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Re: Study: Platypus Retains Bird Sex Link

Quoting Dora Smith <villandra@austin.rr.com>:

> I thought that all reptiles had the basic sex chromosome structure of birds.
> It is backwards from taht of mammals; the letters are different, for
> different chromosomes, I guess; but with birds, two of a particular
> chromosome makes one a male, and one of each makes one a female.

This is correct as far as birds go, but I was under the impression that other
reptiles did not generally have chromosome-based sex determination.

> I understand that mammals are descended from a very primitive reptile near
> the foot of the reptile family tree.

If that ancestor were alive today, people might (might!) refer to it as a
"reptile" based on its looks, but under the definition usually used today, that
ancestor was not a member of Reptilia.

> Dinosaurs are very highly evolved
> reptiles near the top fo the reptile family tree.   Birds belong to one of
> the major branches of dinosaurs, and they are highly evolved relative to
> their feathered cousins.

If you mean modern birds have changed a lot since they diverged from their
closest non-bird theropod relatives, then yes.  But bear in mind that that
doesn't mean that birds are "better" or "more fit" than other dinosaurs in any
absolute sense.

> So if tehre are similarities between a promitive mammal, probably LONG
> removed from its common ancestor with other mammals, and birds, then
> wouldn't the reason why be that reptiles were our ancestors?

Any features in which platypus are more like birds than other mammals were
probably present in the common ancestor of mammals and reptiles, unless those
features were acquired independently in the platypus and bird lineages.

Nick Pharris
Ph.D. Candidate
Department of Linguistics
University of Michigan