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The continuing discussion concerning whether or not the avian respiratory
complex could have evolved from a crocodilian-like liver pump is irrelevant,
in that there is no compelling evidence that any theropod had a liver pump.
Large liver - Some birds have livers as large and deep as those of
crocodilians, so the presence of a deep liver in a theropod is not definitive.
Low trachea - A posterior cervical vertebra is displaced ventrally in
Scipionyx, so the trachea may be as well. The low set trachea is therefore not
Diaphragmatic muscles - These tissues are poorly preserved, to the point that
the fine muscle fibers seen in other muscles on the specimen are not present.
The longitudinally oriented undulations may be superfical, and resemble those
of pubis based muscles in birds. The muscle evidence is therefore not
High colon - Most theropods retain the basal archosaur condition of a pelvic
canal set immediately below the sacrum, and their pubes are extremely
elongated ventrally, so the colon is set high in the belly. In birds including
Archaeopteryx the pelvic canal is deeper, so the colon is set lower. However,
oviraptorosaurs, alvarezsaurs and avimimids also had deep pelvic canals, so
their colons should have been set low. The position of the colon is therefore
The only tetrapods that can be definitively assigned a liver pump are
crocodilians that possess the required skeletal adaptations, including mid
dorsal ribs that articulate with only the ends of wing-like transverse
processes, a well developed lumbar region, and mobile pubes. The gracile
sphenosuchian protocrocodilians lacked these features and probably lacked a
liver pump. The same is true of all dinosaurs. Theropods progressively
developed the skeletal features associated with and required by avian air-sac
ventilation. These included short chest ribs, elongated posterior ribs with
mobile double heads, a deeply corrugated ribcage ceiling, and pneumatic
vertebrae. The most bird-like theropods also had a elongated sternum with a
hinged coracoid articulation, ossified sternal ribs and ossified uncinate
processes - all of these avian features are absent in even Archaeopteryx. As I
pointed out at Yale, one could hardly ask for a better example of progressive
evolutionary development. In contrast the theropod liver pump has no
evolutionary context, and no compelling evidence supports it.
If Charles Darwin saw the evidence for the evolution of avian respiration in
dinosaurs, he would dance a jig. And Tom Huxley would join him.
Archaeopteryx acetabulem -
On another subject, Larry Martin restores Archaeopteryx with an extremely thin
rim of the posterior acetabulem. He uses this feature to justify his
restoration of the urvogel with a vertical body posture. This is quite
incorrect. The supraacetabular shelf of the London Archaeopteryx is fairly
broad and well developed, more so than those of some theropods, including
dromaeosaurs and even tyrannosaurs. There is no evidence that Archaeopteryx
walked hither and yon with its body erect.