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Re: AW: Heterodontosaurid with protofeathers



On Tue, Mar 31, 2009 at 7:04 PM, Dann Pigdon <dannj@alphalink.com.au> wrote:
> Quoting "T. Michael Keesey" <keesey@gmail.com>:
>
>> On Tue, Mar 31, 2009 at 4:01 PM, Dann Pigdon <dannj@alphalink.com.au> wrote:
>> >
>> > There's no reason to assume that fur evolved just once amongst
>> > mammals
>>
>> Given that we have direct evidence of fur in a stem-mammal (the
>> _Castorocauda_ holotype, a docodont mammaliaform), there's a pretty
>> good reason to assume that. (The picture *could* be more complex, but
>> at this point there's no reason to *assume* so.)
>
> Provided of course that some mammalian lineages didn't become secondarily 
> hairless, then their
> descendants re-evolved integument. That might be highly unlikely though (but 
> not necessarily
> impossible).

Molecular analysis should be able to determine this.

> Then again, who's to say that the fur traces that have been fossilised were 
> in life chemically similar
> to modern mammalian fur? All we really know is that it was physically very 
> similar in appearance.
> The original chemical composition, or the genes responsible for producing it, 
> could have been very
> different.

Yes, but we're talking about assumptions. Had you said "assert"
instead of "assume", you'd have a point, maybe. There's plenty of good
reason to assume that hair is a plesiomorphy for _Mammaliaformes_ (and
even without _Castorocauda_ there'd be plenty of good reason to assume
that it's a plesiomorphy for _Mammalia_). Of course we can't ever
assert such things with 100% certitude, but it's still a good,
parsimonious assumption until such time as new evidence trumps it.

-- 
T. Michael Keesey
Technical Consultant and Developer, Internet Technologies
Glendale, California