Re: Can we ever recover ancestors? (Was: Fwd: Are dinosaurs really reptiles? (2))

[David Marjanovic <david.marjanovic@gmx.at>]

> The example that springs to mind is the stratigraphic sequence of
> centrosaurines in Horner's book _Dinosaur Lives_.  I'm not saying that
> it's a slam-dunk, but I would at least be slow to dismiss the
> possibility that they really do constitute an ancestor-descendent
> chain.

The possibility certainly deserves consideration -- but only if none of these 
centrosaurines (except the last) have autapomorphies. This part was routinely 
overlooked before the introduction of cladistics.

> The problem is not so much that we can never find an ancestor; it's
> that we can never know when we _have_ found it.

Yep. The basic limitation of science.

And _then_ comes the stochastics. If we find a metataxon ( = lacking 
autapomorphies) that is older than its possible descendants, what is the 
probability that it really is an ancestor? The two considerations here are
-- the completeness of the fossil (most potential autapomorphies are in DNA or 
soft anatomy and don't fossilize)
-- and the completeness of the fossil record in time and space: how probable is 
it that we haven't overlooked an even more suitable candidate (or that it was 
never preserved in the first place)? This point is what I was talking about.
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