Why giant sea scorpions got so big

ResearchBlogging.orgResearch on giant sea scorpions (eurypterids) – the largest bugs that ever lived – has shed new light on why eurypterids became so large and eventually died out.

In the 1930s, Romer argued that eurypterids became larger and larger over time because they were in a kind of arms race with giant fish called placoderms. Presumably the larger would be the predator of the smaller, so the smaller grows. Others have suggested that a greater abundance of oxygen in an ancient atmosphere (and thus sea) just made things big. New research seems to indicate that both ideas are still valid.

There are two main evolutionary lineages of eurypterids. One of them, the predatory eurypterids, grew to up to 2.5 meters in length and may have done so in competition with placoderms. The second group did not grow as long, and were sea floor scavengers, may have grown long because of environmental conditions.

According to James Lamsdell, the lead author of the paper describing this finding,

… the evolution of the two main eurypterid lineages was quite different. The giant predatory eurypterids increased in size but decreased in diversity as placoderms become more common, while the other form of eurypterids that were initially small scavengers, only reached their massive size later on when many other invertebrates also increased in size.

Eventually, the giant predatory eurypterids seem to have lost the arms race, about 400 millin years ago. The placoderms won. The scavenging eurypterids lasted another 140 million years or so, and their size increase happened later as well owing to the timing of optimal ecological conditions. They were killed off by the Permian Extinction, which just goes to show that natural selection does not design organisms to cope with events that have not happened yet!

Carsten F. Dormann, Bernd Gruber, Marten Winter, & Dirk Herrmann (2009). Evolution of climate niches in European mammals? Biology Letters

Comments

  1. #1 travc
    October 14, 2009

    A quibble on “natural selection does not design organisms to cope with events that have not happened yet”.

    Yeah, very true and important to point out to folks who don’t really grok ToE. But perhaps more interesting to those who do already generally ‘get it’… Evolution can design organisms to cope with variability in the environment. Part of this is a (normally weak) selection for adaptability… a sort of evolution of evolvability. There is an infinite regression… but we are still arguing over wether the first order effects are strong enough to generally matter.

    Predatory eurypterids appear to have been pushed by strong-ish selection towards larger body sizes. This decreased their variability and the population’s ability to respond to different selective pressures on body size (and probably lots of ancillary traits). That set them up for a fall when the environment changed, since they traded a lot adaptability for optimization to the particulars of a single niche. The scavengers were more diverse and maintained more adaptability as a group… so they lasted longer.

    Hard data would be nice… but it does make a nice story ;)

  2. #2 Zach Miller
    October 14, 2009

    I can’t help but think the paper was poorly titled, as neither placoderms nor eurypterids are mammals, or even tetrapods.

  3. #3 Phillip IV
    October 14, 2009

    I can’t help but think the paper was poorly titled, as neither placoderms nor eurypterids are mammals, or even tetrapods.

    At first they were going to name it ‘J.C. Lamsdell and S.J. Braddy (2009). Cope’s Rule and Romer’s theory: Patterns of diversity and gigantism in eurypterids and Palaeozoic vertebrates. Biology Letters.’ But the focus group wasn’t too hot for that title – fuzzy animals simply sell a lot better.

  4. #4 Rob Jase
    October 14, 2009

    I think you’ve stumbled onto something Zach.

  5. #5 MadScientist
    October 14, 2009

    I’ve always objected to the use of the word “design” because evolution doesn’t design anything. If something happens in the environment, say, much higher temperatures, there is no opportunity for evolution to do anything. The organisms that can withstand higher temperatures will already exist and will be the preferred individuals (part of natural selection). So the (single species) evolution necessary to cope would have already happened or else the species goes extinct; in this case the ability to withstand higher temperatures gives an individual an advantage in selection where previously there was no obvious advantage between the individuals.

  6. #6 Diane G.
    October 14, 2009

    The placoderms won.

    Endoskeleton beats the exoskeleton in size race yet again.

  7. #7 Greg Laden
    October 15, 2009

    travcEvolution can design organisms to cope with variability in the environment.

    I think that is right but it remains to be demonstrated to the degree needed to become part of the dogma. In any event, mass extinction events != variability in the environment than can be tracked by N.S.

    I have a one liner I like to use in teaching evolution because it get people mad and is as true as any other thing people say about evolution, yet flawed, and is thus a good starting point for discussion: “The ultimate outcome of evolution is almost always extinction.” The body size conundrum you point out is an example of that.

  8. #8 Greg Laden
    October 15, 2009

    Right … wrong reference. Well, I’m on the road so this is going to be hard to fix for a couple of days. Was this a glitch in the automatic reference generation system using DOI? Maybe. Maybe not. We shall see…

    Anyway, the paper is the next one over in the same journal.

  9. #10 Kat
    April 3, 2012

    Hello. I’m wanting to post a comment of your statment of natural selection doesn’t design organisms to cope with events that havn’t happened yet.
    It’s just as well. Can you imagine how dangerous it would be for modern day swimmers to go a swim in the sea and get killed by a giant Sea Scorpion. How dangerous it would be for bird life and marine life.
    It’s just as well the Megaladon went extinct as well, how life threatening that would be surfers and luxury cruisers.
    I don’t think Jurassic should be made a reality either, because it would end up happening just the way it happened in the movie.