Below is the abstract of a _Science_ article "Lung structure and
ventilation in theropod dinosaurs and early birds" by Ruben and others
(Vol. 278, p. 1266-1270).
"Reptiles and birds possess septate lungs rather than the alveolar-style
lungs of mammals. The morphology of the unmodified, bellowslike septate
lung restricts the maximum rates of respiratory gas exchange. Among taxa
possessing septate lungs, only the modified avian flow-through lung is
capable of the oxygen-carbon dioxide exchange rates that are typical of
active endotherms. Paleontological and neontological evidence indicates
that theropod dinosaurs possessed unmodified, bellowslike septate lungs
that were ventilated with a crocodilelike hepatic-piston diaphragm. The
earliest birds (Archeopteryx and enantiornithines) also possessed
unmodified septate lungs but lacked a hepatic-piston diaphragm mechanism.
These data are consistent with an ectothermic status for theropod dinosaurs
and early birds."
Note some interesting points made above. According to Ruben, both
theoropod dinosaurs _and_ the earliest birds lacked avian flow-through
lungs. So this part of their interpretation if correct would support, not
reject the dinosaur-bird relationship. The distinction is that the
earliest birds are interpreted by Ruben to have lacked a hepatic-piston
diaphram. This interpretation is based entirely (as far as I can tell) on
the shape and anterior projection of the pubis in these early birds which
is said to be inconsistent with the presence of a hepatic-piston diaphram.
I am not a physiologist, but I do know that the orientation of the pubis in
Archeopteryx has been a topic of dispute for years. In some reconstructions
the orientation of the pubis in Archeopteryx is essentially identical to
that of several theropod dinosaurs.
I also find it interesting that the above description actually puts the
early birds in a transitional position between theropods and modern birds.
The opponents of a dinosaur origin, however, believe that the transition
from a diaphram to an air sac system is not possible. But arguments based
on "I don't see how this transition could happen" have repeatedly failed in
Finally, the opponents of a dinosaur ancestry argue for a thecodont
ancestor. Thecodonts are more primitive than crocodiles or theropod
dinosaurs! So if they cannot envision how this anatomical transition could
have been possible for dinosaurs, on what basis can they argue for it with
thecodonts? The currently strongest argument against their proposed
theocodont ancestry is that they are completely without any fossil support
Keith B. Miller
Department of Geology
Kansas State University
Manhattan, KS 66506