Careful, you’re starting to say things I actually agree with. You’re starting to sound like a creationist. After all, dinosaurs supposedly evolved into birds, but that’s a total redesign. The bird lung is completely unlike the dinosaur lung. It’s not just a stepwise change, but it would have to represent a total redesign. Reptile lungs are bi-directional, while birds’ are unidirectional.
The avian respiratory system is physically distinct from the mammalian respiratory system, both in structure and in its ability to exchange gas as efficiently as possible.
This pattern of airflow through the respiratory system creates unidirectional (one-way) flow of fresh air over the gas exchange surfaces in the lungs. Furthermore, fresh air passes over the gas exchange surfaces during both inhalation and exhalation, resulting in a constant supply of fresh air enabling the bird to experience a near-continuous state of gas exchange within the lungs. This contrasts with mammalian lungs, which experience bidirectional airflow over the gas exchange surfaces.
Abstract: The Archosauria are a highly successful group of vertebrates, and their evolution is marked by the appearance of diverse respiratory and metabolic strategies. This review examines respiratory function in living and fossil archosaurs, focusing on the anatomy and biomechanics of the respiratory system, and their physiological consequences. The first archosaurs shared a heterogeneously partitioned parabronchial lung with unidirectional air flow; from this common ancestral lung morphology, we trace the diverging respiratory designs of bird- and crocodilian-line archosaurs. We review the latest evidence of osteological correlates for lung structure and the presence and distribution of accessory air sacs, with a focus on the evolution of the avian lung-air sac system and the functional separation of gas exchange and ventilation. In addition, we discuss the evolution of ventilation mechanics across archosaurs, citing new biomechanical data from extant taxa and how this informs our reconstructions of fossils. This improved understanding of respiratory form and function should help to reconstruct key physiological parameters in fossil taxa. We highlight key events in archosaur evolution where respiratory physiology likely played a major role, such as their radiation at a time of relative hypoxia following the Permo-Triassic mass extinction, and their evolution of elevated metabolic rates.
Is this supposed to be an insinuation that dinosaurs had unidirectional lungs? (For that matter, is this an insinuation that there was never any transition between bi-directional and uni-directional lungs at all, but somehow the lung has fundamentally evolved twice, independently)?
You cite a paper that says that dinosaur lungs were likely similar to crocodilian lungs as evidence that dinosaurs had tidal respiration, ignoring the fact that crocodilians have unidirectional respiration. Congratulations, you have argued against yourself.
Recent research shows that some reptiles, including iguanas, monitor lizards, and alligators, have a form of unidirectional airflow. However, they don’t need the avian system of air sacs and parabronchi; instead, the airways are shaped and angled to generate jets that produce one-way airflow, an ingenious design that humans may never have imagined possible.
This has generated more problems than it solves: First, it means that the energy requirements for flight could not have provided selection pressure for one-way air flow, because it also occurs in heavy, non-flying creatures. Second, one-way airflow could not have been a selective force for driving the evolution of hernias, air sacs, parabronchi, and the fixed thigh, since it can exist without these features.
Dinosaurs also had feathers. The skeletal system of birds is just a tweak of what is found in dinosaurs. In fact, most creationists would probably call it devolution because bird evolution required the loss of digits and a loss of grasping function in the wings.
It is also interesting to note that bats have different adaptations for flight, such as a membrane wing stretched between long fingers. Bats don’t have feathers, and bats have tidal lungs like you, me, and the mammalian ancestor of bats. So if birds are optimal, then are bats not?
I’m not convinced of that by a longshot. However if it were proven, it wouldn’t represent any kind of falsification of creation or a proof of evolution.
I never made a claim that birds’ lungs are the only possible design that can accommodate flight. My statement had to do with the transition between two totally different designs. I’ll have to look further into these claims that dinosaurs had bird-style lungs.
How many of those air breathers have lungs? How many of those are independent origins of lungs in vertebrates? Lungs, as far as I can tell, evolved once in an early gnathostome. In some taxa, the lung was lost. In one (some early , it evolved into a swim bladder. In a few taxa, the swim bladder secondarily acquired some lunglike functions. I suppose the last could be
How many of those air breathers have lungs? How many of those are independent origins of lungs in vertebrates? Lungs, as far as I can tell, evolved once in an early gnathostome. In some taxa, the lung was lost. In one (some early acanthopterygian), it evolved into a swim bladder. In a few teleost taxa, the swim bladder secondarily acquired some lunglike functions. I suppose the last could be considered an independent evolution of lungs, but is that what you were talking about?
I’m about to say something with which some might strongly disagree. I’m aware of this risk. Anyway…
There is a serious problem with the discussion of lungs in vertebrates, namely that mammalian lungs and the homologous structures in other species are seen as ‘true lungs’, regardless of the actual function of the structure in the species, while structures that behave exactly as lungs do (and would be called such in invertebrates) are often given other names. If we understand a ‘lung’ as an internal chamber for mechanically assisted exchange of gases between the organism and the atmosphere for the purpose of respiration, then there are clearly many independent origins of lungs in vertebrates.
This would exclude some that use gills reinforced to allow aerial respiration (as in Synbranchus), but would include
since it developed a lung from something that wasn’t a lung, as well as several independent developments of respiratory chambers in Siluriformes, and labyrinth organs of various descriptions.
So to actually answer your question… a few of them? Enough at least to justify my use of the word ‘multiple’.
Wait. I missed that entirely. Whatever does @PDPrice know about dinosaur lungs? I don’t know of any case of preserved dinosaur lungs*, certainly not in enough detail to determine the direction of air flow. However, we do know that plenty of dinosaurs had hollow bones, presumably with air sacs, a feature of avian unitary flow. Even better, there are all manner of intermediate fossils connecting modern birds to the earliest dinosaurs. And the feathers, of course. Archaeopteryx alone kills creationist nonsense.
.* Scipionyx has been claimed, but I don’t think so.
