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Jaws did not dominate early oceans

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fossil fish jaw mandible show evolutionary stability in oceansDeep in the Silurian seas, some 420 million years ago, a strange structure had just emerged in the bodies of many new vertebrates. Some fish began developing a defined upper and lower jaw that allowed them to devour large and hard-shelled organisms.

Today more than 99 percent of vertebrates have these handy eating apparati. But new research shows that for all of their utility, mandibles did not take over the oceans in one swift chomp, as many scientists had previously assumed. Rather, the early, jawed fish didn’t make too much of a dent in their jawless compatriots’ success for some 30 million years. The new findings were published online July 6 in Nature (Scientific American is part of Nature Publishing Group).

And "when the jawless fishes do decline, we see no indication that their jawed cousins took up new functional roles," Philip Anderson, of the University of Bristol’s School of Earth Sciences and co-author of the new study, said in a prepared statement. That finding has theoretical implications that reach far beyond the layers of Silurian sediment. It calls "into question old ideas of ecological replacement," that as a niche opens up, other species will fill it, he noted.

After analyzing fossilized Silurian and Devonian mandibles from 198 genera for different functional traits, the researchers also found that the emergence of this revolutionary mechanism did not set off a wild spree of evolutionary experimentation in design. Instead, after an initial bump of diversification—and a few exceptional mandibles in extinct placoderms and lungfishes—the major groups of jawed creatures whose descendents persist today, such as the ray-finned fishes and early tetrapods, seemed to stay relatively uniform. And all of this stability persisted despite "fluctuating global temperatures, shifting continental weathering patterns, sea level changes and a major extinction event," the researchers noted in the new paper.

Image of early fish jaw fossils, which show more diversity in lineages that later went extinct than in those that produced many of today’s fish and terrestrial vertebrates (bottom two); courtesy of Simon Powell/Philip Anderson/Matt Friedman

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  1. 1. jtdwyer 12:01 am 07/7/2011

    Is the only measure of an adaptation’s success its success in wiping out competing and/or prey species? I would have though a better measure of success might be the populations numbers numbers of varying adapting species, even though accurate species population numbers are almost impossible to derive from the fossil record.

    Humanity might survive a little longer if we didn’t simply consume everything and reproduce to the exclusion of all others.

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  2. 2. E-boy 1:54 am 07/7/2011

    Ever since Malthus people have been predicting doom and gloom with what I can only call no real evidence. Yes, there is plenty of reason to suspect that there may be some major population bumps along the road, but even with disasters, global warming, and killer bees there’s no reason to think humanity is in imminent danger of extinction. That stance shows just as much hubris and is based on just as little evidence as the stance of people who think we can go on living unsustainably. The truth is we really have no idea how long our species will last.

    Moving on to your earlier comment, it’s very easy in hindsight to call a particular feature adaptive. Especially when you are dealing with fossils. Sometimes we have no idea what "purpose" a given feature served (An example would be the odd bony projections in the nasal anatomy of Neanderthals). In point of fact, it’s probably a mistake to think too much like engineers when looking at skeletal or other anatomical features. Genetic drift can play some interesting tricks in feature distribution, and there are a lot of features that are there as a consequence of some other structural interaction. This happens with genes too. It’s all well and good to try and pin one specific function on one specific gene, but from what they have found so far genes exist in a sort of ecosystem all their own and can have far reaching and downright weird effects. For example some genes for muscle fibers can have an impact on brain function. For reasons that are not firmly understood the genes associated with red hair seem to have an impact on pain tolerance.

    Gould, in his work showed that a great many features appear as a sort of unrelated consequence of other features, and that some features were almost utterly dependent on environment (Like the shape of some species of snails). He also would not have been surprised by the failure of jawed fishes to race into new niches.

    The best measure of a particular features adaptiveness is the survival of it’s posessor. Replacements occur when there is competition for a niche. All this article appears to be saying is that there was initially no strong competition for already occupied niches by jawed fishes and that they stayed in their own even when others opened up. Adaptations don’t always create competition, sometimes they avoid it.

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  3. 3. jtdwyer 5:24 am 07/7/2011

    I haven’t read or studied Mathus’ philosophies, only population data. In Malthus’ time (around 1800) the population of the world was less than 1 billion people. By 1900 it had only increased to 1.6 billion. Since that time, the world population has more than quadrupled. Moreover, the largely dispersed rural population has increasingly migrated to vast urban centers, most often located with fifty miles of a sea shore. Perhaps Malthus was ahead of his time in some respects.

    Continually dismissing any concern regarding overpopulation because Malthus seems to have jumped the gun a couple of hundred years ago is foolish, unless there is no possible population that cannot be supported by the Earth’s resources. I think that is not the case.

    Regarding my first remarks, they were responses to the article, which began: "…fish began developing a defined upper and lower jaw that allowed them to devour large and hard-shelled organisms." I understood that statement to fit the definition of an adaptation, regardless of how it originated.

    You eventually stated: "The best measure of a particular features adaptiveness is the survival of it’s posessor."
    That was my point.

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  4. 4. Bill Crofut 11:29 am 07/9/2011

    Prof. Ehrlich, in the sequel to "Population Bomb," bemoaned the fact that no one seemed to be paying any attention to his initial warning. Gee! One can only wonder, Why not?

    What, precisely, is meant by "…an initial bump of diversification…"?

    What would seem to be indicated by persistence of relative uniformity?

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