The last perambulation of an ancient arthropod

September 8, 2012 • 8:46 am

From BBC Science News via a series of tweets, including Barbara King, Steve Ashley, and finally an email from Matthew Cobb, an amazing fossil finds its way to us.

As reported by Nick Crumpton, a fossil “death march” of a horseshoe crab was found in the Solenhofen limestone—the same formation that yielded the famous transitional fossil Archaopteryx.  The fine-grained sediments from what was once a quiet lagoon produced exquisite preservation, and in this case we have what is interpreted as the final walk of a horseshoe crab flung into the lagoon by a storm (the storm part is speculative) 150 million years ago.  Here’s the animal:

Note the phenotypic similarity to modern horseshoe crabs, a similarity which makes this animal a famous “living fossil.” Of course they’re not externally identical to modern ones, and we know nothing about the changes in its anatomy, biochemistry, or simply DNA sequence, which presumably has changed via the molecular clock in the last 150 million years.

After it purportedly landed in the lagoon, the crab began to walk, and made 9.7 meters before it died. (Remember, horseshoe crabs are in the subphylum Chelicerata, not Crustacea, so they’re more closely related to spiders and scorpions than to “real” crabs, which are crustaceans.)

As the BBC reports:

The fossil records an entire walk, and the researchers believe that the abrupt beginning of the trace can be explained by the animal being “flung” into the lagoon during a storm, although they cannot be certain of this interpretation. . .

“The lagoon that the animal found itself in was anoxic, so at the bottom of these lagoons there was no oxygen and nothing was living,” Mr Lomax [Dean Lomax of the Doncaster Museum and Art Gallery] told the BBC.

“This horseshoe crab [Mesolimulus walchi] found itself on the lagoon floor and we can tell by looking at the trace that the animal righted itself, managed to get on to its feet and began to walk,” he explained.

However, the anoxic conditions of the lagoon floor quickly proved fatal to the arthropod and it soon began to struggle.

“We started to study the specimen closer and saw that the walking patterns and the animal’s behaviour started to change. The leg impressions became deeper and more erratic, the telson (the long spiny tail) started being lifted up and down, up and down, showing that the animal was really being affected by the conditions,” he said.

And here’s the fossil trackway, also preserved in those optimal conditions. The animal moved from right to left in the picture (tracing is below), and you can see the fossil at the end.

RIP, ancient arthropod.

37 thoughts on “The last perambulation of an ancient arthropod

    1. yeah. They’re tough.
      I suppose that they don’t get much oxygen when out of the water for breeding purposes. That would select for anaerobic exercise capacity.

  1. Thanks for the great post. Not to be pedantic, but horseshoe crabs are members of the Chelicerata, which along with Crustacea and some other groups are all members of the Arthropoda. Also, your first “150” is missing a 10^6.

  2. RIP, ancient arthropod.

    Surely you don’t really want it to rest in peace, surely you’d prefer it to rest in an exhibit somewhere, being ogled by scientists, the public, school kids … and website readers. 😉

    1. RIP, ancient arthropod

      surely you’d prefer it to rest in an exhibit somewhere, being ogled by scientists, the public, school kids…

      but, er, peacefully… :]

  3. (Remember, horseshoe crabs are arthropods, not crustaceans, so they’re more closely related to spiders than to “real” crabs.)

    I think you meant to say “chelicerates”, not “arthropods” there.

    But thanks for the tragic tale.

  4. Sorry, I am not usually this pedantic, but:

    “horseshoe crabs are arthropods, not crustaceans”

    Of course, all crustaceans are arthropods, too, just as insects and arachnids are. Presumably you meant they are chelicerates, related to arachnids?

  5. 150 years? Is WEIT going YEC on us?

    Such was my first reaction on reading the post in my email. I have noted that it has since been fixed.

  6. Of course they’re not externally identical to modern ones, and we know nothing about the changes in its anatomy, biochemistry, or simply DNA sequence, which presumably has changed via the molecular clock in the last 150 million years.

    I’d love to see another post that goes into more detail on this topic.

    At the very least, I’d assume that the immune system is much more sophisticated in the modern descendants than their ancestors. Modern pathogens are presumably much more sophisticated than ancient ones, and, presumably, many modern pathogens that are little more than annoyances today are easily capable of devastating ancient hosts. Of course, the ancient ancestors of modern pathogens wouldn’t have been so lethal to their ancient hosts.

    What the morphological similarities tell me is that, much as the body plan for fast predatory dolphins / sharks / tunas is close to optimal and, in the case of sharks, hasn’t changed significantly in hundreds of millions of years, the body plan of horseshoe crabs is likely already close to optimal for its own ecological niche.

    What I don’t get is what’s so special about the horseshoe crab body plan. With sharks, it’s obvious; it’s about as hydrodynamically-efficient as an aquatic animal can get. But the body plan of the horseshoe crab is as old as that of the shark…and I have no clue what physics or whatever else it is that makes it so special.



    1. I can only speculate that the domed shape is particularly suited to the task of travelling between the ocean and the shore without being tossed around.

      Perhaps the tail spike plays a part as well. It might provide some lateral stability against cross-currents, especially if they push it down when they feel themselves moving off track.

      Make me wonder if anyone has ever tested notions like these.

        1. Not a wind tunnel, but a water tunnel, of course…and you’d probably be better off with models at the least, and probably jumping straight to the computer modeling.

          I smell a really, really interesting interdisciplinary paper or dissertation here, on the hydrodynamics of horseshoe crabs. A bit of Googling suggests at least one such paper has already been written:

          but I imagine there may well be room for lots more, in case I’ve piqued the interest of anybody with the necessary background….



      1. A device that is well designed for getting between sub-marine level and well-up a beach. If you can’t see the military potential there, I can. Smell that lovely grant money!

    2. Why would immune system and pathogens be more “sophisticated?” Isn’t the point of evolution that all is locally optimized not progressively?

      ‘Course there are accumulations. Maybe more complex.

      1. A pathogen infects its host by exploiting some weakness. For example, in the case of AIDS and many other diseases the host’s immune system might not recognize the pathogen as a threat and so doesn’t mount an immune response.

        The survivors are those either without the initial weakness or with some other mechanism that compensates for it.

        A modern horseshoe crab has the immune system of its ancestor that was able to fight off the diseases that killed off the ancient victims. Introduce one of those ancient diseases to a modern horseshoe crab, and the crab already has everything it needs to fight off not only that particular disease, but, most likely, after so many hundreds of millions of years, that whole class of diseases.

        Modern pathogens are those which still manage to get around even the defenses of a modern horseshoe crab. They exploit weaknesses that ancient horseshoe crabs never faced, and they exploit those weaknesses while dodging all the defenses modern horseshoe crabs have in their arsenals.

        In short, it’d be like the entire ancient Roman military failing to fend off a single modern aircraft carrier’s invasion of the capital, or a lone ancient Roman battalion trying to storm a modern military base. In the former case, the Romans would be hit with devastatingly incomprehensible magical weapons that strike out of nowhere and dragons spewing armored soldiers whose spears throw lightning; in the latter, even the full might of an army armed with nothing but spears, swords, arrows, and Greek fire can’t even make it through the perimeter minefield, let alone get close enough to use its own weapons.



        1. Let’s set aside the analogies, this is biology.

          So the proposition is that pathogens and the immune system gets more “sophisticated” which means more complex? More moving parts?

          Different threats doesn’t mean more sophisticated.

          There does seem to be more complexity in terms of number of cells and more ecosystems exploited. But don’t see the more sophisticated.

          Is a mammal more sophisticated than a bacteria? Suppose so. Is an ant immune system more sophisticated than a rat’s? Guess I don’t see that a priori.

          1. So the proposition is that pathogens and the immune system gets more “sophisticated” which means more complex? More moving parts?

            No, by “sophisticated,” I merely mean, “better able to withstand a wider array of threats.” It wouldn’t at all be hard to imagine ways that less complexity and fewer moving parts would equate to better fitness, and therefore more sophistication.

            Is a mammal more sophisticated than a bacteria? Suppose so.

            Not only would I not suppose so, I’d initially lean towards the bacteria having the edge in sophistication. After all, bacteria are far more prolific and have retained their same basic body plan far longer than mammals. Indeed, some surprisingly-large percentage of your own body is comprised of bacteria.

            Is an ant immune system more sophisticated than a rat’s?

            At least to a first-order approximation, no.

            But a modern ant’s immune system is likely much more sophisticated than the immune system of a rat from a dozen million years ago, and a modern rat’s immune system is likely significantly more sophisticated than the immune system of an ant from a hundred million years ago.



            1. Well, in fact I no remember that there is a jump in “sophistication” from primates to humans. I forget what it is.

              It’s an interesting idea though. Something being more sophisticated in biology based on time passing.

              1. We are primates, one of four (or five) extant species of great ape (chimps / bonobos, gorillas, and orangutans being the others), with many species of new- and old-world monkeys and out the rest of the primate family tree.

                And this idea, though certainly interesting, is hardly novel. Darwin wrote a whole book on it, in fact — a very famous one. He wasn’t aware of microbiology, of course, so he only described this prevalence, or survival, if you will, of the most sophisticated, or most fit, macroscopic traits. But the same principles certainly apply at all scales of life.



              2. It seems like the principals of evolution are often best illustrated at the molecular level.

                I have a special interest in the immune system because of this. Been spending some time studying.

                The principals of evolution or descent and selection seem to be general characteristics of energy systems.

                There are some kool ideas about the immune system trading energy with behavioral adaptions. Robert Trivers talks about this.

    1. One of my favourite fossils in my rock pile (currently in the garage, buried after moving) is a collection lf worm burrows choked by a storm’s sand influx. I call it “a day in the death of a worm”.
      I’ll have to post some pictures, but not until I’ve got my broadband access sorted out properly.

  7. Nice! I am reading Fortey’s “Horseshoe Crabs and Velvet Worms” at the moment, so it’s quite fitting to stumble upon this now.

  8. There is a similar Devonian horseshoe crab trackway and trackmaker at AMNH, Hall of Ocean Life, mezzanine. The track is an inward spiral with the dead animal in place at the end. Most poignant fossil in the place.

  9. sad – but I guess all fossils record the death of an individual creature. Still, this description brought it alive for those moments before it died again. Yes, RIP, horseshoe crab relative.

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