Hi Arachnophilia,
this is some nice circular logic here, and it needs to be pointed out.
why are feathers not a defining characteristic of a bird? no other animal today has them. do reptiles have them? mammals? amphibians? fish? if you ask any biologist the single most defining feature of a bird what will they say?
Feathers are not definining characters of birds because non-birds have feathers. If you ask a biologist who has a knowledge of therapoda, he will probably reply that feathers are not defining characters of birds. Extant birds have feathers & no other extant organisms do, but that's neither here nor there. Extinct non-avians, as identified cladistically, have feathers, ergo; feathers do not define birds in the most inclusive sense.
for something to be partway between a bird and something else, wouldn't it have to feathers? why should that rule out feathers as a defining feature? it is, afterall, what we're looking for.
In this case all birds have feathers, but not all non-avians lack them. In other words, feathers evolved before birds. But speaking generally & hypothetically, suppose early birds lacked feathers, so what? It doesn't make them non-birds. What makes them birds are that they are on or above the node on a cladogram called "Aves". Running with this hypothetical, all extant birds which share characters that are exclusive to other organisms are crown group birds, everything else on or above the Aves node are stem group birds. Characters that are common to crown & stem groups are defining characters of clade Aves. A good example of this is the fused pygostyle, ALL extant birds have this, Archy lacks it. If Archy is to be considered a bird (& not all cladograms agree, it is often placed in the next node lower than Aves; Aviala) then it is a stem group
bird.
In short, there is nothing that cladistically says that all extinct organisms in a clade must have the same characters as all extant members of that clade.
but you can't say it's not a defining characteristic of a bird because the transitional form that led up to birds also had feathers. that just makes no sense.
No I didn't. The feathered non-avians are not birds, they are below Aves, furthermore, after the aves/dromaeosaur/troodontid split there was then a sister group that possessed feathers. Ergo if feathered therapods exist that are non-ancestral to Aves, then feathers are not a defining character of birds by definition.
Mark
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