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Re: Pro(to)avis

How would this hypothetical first bird be DIFFERENT from Archaeopteryx?

Just to really frustrate you...

This depends on your definition of "bird". <g>

If, however, we happened to agree on "Archie is the oldest known bird, and the sister-group of all other known birds", then the first bird wouldn't differ much from it. *Archaeopteryx* does have a few peculiarities of its own, though...

Identifying apomorphies in Archaeopteryx seems to be "challenging" more because people want it to be the perfect bird ancestor (a metataxon if you will). Two very obvious apomorphies are the distally bifurcated ischium and the lack of caudal neural spines.

Apomorphy = derived feature ( = which its ancestors lacked).
Metataxon = taxon without apomorphies of its own (which are called autapomorphies).