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Re: Some thoughts on AVES (& gaps)





Sorry Demetrios, but Nick is right. We can never really hope to discover "the" fossil species that gave rise to any particular clade. We'll be lucky to even narrow it down to where we can identify a particular sister group at genus level. The gaps are there (and always will be, due to extinction and a sparse fossil record), so we might as well get some advantage out of that "lack of information".
What makes the mammal transition so useful is that it is NOT a single (population or species level) change. It is a series of character transformations that followed in relatively rapid succession (at least fast in geological time). This gap is huge compared to the little gaps which we neotologists deal with at species level.
Punctuated equilibrium is not always marked by just a single isolated "spike" of change. A quick series of such "punctuation" events can sometimes give us a sizeable gap that will probably not even be partially bridged for many decades (if ever). That's why the character-based definition of Mammalia is still so widely used. It's not really just a single character, but a series of character transformations which happened in a geologically short amount of time.
There might not be anything quite that clear-cut for an apomorphy definition of Aves, but I think we should look for the best one we can find, and hopefully it will be less arbitrary than just anchoring it on Archaeopteryx (which was a good tradition, but one whose usefulness is fading fast).
------Ken Kinman
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NJPharris first wrote:
> Fortunately, I don't have to. We only have to classify the specimens we actually discover. For this reason, abstract apomorphy-based definitions like the one I gave do not bother me overly much.

Demetrios responded:
Again, not only are you relying on a lack of knowledge (which we all labor to fill), but are setting up a system which inherently has limitations. One of these limitations is the fact that you can only work with fossil taxa. Yeah, the biases in the fossil record are great for this type of character-based classification system, but it doesn't work for extant organisms. Evolution is still happening, and we'll need to deal with speciation in the future. But contemporaneous speciation won't happen with those gaps that are inseparable from the fossil record; the overnight-character-acquisition system cannot apply today.




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