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feathers, fibers, endothermy...
> Thomas R. Holtz, Jr. <email@example.com> wrote:
> People seem to be putting relatively high importance on integumentary
> structures whose presence or absence in most coelurosaurian dinosaurs can
> neither be confirmed nor denied, as they have not been found in sediments
> which such things can be preserved.
Absolutely. The fossil integument record for several large dinosaur
individuals has been most instructive, and has provided a glimpse of the
tubercle and (in some groups) osteoderm look which has become so familiar.
The record for small (non-avian) dinosaur integuments is, by contrast,
paltry. The _Pelicanimimus_ fossil suggests a skin with minute wrinkles
and neither tubercles nor fibers, although these structures may, in fact,
represent subcutaneous anatomy. _Proarchaeopteryx_, _Mononykus_, and
_Sinosauropteryx_ appear to represent the best evidence yet for a fiber
coat on very small theropod dinosaurs (and actual -- albeit assymmetrical
-- "feathers" on the tail of the former), and appear to be close enough to
the bird lineage that their appearance is not altogether unexpected. Yet
the phylogenetic distance between _Protarchaeopteryx_ and _Sinosauropteryx_
on the one hand, and the lineage leading up to modern birds on the other,
certainly is sufficiently expansive to suggest that quite a number of other
small theropods may have been similarly attired.
Consider the case of _Archaeopteryx_, so famous for revealing its flight
feathers, yet much less informative on the subject of other body
integument. It may be that the large, fibrous wing and tail feathers were
more permanently anchored to the wings and tail than the body feathers, as
is the case in the (obviously much better anchored) feathers of modern
avians. On a modern bird that is not immediately devoured or scavenged,
the activity of beetles and yet smaller life forms break down or scatter
the main body feathers and tissues and leave the flight feathers relatively
undisturbed for some time. I recall seeing a sequence in Lennart Nilsson's
miniseries _The Odyssey of Life_ which showed a remarkable time lapse of
beetles and such beginning to work on a bird carcass. In two weeks, the
body was stripped of all feathers except those of the wings and tail. In
time, if this bird were to become fossilized, it, too, would withhold the
secret of its body feathers, while revealing the flight feathers. We
cannot presume that _Archaeopteryx_ was a "naked" bird simply because we
haven't found a full integument.
On the metabolic status of early birds, there is -- as we are well aware
-- some controversy. Ruben, Jones, and Geist of Oregon State University,
and Hillenius of the College of Charleston have published their collective
opinion that _Archaeopteryx_ and other early birds (the enantiornithines),
may have been ectotherms or near-ectotherms. _Archaeopteryx_ could, in
theory, make brief flights as an ectotherm because biomechanical
calculations render it a possibility. Further correlational support for
the hypothesis of early ectothermic avians was provided by Chinsamy's bone
histology studies that revealed, for example, that _Patagopteryx_ and other
early bird fossils exhibit LAGs (lines of arrested growth). Evidence for
respiratory turbinates in birds has only been found as far back as the
advanced Cretaceous _Hesperornis_ by the aforementioned scientists, but, as
they wrote in their paper in _The Complete Dinosaur_, such bird fossils are
rare and such study is most difficult. This is about all that can be said
at this time on RT's in early birds, contrary to impressions given in the
popular press, and contrary to a posting I made last year. The scientists
did not find sufficient evidence to state that these birds did not have
respiratory turbinates. On the other hand, their take on the corelational
data leads them to the conclusion that these birds "may have been
ectotherms or near-ectotherms." I hope I cleared that up.
I am hopeful that scientists will some day analyze some of the over 800
specimens of Cretaceous fossil birds recently discovered in the Liaoning
province in China for evidence of respiratory turbinates. Though
flattened, the sheer preponderance of specimens may enable scientists to
accurately reconstruct the birds in three dimensions, as would be required
for careful sinus studies. Positive identification of respiratory
turbinates in these specimens should positively prove that they were
endotherms; proof that they could have had no respiratory turbinates would
call into question a methodology that would lead us into viewing them as
ectotherms (or near-ectotherms) covered with an insulating integument, and
could lead to studies considering how these creatures could support a high
rate of respiration without the turbinates which are theoretically required
to prevent dehydration.
Gregory S. Paul has stated that the extant kiwi has no respiratory
turbinates; Terry D. Jones has written to me that the kiwi does have them.
Gregory S. Paul has written that vulture neck feathers are simple shafts,
but Alan Brush has written that every feather (and this includes eyelashes
and facial bristles) of every extant bird he has studied reveals barbs,
however small they might be.
These inconsistencies are troubling, but are to be expected with issues as
contentious as these. The best we can do is read all that we can, share
what we have learned, eagerly await further publication, and draw our own
conclusions, which may change as new information becomes available.
Science is not merely truth, but the search for truth.
The _Sinosauropteryx prima_ paper in _Nature_ has much to offer, and leaves
much yet to be studied and described. It offers that the _Sinosauropteryx_
fibers are lighter at the tips, consistent with the hypothesis that they
are hollow structures. Chen et al. propose that the fibers were most
likely adaptations for insulation. The paper points out that the precise
anatomy of individual fiber structures is difficult to determine because
they are piled up on each other as is the case for the feathers of birds
found in the same deposits. There is no diagram so clear as Philip J.
Currie's reconstruction of the manus (presented at the October '97 SVP
meeting), but there is helpful discussion of some of the most salient
anatomical features. There is no discussion of the third specimen, which
Currie has studied. The paper includes photographs of the twin eggs and
the lizard remains found within the second specimen, and the short Unwin
piece in _Nature_ speaks of the Liaoning deposits revealing "an almost
complete Early Cretaceous ... continental biota..." In time, we will
probably know a lot more about this lizard. Ostrom had been quoted in
_Discover_ magazine, calling the Yixian Formation fossils the discovery of
a lifetime. No doubt!
Ralph Miller III <firstname.lastname@example.org>