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RE: FEATHERS FOR T-REX?



Michael Lovejoy wrote:

>A quote from Walking on Eggs: "...coelurosaurs were feathered; even the
>colossal Tyrannosaurus must have had a feathered body at some early
>stage of its life. 

Using the phylogeny of Xu et al. (2002), Tyrannosauridae falls outside the
Maniraptoriformes.  Hence, it is outside of the clade currently bracketed by
the *known* expression of integumentary structure (dinofuzz, protofeathers
or feathers - whatever you wish to call them).  These structures have been
described for _Shuvuuia_, _Sinosauropteryx_, _Beipiaosaurus_, _Caudipteryx_,
_Microraptor_, Sinornithosaurus_, _Protarchaeopteryx_ and (of course) birds.
According to the phylogeny of Xu et al. (2002), the most recent common
ancestor of all these taxa occurs higher in theropod phylogeny than the
first tyrannosaurid.

To put it another way, possession of feathery integumentary
structures is *minimally* primitive for the Maniraptoriformes (if
_Sinosauropteryx_ is a basal maniraptoriform).  Should adult
tyrannosaurids be found to have feather-like structures, this apomorphy
will slide down the tree even further (assuming the position of the
Tyrannosauridae remains stable - and I wouldn't bet my life savings on
that).  

However, should adult tyrannosaurid skin specimens be found to lack
feather-like structures (i.e. they had "naked" skin - tuberculate or scaly
in texture), several hypotheses may be advanced:

(1) Tyrannosaurids are descended from "naked" ancestors, and feathery
integument is primitive for a clade more derived than Tyrannosauridae.  In
other words, tyrannosaurids don't have feathers and never did.

(2) Tyrannosaurids are descended from feathery ancestors, and feathery
integument is primitive for a clade that includes Tyrannosauridae and other
feathery theropods; i.e. the absence of feathers is a secondary loss in
tyrannosaurids.    

(3) Hatchling and juvenile tyrannosaurids may possess a feathery integument,
but this was shed upon reaching maturity and/or upon achieving a certain
body size (see below).  This means that expression of feathery integument is
an ontogenic character (at least for tyrannosaurids).

(4) Possession of a feathery coat may have persisted in adult
tyrannosaurids, but its expression was seasonal.  Tyrannosaurids wore their
downy coats in winter, but shed them in the spring.

(5) Possession of a feathery integument may have persisted in adults,
but its expression was limited to certain parts of the body, not
represented in known integument samples.  E.g. tyrannosaurids had
feathery crests or frills, but naked bodies.

(6) Adult tyrannosaurids had feathery integument, but the skin specimens
that are known came from individuals that lost it through disease.  Sort of
like as described for Jabba the Hutt in the _Return of the Jedi_ novel.
(OK, I'm kidding for this one - though I guess it is a possibility.)

You can imagine what evidence (in the form of future discoveries) could
refute the above hypotheses.  For example, the discovery of a feathered
_Syntarsus_ (a la Bakker) would refute hypothesis (1), and make feathery
integument a basal theropod character (like the furcula).

>However, scientists do not believe that adult
>tyrannosaurs were feathered because the combination of this insulating
>covering and their large size might have posed a disadvantage in
>regulating the animal's body temperature."

This is conjecture, since nobody can investigate the physiology of _T. rex_,
since tyrannosaurids are long extinct.  The theoretical basis for the
conjecture is sound, however.  With all other factors being equal
(geography, for example) big animals have less reliance upon insulation due
to their lower surface area / volume ratios ("Balloon Theory", etc) and the
minimization of endogenous (or exogenous) heat through dissipation.

References

Xu, X., M.A. Norell, X.-L. Wang, P.J. Makovicky and X.-C Wu (2002). A
basal troodontid from teh Early Cretaceous of China.  Nature 415:780-784.


Tim

------------------------------------------------------------ 

Timothy J. Williams 

USDA-ARS Researcher 
Agronomy Hall 
Iowa State University 
Ames IA 50014 

Phone: 515 294 9233 
Fax:   515 294 3163