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A TICKLISH SCIENCE LESSON (long)
I have over the years been concerned that paleontology is not always
practised as a rigorous science (
some nonpaleontologist scientists have expressed the same concern to me). The
issue of feathered dinosaurs well illustrates the problem.
Back in the mid 70's Disco was King, and at that time were presented the
first suggestions that the probable ancestors of birds, the possibly
endothermic theropods, were feathered. This inspired some artists (incl. moi)
to illustrate small dinosaurs with some form of pelage, usually some form of
feathers. This is were the science problem began.
A number of paleontologists vigorously objected to restorations of feathered
dinosaurs. Sometimes they did so derisively and harshly - odd how emotional
people can get about a little dinofluff. The basic scientific argument was
something like this. Dinosaur "mummies" show that dinosaurs had scales, like
ectothermic reptiles, not feathers like endothermic birds. No feathers had
been found adorning small dinosaurs. Parsimony therefore favored scales in
dinosaurs, feathers were too speculative. Some researchers had somewhat
more complex and subtle arguments as to why
This argument was always false. The scaly skin was and is limited to large
dinosaurs. If mammals were extinct, and only the skin of fossil elephants and
rhinos were preserved, then some form of insulation would be ruled out in
small mammals by the above logic. Parsimony would favor scales in small
dinosaurs only if both their ancestors AND descendents were scaly. As it is,
with scaly ancestors, and feathery relatives, parsimony was neutral on the
issue. The question could be answered only by
direct fossil evidence. The evidence was also neutral because no integument
had been clearly preserved on any small dinosaur. Example - claims that the
lack of feathers on the famousCompsognathus specimen contradicted their
presence were spurious because
scales were not found either! Nor are body feathers present on all but one of
the Archaeopteryx specimens from the same sediments. Viewed objectively,
restoring Comp. and other small dinos with feathers, scales or nonscaly skin
was equally speculative and plausible! Yet the majority opinion always
nonobjectively presumed that scales were less speculative than feathers. I
took a gamble and consistently restored small theropods with feathers, a bet
that seems to have paid off at least in part.
What do we currently know about the integument of small dinosaurs? I have
only seen a rather poor color photo of the Chinese theropod. However, Currie
has seen the original and says that it clearly has feather-like structures
similar to those of birds from the same sediments. Although the feathers are
entirely peripherial to the body on the slab, this is typical of bird
fossils. A full body covering is very possible if not probable. There is some
evidence that the structures on the underside of the tail base are
intermediate to scales and feathers. There may be more specimens from the
same sediments, certainly more can be expected and will help solidify the
data base one way or another. The rest of the discussion assumes the
specimens does have short ratite-like feathers, but no contour or long
The E. Cretaceous theropod appears to be broadly similar in form to L.
Jurassic Compsognathus, with short arms, and a long rather heavy tail. If so
it is a basal tetanurian, much less advanced than more bird-like theropods
which show flight adapatations.
A partial small theropod from the E. Cretaceous of S. America was reported to
have naked nonscaly skin in Nature (1996, 379:32). No clear photos of the
skin surface were published, and it is not clear whether the outer skin layer
had been lost. More data is needed.
Also in Nature (1994, 370:363) it was reported that the E. Cretaceous
ornithomimid Pelecanimimus has an integument of uncertain nature. Again more
data is needed.
The only small ornithischian skin reported was a nonscaly, punctured surface
on Thescelosaurus earlier in this century, but the integument was not figured
and is not accessible.
The combined data suggests but does not prove that feathers evolved in
terrestrial theropods well before the evolution of birds, at least by the L.
Jurassic and in tetanurian theropods. How much earlier feathers may have
evolved is not clear, so Triassic and earlier Jurassic theropods may or may
not have been insulated. (Therefore artists should feel free to restore
Coelophysis with or without feathers. If someone tries to tell you otherwise
just subtly roll your eyes, or smile enigmatically yet knowingly - its what I
used to do.) Feathers are highly probable in the advanced bird-like
theropods. What small ornithischians were wearing is up for grabs.
The probable presence of feathers in at least some small theropods has
important metabolic and thermoregulatory implications. Recently it has been
asserted that the presence of feathers does not necessarily indicate
endothermy, contrary to a long opinion otherwise. This is based on the
following arguments. Early birds could only have powered flight with the
small reptile-type muscles their small sternal plates were able to support.
This is false because some modern flying birds have flight muscles much
smaller than the avian norm, and early bird arms and shoulder girdles were
large enough to support them. Another argument is that early bird bones show
they grew slowly like reptiles. However, the thin walled bones of the fossil
birds do not preserve the period of juvenile growth, which may have been
rapid (the growth rings only indicate that the birds continued to grow slowly
as adults). Last, some birds bask. This, however, only shows that endotherms
bask with insulation. The observed temperature rise is too modest for
Which brings us to the point that all the many thousands of insulated
tetrapods are endothermic. Even furry insects tend to be endotherms.
Insulation prevents ectotherms from rapidly absorbing the large amounts of
heat they need from the environment. This problem is so serious that even
desert reptiles cannot use insulation as a solar screen. Only endotherms that
need to retain body heat are insulated. Ergo, insulation is always excellent
evidence for endothermy. Also for resting metabolic rates above the reptilian
level. Therefore, ecothermy is effectively falsified in insulated dinosaurs
(I am properly and strictly defining ectothermy and endothermy to mean that
the majority of body heat is gained from the environment or generated
internally respectively. No other metabolic or thermoregulatory
characteristics are implied.)