The things you can tell from a pile of corpses…

I’m really late to this party, but I never claimed to be timely, and the thing about the reproductive habits of Fractofusus is too interesting not to cover.* Rangeomorphs  like Fractofusus are really odd creatures. They lived in that Ediacaran twilight zone between older Precambrian seas devoid of macroscopic animals and younger Cambrian seas teeming with recognisable members of modern groups. Rangeomorphs such as RangeaCharnia and Fractofusus itself have such a unique fractal body plan (Narbonne, 2004) that no one really knows what they are. Although they were probably not photosynthetic like plants or algae (they are abundant in deep sea sediments where there wouldn’t have been enough light), their odd body architectures are equally difficult to compare to any animal that we know.

Mitchell et al. (2015) don’t bring us any closer to the solution of that mystery; they do, however, use the ultimate power of Maths to deduce how the enigmatic creatures might have reproduced. Fractofusus is an oval-shaped thingy that could be anywhere from 1 cm to over 40 cm in length. Unlike some other rangeomorphs, it lay flat on the seafloor with no holdfasts or stalks to be seen. Fractofusus fossils are very common in the Ediacaran deposits of Newfoundland. Since there are so many of them, and there is no evidence that they were capable of movement in life, the researchers figured their spatial distribution might offer some clues as to their reproductive habits. A bit of seafloor covered in Fractofusus might look something like this (drawing from the paper):

clusters within clusters

(The lines between individuals don’t actually come from the fossils, they just represent the putative connection between a parent and its babies.)

Statistical models suggest that the fossils are not randomly distributed but clearly clustered: small specimens around medium-sized ones, which are in turn gathered around the big guys. Two out of three populations examined show these clusters-within-clusters; the third has only one layer of clustering, but it’s still far from random. As the authors note, the real populations they studied involve a lot more specimens than shown in the diagram, but they “rarefied” them a bit for clarity of illustration while keeping their general arrangement.

The study looked not only at the distances between small, medium and large specimens, but also directions – both of where the specimens were and which way they pointed. If young Fractofusus spread by floating on the waves, they’d be influenced by currents in the area. It seems the largest specimens were – they are unevenly distributed in different directions. In contrast, smaller individuals were clustered around the bigger ones without regard to direction. Small and large specimens alike pointed randomly every which way.

What does this tell us about reproduction? The authors conclude that the big specimens probably arrived on the current as waterborne youngsters, hence their arrangement along particular lines . However, once there, they must have colonised their new home in a way that doesn’t involve currents. Mitchell et al. think that way was probably stolons – tendrils that grew out from the parent and sprouted a new individual at the end. This idea is further strengthened by the fact that among thousands of specimens, not a single one shows evidence for other types of clonal reproduction – no fragments, and no budding individuals, are known. (Plus if a completely sessile organism fragments, surely the only way the pieces could spread anywhere would be by riding currents, and that would show up in their distribution.)

Naturally, none of this tells us whether Fractofusus was an animal, a fungus or something else entirely. Sending out runners is not a privilege of a particular group, and while there is evidence that the original founders of the studied populations came from far away on the waves, we have no idea what it was that floated in to take root in those pieces of ancient seafloor. Was it a larva? A spore? A small piece of adult tissue? Damned if we know. Despite what Wikipedia and news headlines would have you believe, there is nothing to suggest that sex was involved. It may have been, but the evidence is silent on that count. (Annoyingly, the news articles themselves acknowledge that. Fuck headlines is all I’m saying…)

While sometimes we gain insights into ancient reproductive habits via spectacular fossils like brooding dinosaurs or pregnant ichthyosaurs, this study is a nice reminder that in some cases, a lot can be deduced even in the absence of such blatant evidence. This was an interesting little piece of Precambrian ecology, and a few remarks in the paper suggest more to come: “Other taxa exhibit an intriguing range of non-random habits,” the penultimate paragraph says, “and our preliminary analyses indicate that Primocandelabrum and Charniodiscus may have also reproduced using stolons.”

An intriguing range of non-random habits? No citations? I wanna know what’s brewing!

***

*Also, I’ve got to write something so I can pat myself on the back for actually achieving something beyond getting out of bed. Let’s just say Real Life sucks, depression sucks worse, and leave it at that.

***

References:

Mitchell EG et al. (2015) Reconstructing the reproductive mode of an Ediacaran macro-organism. Nature 524:343-346

Narbonne GM (2004) Modular construction of Early Ediacaran complex life forms. Science 305:1141-1144

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4 thoughts on “The things you can tell from a pile of corpses…

  1. johnnymorales March 26, 2016 / 07:57

    Hi, I love your scientific meandering.

    I was wondering if you’d do a very easy and quick meander on the ediacaran fossil fossil spriggina.

    If I’m right about this, it could be very interesting for you, because if I’m right a clear descendant of an ediacaran is alive today.

    If not you’ll be able to dismiss it within a few seconds.

    I always thought it must be a direct ancestor, because I’ve been lucky enough to see the living critter’s ventral side, something no paleontologist has done apparently.

    Otherwise the shared features between the fossil and the living critter which are clear and abundant would have been an eureka moment immediately leading to a professional paper to write a paper to get credit for the discovery of a living descendant of an ediacaran creature.

    Virtually all the images you can find of this critter show the dorsal side, and from that perspective the resemblance is superficial at best and wouldn’t generate any interest.

    I know I’m being a bit nebulous, but that’s because I think pictures will say everything there is to say far better than typing it all out for you to read then still have to wait for the 2 pictures of spriggina and the current living critter to see if you agreed or disagreed.

    I wanted to send you the photos in this message, but I could find no way to do it. I wasn’t able to create my own blog, because I lack something called an “active token” about which I can find no information.

    I read your most recent blog, and I also thought I’d ask you to do this, because if I’m right it should cheer you up. I’m not a paleontologist, if that’s not obvious by now. So I am unable to use this info in any way.

    So if you can tell me how you’d like to get the comparison photo…..

    I can email it to you if you give me your email, or I can post it on one of those picture sites like pinterest or Instagram or the site of your choice if you’d rather not give me an email.

    • Naraoia April 9, 2016 / 17:06

      The problem with things like Spriggina is that what looks like a dead ringer to a layperson (and I definitely count myself among those in this case) is probably no more than a superficial similarity to an expert. The look of these fossils depends on a lot more than the look of the living organism – the nature of the sediment, the process of decay, etc. There is also relatively little preserved detail in fossils like Spriggina, so it’s very hard to identify the kinds of traits that could securely link it with a living group.

      If there is one thing I’ve learned from being a scientist, it’s that the correct answer is very rarely “obvious”. When you don’t have a deep knowledge of a field, you inevitably miss *so many* important things, and you don’t even realise that you are missing anything.

      If you want a *useful* response, I would suggest contacting a real expert on Ediacaran fossils, someone who understands how they were formed and knows their existing interpretations in more depth than I do. I’m not one of them – like you, I’m not even a palaeontologist, so my opinion would not have the necessary knowledge behind it.

      I’m not sure who the best people for Spriggina might be, though. Budd and Jensen’s 2015 review (http://onlinelibrary.wiley.com/doi/10.1111/brv.12239/full) is one recent paper that gives it more than passing mention; Jim Gehling (http://www.samuseum.sa.gov.au/about/staff/dr-jim-gehling) is one researcher who actually studies the fossil site where Spriggina comes from. TBH, Google Scholar is probably going to be a bigger help than me here 🙂

      • johnnymorales April 10, 2016 / 02:03

        Everything you say is right of course, with one caveat.

        There is a tremendous lack of cross expertise or cross fertilization among the various scientific fields.

        Paleontologists in particular don’t necessarily have better knowledge of what living species look any more than the average person beyond those that are well known.

        So while they as a group are very likely to recognize a fossil reptile, they are far less likely to make a connection to obscure living, poorly researched groups almost unknown to anyone save those who study them especially one that is as minor a part of its phylum as this one is. One that has 10,000s of different ones alive today.

        That they there are paleontologists who have postulated that spriggina is an early arthropod makes me think that rather than being mislead by my ignorance, it is far more likely that no paleontologist who is very familiar with spriggina has ever seen the specific photo in question.

        That you found it difficult to think of any who are knowledgeable about spriggina underlines that point.

        All too often in paleontology, the fact that the right person never sees the right information to put all the pieces together about an extinct organism ends up the basis of a mystery that can take a century to solve.

        Of course, I thought you were a paleontologist. That’s why I asked if you wanted to see the photo. Now that I know differently, I understand and respect your lack of interest. Thanks for replying.

      • Naraoia May 4, 2016 / 15:25

        I won’t dispute the point that obscure groups can escape the notice of those not specifically focused on studying them, but the general impression I get is definitely that palaeontologists are keen to learn about the biology of modern organisms to better understand extinct critters, be the question the behaviour of predatory dinosaurs or the decay of dead chordates. The right people are often receptive, they just might need the right nudge to see an opportunity for collaboration 🙂

        Anyway, I’m definitely not a palaeontologist. My degree involved bits and bobs of ecology, evolution, molecular biology, genetics, developmental biology and zoology, and my PhD was about appendage regeneration in a modern polychaete worm, not much fossil connection there. Nearly everything I know about palaeontology, I picked up in my spare time. The fact that I can’t easily direct you to the right people to talk to about Spriggina probably says more about the sporadic nature of my reading than the state of knowledge on Spriggina 😉

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