin is most closely related to.  But because it’s so long diverged, that’s been problematic too. Various analyses, depending on which parts of the DNA you look at, have suggested that it’s most closely related to turacos, or maybe to cuckoos, or maybe to rails, or maybe, as this article suggests, to the ancestor of cranes and shorebirds. The problem is that the DNA is so long diverged from that of other bird species that, depending on what part of the DNA you look at and which bird species you use for comparison, you get different answers.

But this is not your typical bird, and we can usually place a bird near its closest relatives if we use a lot of DNA. That kind of analysis sometimes yields surprising results: one I like to mention is that peregrine falcons are more closely related to parrots than to other birds of prey (“raptors”) like hawks, ospreys, and eagles. That’s based on a lot of DNA, and that is the correct placement of falcons on the bird evolutionary tree.

Of course the hoatzkin kind of difficulty does not invalidate the idea of a “branching tree of life”, for we are using a phylogeny of genes (one branching scheme) to determine the phylogeny of species (or populations), which can be a different branching scheme. And if you look at some animals and some genes, you’ll find that the family tree of genes, whose own evolutionary history branches when they mutate, does not match the phylogeny of the species themselves: the evolutionary history of the organisms that contain the genes.

These discrepancies between the “tree” of some genes and of the organisms that contain them have several causes.

The first is what we call “incomplete lineage sorting”. Let me explain. Suppose that an ancestral species has two mutant forms of a gene or DNA segment. Let us call them A, and B. Let us then suppose that the ancestor branches in such a way that it produces species X, Y and Z, and the branch that eventually splits to produce Y and Z comes off separately from the branch that leads to X. Now assume that each of these ancestors has all the A and B mutants, but then species Z loses the A form via genetic drift, while species X and Y lose the B form.  This happens, for genes can change their frequencies by random processes not involving natural selection (“genetic drift”).

The history of the species themselves shows that Y and Z are more closely related to each other than either is to X, since they share a more recent common ancestor. But if you look at the one gene that had the two forms A and B, you’ll see that species X and Y are more closely related than either is to Z in terms of that gene, for they both have form A, while Z has form B. In other words, a tree for this gene shows an evolutionary history different from that of the tree for the species themselves. And if you look at another gene, which drifts independently, you may find that species X and Z are more closely related to each other than either is to species Y.

Since there are a gazillion genes to look at, it’s not unlikely that you would find such discrepancies. The genes can show three different evolutionary histories while there is only one for the species themselves, based on which populations evolved into new species. Gene trees and species trees can be discordant.

The way to solve this, of course, is to use lots and lots of genes, for together they should show a preponderance of phylogenies that match the tree of species themselves, since genes (with two exceptions mentioned below) stay within the borders of species. (The definition of “biological species” involves barriers to gene exchange.)

And, in general, that’s what we find. When we sequence whole genomes of species, like humans, chimps, and gorillas, we find what we knew from other data: humans and chimps (I lump bonobos with chimps) are more closely related to each other than either is to the gorilla, which is the more distant ancestor or “outgroup”. And all four of these species are yet more distantly related to the orangutan.  The more genes we use, the closer we get to reconstructing the true evolutionary history of the species themselves.  

Since the hoatzin is so long removed from other species of birds, it’s evolved nearly independently for nearly sixty million years, and so, depending on which species and which genes you look at, you could find that the hoatzin has genetic similarities that are discrepant if you use different comparison groups. There is no species closely enough related to the hoatzin to allow us to show a general similarity between its genes and that “sister” species. Ergo we don’t know the closest relative of the hoatzin. And we may never know.

I haven’t done a great job explaining this, but perhaps you’ll understand. Still, author Crair doesn’t try to explain this at all, referring in one sentence to incomplete lineage sorting as a “kind of genetic scrambling.” The key, though, is to understand that the evolutionary history of individual genes or segments of the genome is not the same thing as the evolutionary history of the populations of organisms that contained those genes—the species themselves.

The other factors that causes discrepancies between gene trees and species trees are hybridization, which can transfer genes between species that aren’t all that closely related, or horizontal gene transfer (“HGT”) via vectors like viruses. Both of these transfer bits of DNA into species that don’t reflect their evolutionary history, and trying to suss out species’ history from such wide gene exchange is confusing. That, too, could have been a problem with placing the hoatzin, but I doubt it.

Here’s one example from humans. Because of ancient hybridization between Homo sapiens and Neanderthals—I won’t get into the issue of whether they’re different species, though I don’t think they are—if you looked at the right gene in me, Jerry, and compared just that one gene to other gene forms in my species and to the Neanderthal genome, you might find that “for gene X Jerry is more closely related to Neanderthals than to his fellow H. sapiens.” And that would be true. Most of us probably carry a different set of gene bits from Neanderthals, and if you wanted to make a tree using just those bit, you’d find out that every one of us is more Neanderthal than sapiens. But of course this is only for the bits of genome that we’ve inherited after ancient hybridization between Neanderthals and our own ancestors. If you look at whole genome analysis, you’ll see that this small discrepancy washes out and you get the correct answer: all of us are more closely related to each other than to Neanderthals.

It is these issues that allow the author to conclude that the idea of Darwin’s branching evolutionary tree may be way overrated. Here’s how he states it:

The tree is so ingrained in evolutionary biology that scientists encourage “tree thinking.” By learning to think in terms of trees, students can avoid the common fallacy of reading evolution as a ladder in which simpler organisms become more complex, as in the famous image “The Ascent of Man,” which shows a knuckle-walking ape evolving into an upright human. For all its pedagogical value, however, the tree also embeds subtle assumptions about evolution. The tree tends to downplay the genetic variation within species, which can obscure the fact that common ancestors are actually diverse populations that can pass on different versions of a gene to different descendants. It tells a story of endless partition and diversification, with branches that diverge and never reticulate.

Now doesn’t that imply that the idea of evolutionary trees is dubious? In fact, the idea of trees for species, which is the way Darwin meant it to be construed, is doing fine; it just doesn’t always comport with trees for some genes within a species. Tree thinking is well and good and isn’t likely to go away.

And yet. . . and yet Crair admits this in one place:

The outlines of animal evolution still look a lot like a tree in many places, which is why scientists continue to spend so much time developing and debating different branches. But, if tree thinking taught biologists that everything is connected, genes are suggesting that the connections can run even deeper than a tree can capture. To gain a more complete picture—and to answer questions like how such an unusual mix of traits came together in the hoatzin—scientists may need to think outside the tree.

If Crair was careful to distinguish between species trees on the one hand and gene trees on the other, he wouldn’t have to create these apparent “discrepancies”. We’ve known about this issue for years; in fact, I wrote about it in my book Speciation with Allen Orr, and that came out in 2004. (See the Appendix.)

I don’t know if sloppy editing exacerbated the confusions in this article, or whether the author didn’t clarify them (I think he understood them). In the end, we’re simply left with these facts:

a. The hoatzin is a damn weird bird.
b. We haven’t been able to deduce its closest bird relatives.
c. There are evolutionary/genetic reasons for this difficulty.

But that wouldn’t make as click-worthy an article as the one that the New Yorker published—to wit:

a. The hoatzin is a damn weird bird.
b. We haven’t been able to deduce its closest bird relatives.
c. Therefore there must be something wrong with Darwin’s idea of a “tree of life”.

“a” and “b” are what you should remember, and also remember about why the hoatzin smells bad and the weird claws its chicks use to climb up trees.

Now it’s time to feed the ducks!

Human Phylogeography

February 23, 2019 • 11:33 am

by Greg Mayer

For the spring semester, my colleague Dave Rogers and I are teaching a seminar class entitled “Human Phylogeography.” Phylogeography is the study of the history of the genetic variation, and of genetic lineages, within a species (or closely related group of species), and in the seminar we are looking at the phylogeography of human populations. DNA sequencing now allows a fine scale mapping of the distribution of genetic variation within and among populations, and, remarkably, the ability to sequence ancient DNA from fossil remains (including Neanderthals). The seminar is based primarily on a close reading of David Reich’s (2018) Who We Are and How We Got Here (published by OUP in the UK).

A Krapina, Croatia, Neanderthal woman, photo by Jerry.

Although rarely under that rubric, human phylogeography has been a frequent topic of discussion here at WEIT, by Jerry, Matthew, and myself, including our several discussions of Neanderthals (or Neandertals) and Denisovans. So it may be of interest for WEIT readers to follow along. Below the fold I’ve placed the course syllabus, which includes the readings, and links to many newspaper articles of interest, and online postings, including many here at WEIT, and also from John Hawks Weblog, a site we’ve recommended on a number of occasions when discussing human evolution. (The newspaper links appear as images; just click to go to the story.) We just finished our third meeting, and I’ve been quite impressed by the students’ discussion and writing. We’re fortunate to have some students from anthropology or with some anthro background.

Please read along with us, or browse what seems interesting below. If you have questions or comments, post them here, and I’ll be looking in.

Continue reading “Human Phylogeography”

What were the first animals?

October 31, 2018 • 11:45 am

by Matthew Cobb

I’ve just finished making a BBC World Service radio programme about the first animals. Anyone, anywhere in the world, can listen to it (it’s only 28 minutes long!) – you just have to register with the BBC (free, rapid and cost- and spam-free). Click on the pic to go to the BBC website:

The programme deals with two different ways that researchers are studying this question – by looking at fossils, and at DNA. In both cases I interview researchers and – in the case of the Ediacara – get to handle some fossils. I also ate some 600 million year embryos at Bristol University (to see what they tasted like, obviously), but we didn’t include that in the programme. . .

The fossil data relate to what are called the Ediacaran biota – strange fossils from before the Cambrian, around 570 million years ago. The fossils are very hard to interpret – they don’t look like much alive today – but an amazing technique for analysing cholesterol molecules in the rock, so organic molecules preserved for all that time, has confirmed that Dickinsonia, the thing in the picture above, was an animal. Other techniques involve looking at large numbers of Ediacaran fossils and seeing how their distribution relates to those of modern animals. All the data suggest that some of the Ediacaran weirdos were indeed animals, although we cannot know if they are the ancestors of any animal alive today.

The DNA data focuses on a different question, which DNA can answer – which of the groups of animals alive today was the first to branch off the tree of life? Traditionally there has been a straightforward answer to this: sponges, which are nerveless and tissueless. But 10 years ago comparative genomic studies dropped a bombshell – they suggested that the first group to branch off were the ctenophores or comb jellies. This has caused a huge row because it would mean either that nerves evolved twice – once in the ctenophores, and once in our ancestors, after the nerveless sponges branched off – or that the huge sponge group somehow lost the genes for producing nerves.

Many biologists (myself included) don’t like either of these options, and prefer the sponges as the first model, but the data are persistent. Or are they? I spoke to experts on both sides of this argument, which has caused quite a hoo-haa in the zoological community for the past decade.

Anyway, go ahead and have a listen – download it and listen to it on public transport or while you are exercising. NB: I made the programme with ace producer Andrew Luck-Baker.

If you are a teacher, especially if you teach animal evolution, please get your students to listen to it.

A new phylum of very weird sea creatures

September 7, 2014 • 6:57 am

Read some biology today; it’s good for you!

It’s not often that a new animal phylum has been described, but a new paper in PLoS ONE apparently does just that, basing the phylum on two enigmatic species, dredged up from the deep sea, that can’t be placed in any existing phylum. This may add one more to the 35 phyla that already exist (see the list here, and please look. It’s nice to review the major divisions of life.)

The paper is by Jean Just et al. (all authors are from the Natural History Museum of Denmark at the University of Copenhagen), and the reference and pdf, which is free, are below.

What we have is something that looks like a cnidarian (jellyfish, corals, and sea anemones) or a ctenophore, but with a stalk. (Some cnidaria do have a stalk). But it has features that keep it from being placed in the phyla Cnidaria or Ctenophora.  Its placement on the tree of life is further complicated by two things: we don’t really know where some major groups fit on the tree of life already (see below), and we don’t have any DNA or molecular data from this group to see what it’s most closely related to, or whether it’s an outgroup (a more distant ancestor) to all metazoans (multicellular animals).

The problem is that these creatures, which I’ll show shortly, were dredged up off of Victoria, Australia in 1986 from 400-1000 meters down. They were then fixed in formalin and later transferred to 80% ethanol. I’m no molecular biologist, but I think that would pretty much destroy the DNA, preventing any molecular analysis. And the samples are now old, shrunken a bit and degraded, and so some features may be effaced.

What we have are two species placed in a new genus, Dendrogramma, which the authors consider members of a new phylum as well, though they didn’t formally name one in this paper—probably because the placement of these creatures is uncertain.  Two species were named. Here’s the first, Dendrogramma enigmatica:

Screen Shot 2014-09-06 at 12.55.41 PM

Like the other speciers, it has a flattened disc with a notch in it, a stalk (so it was attached to the substrate), and a mouth-like opening that leads into an “gastrovascular” canal in the stalk that also feeds into the radiating canals in the disc. The tissue types were not examined, so we can’t draw homologies between the types of layers and those of other metazoans.  Here’s the other species, Dendrogramma discoides:

Screen Shot 2014-09-06 at 12.56.07 PM

And both species together. You can see from the scale (1 mm) that they were very small (10 mm = 1 cm, and there are 2.54 cm per inch).

Screen Shot 2014-09-06 at 12.55.17 PM

Because of the stalk and the inflexible disc, these things were probably unable to swim but attached to rocks or the sea floor. Given their mouthlike opening, the authors suggest that “they fed on microorganisms, perhaps trapped by mucus from the specialized lobes surrounding the mouth opening.”

Why aren’t they members of existing phyla like cnidarians and ctenophores? Because they lack features found in those phyla. As the authors say (my emphasis):

Dendrogramma shares a number of similarities in general body organisation with the two phyla, Ctenophora and Cnidaria, but cannot be placed inside any of these as they are recognised currently. We can state with considerable certainty that the organisms do not possess cnidocytes, tentacles, marginal pore openings for the radiating canals, ring canal, sense organs in the form of e.g., statocysts or the rhopalia of Scyphozoa and Cubozoa, or colloblasts, ctenes, or an apical organ as seen in Ctenophora. No cilia have been located. We have not found evidence that the specimens may represent torn-off parts of colonial Siphonophora (e.g., gastrozooids). Neither have we observed any traces of gonads, which may indicate immaturity or seasonal changes. No biological information on Dendrogramma is available.

Given the absence of DNA data or complex characters that might help us decide where these things fit in the tree of life, the authors can only speculate. One big problem is that we don’t really know where the major phyla of multicellular animals fit on the tree. For example, some biologists claim, based on both molecular and morphological data, that the “outgroup” (the most unrelated phylum) to all metazoa is the Porifera (sponges). Others (and the authors of this paper take this position) claim that the outgroup is really Ctenophora (which, based on morphology alone, I would have thought were more closely related to the cnidarians, as biologists once thought [they’re really distantly related groups, though]). So here’s the phylogeny presented in the paper, showing cetophores as the outgroup to other metazoans (including the Bilateria, the group of phyla that includes all bilaterally symmetrical animals, including us:

Screen Shot 2014-09-06 at 12.56.24 PM

To hedge their bets, the authors have also included ctenophores within other groups, as its placement is uncertain. They’ve put Dendrogramma as either an outgroup to all other phyla, or perhaps more closely related to the ctenophores or cnidarians. We just don’t know yet.

Molecular evidence could potentially resolve the placement of all these groups, and, frankly, I’m surprised that we haven’t settled the issue. For Dendrogramma we clearly need fresh material to get DNA (the authors plead for someone to get more specimens), but we could get plenty of DNA from the other species.  Either that hasn’t been done (which I strongly doubt), or the lineages diverged so long ago that DNA evidence is inadequate to settle the question of, say, whether sponges or ctenophores are the outgroup.  Perhaps some reader can explain to us why this major issue remains unsettled.

I noticed that the discs of these species resemble some creatures described from the Ediacaran fauna (also called the “Vendian fauna”), a group that lived from about 580 million years ago to about 545 million years ago, when the “Cambrian explosion” occurred and Ediacaran animals (if they were animals!) disappeared. (For pictures of various weird Ediacaran creatures, see here.)

My friend Latha Menon, who is not only the trade science editor at Oxford University Press (and editor of the British edition of WEIT) but also a Ph.D. candidate at Oxford’s Department of Earth Sciences, would know more about this, as she works on discs that strongly resemble these, but lived hundreds of millions of years ago. I therefore asked her to relate the new finding to the old group, as they could be related. Her answer is below, along with references. As you can see, she’s a very good writer, and I’m grateful for her input on this issue.

by Latha Menon

The discovery of Dendrogramma from the deep sea off Australia has undoubtedly caused a frisson of excitement among researchers on early life. A living fossil? An Ediacaran that has been surviving quietly in bathyal regions for several hundred million years? Let’s not get carried away, but it is an intriguing find.

When Reginald Sprigg discovered, in the 1940s, a set of strange impressions, many of discoidal forms, preserved on surfaces of the sandstone and quartzite of the Ediacara Hills, South Australia,  he called them “medusoids”. Further work by Martin Glaessner and Mary Wade in the late ’60s continued to describe the various discoidal forms as medusoids, while frondose forms such as Rangea were considered to be Pennatulaceans (sea pens), and Dickinsonia was thought to be an annelid. Since then, Ediacaran macrofossils have been found all over the world, including spectacular fossil assemblages from the White Sea coast in Russia, the Nama Group, Namibia, Lantian and Miaohe Formations in South China,  and the remarkable “E surface” at Mistaken Point, Newfoundland (see e.g. Fedonkin et al., 2007). The biota gave its name to the Ediacaran Period (635-541 Ma) ratified in 2004, and the fossils themselves appear from about 579 Ma (perhaps earlier), soon after the Gaskiers glaciation, the last of several widespread glaciations, and stretch up to the Cambrian boundary. Close to the boundary, the earliest biomineralized forms, the “small shelly fossils” appear, along with intense burrowing activity (bioturbation), and the Ediacarans, as far as we know, disappear, perhaps in an extinction. So what were the Ediacarans?

Nearly 70 years after Sprigg’s discovery, with many more fossil impressions, the affinities of the Ediacaran biota remain uncertain. Remember, that’s all we have – impressions (and in some cases, carbonaceous compressions) in the rocks. No skeletons; no biomineralized parts; and certainly no DNA. Molecular clocks provide little help so far back in time; results are notoriously varied and unreliable. Fossils really matter. And in spite of the limitations, a great deal of work has been done to glean information from the often exquisitely detailed impressions and the sedimentology of the surrounding rock, which indicates the setting in which they lived and died. As more evidence accumulated concerning morphology and sedimentary context, the early interpretations of medusoids, pennatulaceans, and annelids was increasingly questioned. Some may reach 30 cm and more in size, but were they necessarily early animals?  The late Dolf Seilacher proposed that these enigmatic forms represented a “failed experiment”.

Discoidal forms are particularly hard to interpret. Some simple forms may be pseudofossils formed by physical processes; others have been persuasively explained as microbial colonies (Grazhdankin and Gerdes, 2007).  Still, some possible affinities with familiar taxa have been suggested, with evidence put forward for bilaterian traces from about 555 Ma, and the claim that Kimberella may have been an early mollusc (Fedonkin & Waggoner, 1997). Our own group has found evidence in the early Ediacaran Avalon assemblage of Newfoundland for horizontal and vertical motion associated with a discoidal form (Liu et al., 2010; Menon et al., 2013), suggesting that some of these discs may indeed have been simple polyp-like forms. Two weeks ago, we published a paper describing Haootia quadriformis n. gen. n. sp. (Liu et al., 2014: – an extraordinary fossil impression that appears to indicate muscle bands, and bears a striking similarity to modern stalked jellyfish (Staurozoa). The idea that some of the Ediacaran discoidal forms may have been stem-group medusoids has made a big come-back.

And then we hear of Dendrogramma. The authors have referred it to Metazoa incertae sedis [“of unknown placement”]. The organism resembles cnidarians and ctenophores, but lacks the characters to establish a certain affinity with either group, though molecular analysis of further individuals might yet show that it belongs to one of these lineages (the existing specimens were damaged in preparation and not suitable for DNA analysis). Whether or not Dendrogramma turns out to represent a new phylum, it does seem to be a relatively primitive form, lacking cnidocytes, colloblasts, and other more sophisticated characters. From the discription, Dendrogramma appears to be a simple diploblastic animal with a disc showing a distinct pattern of gastrovascular branches and, in the case of one of the two species, D. discoides, a stalk with a possibly trilobed mouth-field. Various Ediacaran discoidal forms, particularly those from the diverse assemblages of South Australia and the White Sea, Russia, have trilobed structures within the disc, most obviously Tribrachidium. The authors point out the similarity of D. discoides with Albumares brunsae, and Anfesta stankovskii, as well as the less obviously trilobed Rugoconites from South Australia. There does appear to be a morphological similarity, particularly with the former two forms, both in the trilobed structure and in the pattern of radial ridges compared with the gastrovascular branching on the disc of Dendrogramma.

So can we conclude that Dendrogramma is a living Ediacaran? That’s almost certainly going too far. But it does seem quite possible that some of the trilobed Ediacaran discs may represent stem-group forms of such a lineage, lacking in such modern armoury as cnidocytes (for what would they sting?) and possessing a simple small mouth with no surrounding tentacles. As for all the other kinds of Ediacaran forms, even the many other discoidal forms, well, the work goes on.

Latha’s References:
Fedonkin, M.A., et al. (eds), 2007, The rise of animals: Evolution and diversification of the Kingdom Animalia: Baltimore, Maryland, Johns Hopkins University Press
Fedonkin, M.A., and Waggoner, B.M., 1997, The late Precambrian fossil Kimberella is a mollusc-like bilaterian organism: Nature, v. 388, p. 868–871
Glaessner, M.F., 1959, Precambrian Coelenterata from Australia, Africa and England: Nature, v. 183, p. 1472–1473
Glaessner, M.F., and Wade, M., 1966, The late Precambrian fossils from Ediacara, South Australia: Palaeontology, Vol 9 (4), pp. 599-628
Liu, A.G., McIlroy, D., and Brasier, M.D., 2010, First evidence for locomotion in the Ediacara biota from the 565 Ma Mistaken Point Formation, Newfoundland: Geology, v. 38, p. 123–126
Menon, L.R., McIlroy, D., and Brasier, M.D., 2013, Evidence for Cnidaria-like behaviour in ca. 560 Ma EdiacaranAspidella, Geology, v. 41, p. 895–898

Sprigg RC. 1947. Early Cambrian (?) jellyfishes from the Flinders ranges, South Australia: Trans. R. Soc. S. Aust. 71(Pt. 2):212–24



REFERENCE TO THE NEW PAPER: Just, J., R. M. Kristensen, and J. Olesen. 2014. Dendrogramma, New Genus, with Two New Non-Bilaterian Species from the Marine Bathyal of Southeastern Australia (Animalia, Metazoa incertae sedis) – with Similarities to Some Medusoids from the Precambrian Ediacara. PLOS One DOI: 10.1371/journal.pone.0102976