Asymmetrical trilobites

September 29, 2009 • 8:11 am

by Matthew Cobb

Jerry’s recent post about Steve Marley’s fantastic photo of a trilobite eye reminded me of one of the odder paleontological findings. Many trilobites have chunks taken out of them, presumably by predators. This fantastic CGI video – an extract of a display at the Chicago Field Museum – gives you some idea of what might have happened: at the end, a hapless Cambrian trilobite is snarfed by the top predator of the time, Anomalocaris. (This idea has recently been disputed by Whitey Hagadorn of Amherst College, who has made computer models which suggests Anomalocaris simply couldn’t have crunched the hard exoskeleton of a trilobite. Other people aren’t convinced and point out we’ll have to find some Anomalocaris poop to settle the issue…)

What’s odd about the munched trilobites is that, when you look at the distribution of injuries, you find a 2:1 ratio of injuries on the right hand side of the animals to injuries on the left-hand side of the animal. Like these:


This figure is taken from an article in Nature in 1989 by Loren Babcock and Richard Robison of the University of Kansas. The table showing the data (rather poorly reproduced below) is pretty convincing, but WHY?


The authors suggested that “those scars we attribute to sublethal predation… are significantly more frequently found on the right side of the trilobites, suggesting that predators preferred to attack that side”, and they pointed the finger at Anomalocaris.

There is a precedent for this kind of predator preference. In 1993, Michio Hori of Wakayama Medial College, Japan, reported in Science that scale-eating chichlid fish in Lake Tanganyika showed handedness and that “attacking from behind, right-handed individuals snatches scales from the prey’s left flank and left-handed ones from the right flank”. Maybe Anomalocaris (or some other Cambrian predator) showed similar preferences?

A few weeks after Babcock and Robison published their piece, Nature published two replies, one from David Maitland, the other from Stephen Stigler, contesting their interpretation.  Maitland suggested the apparent preference might be a consequence of the trilobites curling up to defend themselves (I don’t really follow his argument, to be honest), while Stigler, more interestingly, suggests this might be a problem of sample biais. Maybe the underlying assymetry is not in the predators, suggested Stigler, but in the internal organs of the trilobites: “it may be that trilobites were attacked equally often on both sides, but more of those attacked on the right side survived to join Babcock and Robison’s sample.”

That seems to have been the last word on the question, for the moment. But I am reminded of a study I heard about a while back, which was based on thousands of beachcombing records from the English Channel, where it was found (I think in Holland), that significantly more left shoes were washed up than right (or it may have been the other way round). I’ve been unable to track down this article, or any sensible explanation. All suggestions gratefully received!

24 thoughts on “Asymmetrical trilobites

  1. I love that Cambrian vid, and plan to show it in class tomorrow.
    One quibble: those disk-like jellies frisbeeing around at the beginning–weren’t those later revealed to be the mouths of Anomalocaris?

    1. The “disk-like jellies” are actually supposed to be Eldonia. At the time we recreated this supposed echinoderm the most popular interpretation suggested it was a free-floating, pelagic suspension feeder. This interpretation is still controversial.

  2. Jerry,

    I’m totally unfamiliar with the trilobyte phisiology, but I have an idea that expands on the sampling bias idea.

    Is it possible that the phisiology of a trilobyte wasn’t internally symmetrcal or was otherwise vulnerable and maybe a left side attack would have been more likely to take down the trilobyte.

    Maybe I’m being too specific, simplified: left side attacks may have been more successful due to some reason unpondered.

    Personally, I think a predator preferrence is much more likely, but I’m exploring.

  3. Bjørn, why would anyone ship lefts and rights separately?

    I feel smart – I was pondering the same thing as Stigler. Do we know anything about the guts of trilobites?

  4. Matt, was the bite taken during molting? Another whole world in the past that is interesting. I had no idea chichlids were left or right finned.

  5. I’ve been unable to track down this article, or any sensible explanation. All suggestions gratefully received!

    The answer is obvious. They key is that the study is from the UK. As everyone knows leprechauns only make one shoe, and have a preference for making left shoes. They must just throw the shoes in the ocean when they are done with them, since nobody really has a need for a few dozen left shoes, and due to currents some end up in the channel.

    Either that or its due to pirates, who most likely lose their dominate leg predominately. Peg legs don’t need shoes after all.

    1. Thanks! I’m glad you liked it. We’ve also done one of the Ordovician Churchill formation for the Manitoba Museum in Winnipeg. You can see a bit of on our website. And within a year we might have some more.

  6. Wow on the cgi.

    On the shoes, it is a well known fact that left and right shoes go on different sides. You have to look at the other side of the current to find the mirror article.

    Seriously, references please.

  7. Sven DiMilo-

    Nope, those disk-like organisms are Eldonia, thought to be a floating echinoderm. The anomalocarid mouths were previously named Peytoia and thought to be jellyfish at the time.

  8. So the theories include:

    1. Sample size is small so this is a fluke. To test: get more samples. It is also extremely important to include all trilobytes without bites in ’em otherwise you’re rejecting a large portion of samples and “cherry-picking” information. Yes, a study of trilobyte bites should have unbitten trilobytes.

    2. Predator preference: I think this is weak; it works if the predator needs to chase (so it almost always comes from behind) and has a preference to attack from one side. You need to be able to: (1) distinguish bite marks of different predators and (2) have enough samples. Looking at that image included above, sample size is way too small.

    3. Higher survival when bit from the right: To test the credibility of this one the only thing I can think of is to look at the internal anatomy of a trilobyte. Can we do that by gradually grinding/polishing away layers and effectively ‘slicing’ a fossil? If we know the anatomy better and can find similar existing animals we may be able to find more evidence for or against. (Although similarities to other animals remains a tenuous and contentious link and cannot be accepted as a ‘proof’.)

    4. Godditit when he faked the fossil record to fool humans. This is my favorite explanation because it’s always good for a laugh and it’s even more fun when it gets a scientist screaming.

  9. “But I am reminded of a study I heard about a while back, which was based on thousands of beachcombing records from the English Channel, where it was found (I think in Holland), that significantly more left shoes were washed up than right (or it may have been the other way round). I’ve been unable to track down this article, or any sensible explanation. All suggestions gratefully received!”

    A bit of googling showed some articles referencing Curtis Ebbesmeyer, who apparently has some plausible explanation.

    One blog reference is here:

  10. left & right shoes on the beach : See
    page 7 bottom ”
    Naar de schoenen is onderzoek
    gedaan door Mardik Leopold
    van Alterra (als afgeleide van een
    onderzoek naar olievogels). Er lijken
    meer linkerschoenen op het
    strand te liggen, maar onomstotelijk
    bewijs is er nog niet. Bij een cleanup
    op Texel werden in het zuiden
    veel meer linkerschoenen gevonden,
    in het noorden juist meer rechter
    exemplaren”, I translate : Mardik Leopold from Alterra did some research concerning those shoes (as part of researching oil contaminated birds). It seems there are more left shoes than right shoes on the beach, but this is not proven. Doing a cleanup on the island Texel in the south they found more left shoes, but in the north they found more right shoes.

  11. Seems I’m da’ shoe man to go to. 😉

    Thanks Ray! Obviously I didn’t really expect that the asymmetry would show up that clearly. Which is why I asked for such references. But it seems (shoe’s) current drift is a long sorting conveyor.

  12. One other thought: If predators attack, generally, from the rear, and if trilobites divided their vision with, generally, the left eye looking forward and the right one backward, they might tend to veer in such a way that the right side would be exposed to predation more often than the left.

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