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Re: 11th specimen of Archaeopteryx



Am 31.10.2011 14:52, schrieb Jason Brougham:

 In discussing this we are discussing animals, perhaps one lineage of
 animals, where evolutionary transitions were occurring (where one
 lineage gave rise to proper birds). I am not an expert on
 evolutionary theory, but there must be a state that sometimes occurs
 in evolutionary transitions where organisms have not yet adapted
 anatomically to a new function or habitat, but are instead adapting
 by sometimes straining to the maximum of their physical abilities.
 This strain, in turn, provides the natural selection that leads to
 the anatomical adaptation. In other words, some small paravian or
 basal bird was in trees without a hallux, and this provided the
 selection pressure for a hallux to evolve. It is the same with
 powered flight. There must have been animals that were poor and
 tentative fliers, that lacked large sternal keels, and in which the
 selection pressure for a large carina arose. Now, of course, I know
 that this is adaptationism, and I am skeptical of strict
 adaptationism, but there must be a subset of cases in evolutionary
 history where features arose not sheerly through exaptation or evo
 devo consequences, but through simple adaptation: If you have a
 slightly better grip on the branch you are a little more likely to
 survive and leave offspring who also survive.

I fully agree. The reason I don't think all small eumaniraptorans from the ?Middle? Jurassic to the Early Cretaceous are examples of this is that I expect such phases to be very short -- too short to show up at any reasonable frequency in the fossil record of Mesozoic vertebrates.

 There are also known cases where a group of animals perform a
 function which is an important part of their biology and they simply
 never show an anatomical adaptation to this function, it simply never
 arises. I now collect photos of turkeys and other basal neornithines
 brooding their chicks in trees. Looking at these images the parents
 certainly look awkward and we'd all feel a lot more comfortable about
 the situation if they had feet like Cracids. But they don't, they are
 somewhat ill suited to this sub-habitat, and future Paleontologists
 will probably never ever be able to demonstrate that they behaved
 this way.

Still, their feet are better suited to this than Archie's or even *Microraptor*'s.