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RE: the Horner interview - "fluffy" T. Rex??

Yutyrannus got pretty big, and pretty shaggy. 
From: owner-DINOSAUR@usc.edu [owner-DINOSAUR@usc.edu] on behalf of Tim Williams 
Sent: Tuesday, June 16, 2015 3:09 AM
To: dinosaur@usc.edu
Subject: Re: the Horner interview - "fluffy" T. Rex??

dale mcinnes <wdm1949@hotmail.com> wrote:

> Yes. This is interesting to conjecture. First. Feathers are not analogues of 
> hair.

I would have said that bird feathers can indeed be analogs of mammal
hair.  Especially from the perspective of heat conservation.  There
are strong function parallels between down feathers and mammalian
hair, in the sense that both function to retain/trap heat.  It's my
understanding that down feathers are actually much better at this.

> Secondly. The larger an "endotherm", if that's what they were, the
> greater the need to have feathers, really BIG feathers. Feathers have a lot
> of surface area. They are also bristle stiff.

These would be contour feathers...?  To me, the solution would be for
the large archosaur to have no feathers at all - as in sauropods,
hadrosaurs, etc.  For theropod dinosaurs, I think the totality of
evidence thus far suggests that smaller theropods had a "fuzzy" body
covering for insulation, whereas larger theropods had much sparser
protofeathers/feathers.  Thus, _Dilong_ had an extensive body covering
of protofeathers, but _Tyrannosaurus_ did not.  The decreased
surface-area-to-volume ratio in larger endotherms makes heat
dissipation less of an issue.  This has parallels with modern mammals.
Aside from body size, there may be additional factors that governed
the degree and distribution of heat-trapping integumental structures
in theropods - especially ontogeny, but also local climate and
seasonality.  (Note Solnhofen _Compsognathus_ shows no evidence of a
fuzzy body covering, but Liaoning compsognathids do.  Weird, but maybe
related to climate, if not taphonomic.)

> I don't know if it is even
> possible to have a large ARCHOSAURIAN endotherm without feathers 

> clime. Feathers help keep big archosaurs COOL in hot climes. The mechanism
> behind this is called FEATHER FLUFFING [for lack of a better term].

These "feather fluffing "mechanisms (morphological and behavioral) you
describe for deliberate heat *loss* would be expected to be useful for
archosaurs that already have feathers that are used for other purposes
- such as insulation or aerodynamic locomotion.  But having elaborate
pennaceous feathers simply to expedite heat *loss* seems like an
overly complicated solution. Why not just lose the darn things
altogether, and shed heat through the skin?

> Elephants and rhinos aren't
> archosaurs so, the safest bet for them is to drop or shorten the hair [since
> they can't grow feathers].

I may have misunderstood you here (my apologies in advance if I have),
but IMHO dinosaurs might have done the same thing: lose their feathery
integument as a result of increased body size and/or warmer
environment.  (See tyrannosaur example above).  If elephants vs
mammoths, or elephants vs hyraxes, can show very different
integuments, why not different kinds of theropods?

> That's probably, secondarily, why mammals burrow and birds don't.

The burrowing owl (_Athene cunicularia_) nests and roost in burrows
(usually excavated by mammals, particularly prairie dogs).  The
short-tailed shearwater or muttonbird (_Ardenna tenuirostris_) also
breeds in burrows, as does the little penguin (_Eudyptula minor_);
these birds make their own burrows.

Some non-avian theropods apparently burrowed, like _Oryctodromeus_,
_Koreanosaurus_ and related ornithischians.

> In the late premian, therapsids burrowed because of the lack of that 
> adaptation,
> sending mammals onto a different evolutionary path.

Or... as Myhrvold et al. hypothesize, these therapsids evolved sparse
hair to help shed body heat, like pin fins.  This feature was later
co-opted later for a denser, warmer body covering.  So goes this
hypothesis, anyway.