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Re: [Re: [Re: [Re: [Re: Feathered/scaly theropods: trying to make the point.]]]]



On 25 Jun 2001 archosaur@usa.net wrote:

> > It's over (possibly well over) 120 Mesozoic species and counting. (And
> > that's not including about 9,000 Cenozoic species....) And remember,
> > Clade(_Sinosauropteryx_ + _Passer_) is only the *smallest* clade which
> > feathery integument might be a synapomorphy of.
>
> Compared to the 6 or 700 other dinosaur species known (don't know what the
> official dino count is up to now), it still ranks as pretty small (and yes,
> this is obviously not counting Aves itself).

*At least* 20% of all Mesozoic dinosaurs. I guess it's subjective, but I
wouldn't call that small. And there's about 100 more that could have had
feathery integument as well (tetanurans outside or possibly outside
Clade(_Sinosauropteryx_ + _Passer_)), so it may be over 30%. (Again, not
including Cenozoic neornitheans.)

> > > > And the group may be as large as _Tetanurae_, or even larger if
> > > > certain all-scaled large forms are secondarily so.
> > >
> Again, unless scutes are derived feathers, I don't see any reason to
> believe that a group of animals that were feathered, would do a complete
> reversal instead of just losing the feathers and sticking with skin.

I tend to doubt it, too, but I wouldn't discount it.

> > No feathered animal ever completely lost scales/scutes, TMK. (Snow owls,
> > maybe?)
>
> Likewise; no secondarily featherless bird has ever re-evolved a scaly covering
> (or even went so far as to become truly featherless, but that's another
> story).

True, but it *may* be that when feathery integument had just evolved, it
was easier to lose.

[Tyrannosaurs]
> > A few small patches don't necessarily indicate scaliness for the whole
> > body. (I've seen some interesting speculative restorations showing
> > feathers only on the arms, used for display. And, of course, we all know
> > the "elephants aren't hairy; why should an elephant-sized coelurosaur be
> > fuzzy?" argument.)
>
> The inherent problem with the elephant argument, is that elephants never
> evolved (or re-evolved) a new (or old) form of integument in place of
> feathers.

Well, what old form would that be? Is there one for mammals?

> If _T.rex_ really was secondarily featherless, then it re-evolved
> scales to cover its body. To me, that seems to be stretching it. Why evolve
> scales again ,when one can just remain bare skinned (it works for the elephant
> sized animals)?

But no feathered animal ever lost scales entirely (TMK), so the genes are
obviously still present.

> > > As for the vast majority, is this not also true for most of the
> Wait a minute; why is _Sinosauropteryx_ + _Passer_ all of a sudden exempt from
> this. We don't know the skin type of most of the animals between these two
> either, yet you seem to be stating, that it is safe to assume that they were
> all feathery (or protofeathery).

It stands to reason, unless
A) You don't consider _Sinosauropteryx_' integument to be homologous with
avian feathers, or
B) You think some of these forms may have been secondarily featherless
(and you don't appear to).

If the structures of _Sinosauropteryx_ and fully-feathered animals such as
_Caudipteryx_, _Protarchaeopteryx_, and avians are homologous, then it
means the ancestor of Clade(_Sinosauropteryx_ + _Passer_) had feathery
integument, and every one of its descendants had it as well, unless they
lost it.

> Meanwhile we have scale impressions from
> ceratopians, ornithopods, sauropods a carnotaurine and a tyrannosaur.

And, indeed, all dinosaurs probably do have scales somewhere.

> Even if the tyrannosaur impressions turn out to be wrong (which I
> doubt) that still gives us a wide range of dinosaurs here.

Even if they are not wrong, they only tell us about the tail of adults. A
small piece of the tail, even.

> Their last common ancestor would *at least* be between the divide
> between ornithischia and saurischia, which makes for practically all
> dinosaurs.
>
> Are you saying that all these dinosaurs just evolved scaly coverings
> convergently?

No, of course not. Scutes were present in the ancestral dinosaur (and
further back), and feathery integument *probably* was not. No dinosaur
(AFAIK) ever lost scutes, but one clade (probably something within
Clade(_Passer_ <-- _Carnotaurus_)  and including Clade(_Sinosauropteryx_ +
_Passer_)) exhibits feathery integument in addition to scutes.

_____________________________________________________________________________
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