Dinosaurs tended to be either massive
(sauropods) or quick (ornithomimids), and sometimes even both
(carnosaurs). These traits require robust cardiopulmonary systems to
circulate adequate oxygen to maintain metabolism. Mammals and avians
have four-chambered hearts and muscular diaphragms, while reptiles
have neither. While fossil evidence strongly suggests that dinosaurs had
four-chambered hearts, they likely lacked diaphragms; clearly,
without special adaptation, it is difficult to adequately explain
their mass and activity levels, even taking into account the
comparatively higher atmospheric oxygen levels of the Mesozoic.
Paleontologists have long noted the hollow bones of many dinosaurs, but did not seriously
consider these hollows to serve any other purpose than mass reduction. However, many biologists have started to
seriously entertain the notion that dinosaurian respiration was more
akin to that of birds than of mammals. In addition to the strong
fossil evidence for four-chambered hearts and respiratory air sacs,
further evidence suggests nasal turbinates to reduce respiratory
water loss (a trait largely shared by birds and mammals).
Evidence strongly suggests a
unidirectional respiratory system at least somewhat similar to that
of birds, even though researchers have not formed an adequate
hypothesis for the mechanism by which breathing took place. A
reptilian hepatic piston would be unlikely to
move adequate volumes of air through the lungs using
mammalian metabolism as a model, but a cardiopulmonary system more
reminiscent of that of birds yields better explanatory adequacy while
still allowing for the mass and/or activity levels of many families
of dinosaur.
However, ultimately this adaptation led to the demise of the dinosaurs, as it allowed them to have more mass for lung space, making them less buoyant, and causing them all to perish in the Flood.
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