Precambrian muscles??? Oooooh!

Okay, consider this a cautious squee. I wish at least some of those Ediacaran fossils were a little more obvious. I mean, I might love fossils, but I’m trained to squirt nasty chemicals on bits of dead worm and play with protein sequences, not to look at faint impressions in rock and see an animal.

Most putative animals from the Ediacaran period, the “dark age” that preceded the Cambrian explosion, are confusing to the actual experts, not just to a lab/computer biologist with a fondness for long-dead things. The new paper by Liu et al. (2014) this post is about lists a “but see” for pretty much every interpretation they cite. The problem is twofold: one, as far as I can tell, most Ediacaran fossils don’t actually preserve that much interpretable detail. Two, Ediacaran organisms lived at a time when the kinds of animal body plans we’re familiar with today were just taking shape. The Ediacaran is the age of ancestors, and it would be more surprising to find a creature we can easily categorise (e.g. a snail) than a weird beastie that isn’t quite anything we know.

Having said that, Liu et al. think they are able to identify the new fossil they named Haootia quadriformis. Haootia comes from the well-known Fermeuse Formation of New Foundland, and is estimated to be about 560 million years old. The authors say its body plan – insofar as it can be made out on a flat image pressed into the rock – looks quite a lot like living staurozoan jellyfish, with a four-part symmetry and what appear to be branching arms or tentacles coming off the corners of its body. The most obvious difference is that Haootia seems to show the outline of a huge circular holdfast that’s much wider than usual for living staurozoans.

However, the most exciting thing about this fossil is not its shape, but the fact that most of it is made up of fine, highly organised parallelish lines – what the authors interpret as the impressions of muscle fibres. The fibres run in different directions according to their position in the body; for example, they seem to follow the long axes of the arms.

(Below: the type specimen of Haootia with some of the fibres visible, and various interpretive drawings of the same fossil. Liu et al. is a free paper, so anyone can go and look at the other pictures, which include close-ups of the fibres and an artistic reconstruction of the living animal.)

If the lines do indeed come from muscle fibres, then regardless of its precise affinities, Haootia is certainly an animal, and it is probably at least related to the group called eumetazoans, which includes cnidarians like jellyfish and bilaterians like ourselves (and maybe comb jellies, but let’s not open that can of jellies just now). Non-eumetazoans – sponges and Trichoplax – do not have muscles, and unless comb jellies really are what some people think they are, we can be almost certain that the earliest animals didn’t either.

Finding Ediacaran muscles is also interesting because it gives us further evidence that things capable of the kinds of movement attributed to some Ediacaran fossils really existed back then. Of course, it would have been nicer to find evidence of muscle and evidence of movement in the same fossils, but hey, this is the Precambrian. You take what you get.

(P.S.: Alex Liu is cool and I heart him. OK, I saw him give one short talk, interviewing for a job at my department that he didn’t get *sniffles*, so maybe I shouldn’t be pronouncing such fangirlish judgements, but that talk was awesome. As I’ve said before, my fangirlish affections are not very hard to win 🙂 )

***

Reference:

Liu AG et al. (2014) Haootia quadriformis n. gen., n. sp., interpreted as a muscular cnidarian impression from the Late Ediacaran period (approx. 560 Ma). Proceedings of the Royal Society B 281:20141202

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5 thoughts on “Precambrian muscles??? Oooooh!

  1. dwbapst August 27, 2014 / 17:20

    “not to look at faint impressions in rock and see an animal”

    Not to give away our holy secrets, but most paleontologists I know (including myself) are awful at this unless it is our actual study group. Which means, of course, I sometimes see graptolites where there aren’t…

  2. johnnymorales June 27, 2015 / 21:28

    why not an ancestor of cubazoa aka the very deadly box jellyfish which shares the 4 sided symmetry instead of starozoa which has 5 sided symmetry?

    If I understood in your column is correct (that jellyfish use such muscles to pulse) then a much closer match to the muscles in the fossil would be found in a cubozoan

    • Naraoia July 4, 2015 / 13:26

      I don’t think Staurozoa have fivefold symmetry, not generally, anyway. I’m not a cnidarian expert by any stretch, but all photos I’ve seen of staurozoans show fourfold symmetry. More importantly, though, I don’t think Haootia being a cubozoan ancestor would change the main issue – which is that it appears to be a sedentary animal with strong “swimming” muscles. Box jellies are not sedentary, they are pretty strong swimmers. (Unless their polyp forms pulsate while sitting on the seafloor. It’s entirely possible that I just haven’t heard about that, in which case a citation to look up would be most welcome ;))

      • johnnymorales July 5, 2015 / 09:50

        You are right of course. As for whether or not their polyp forms pulsate, as you know info on that is hard to come by, BUT i did run across a page on a professor who is most likely to know.

        Vicki J. Martin, a professor of biology at Appalachian who has successfully grown a species of box jelly common of the Eastern coast of the USA in her lab for years now.

        If you aren’t already aware of her and her studies a link to a university news article about it is:

        http://www.news.appstate.edu/2009/04/10/box-jellyfish/

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