The Biology, Agriculture, and Paleontology thread

See this.

In short, cetaceans' ability to hold their breath for long comes not from the large volume of their lungs (it's lungs-to-body size ratio is actually small), but from their ability to store a large percentage of inhaled oxygen inside their circulatory system.

Huh. I guess the comparison should've been "elephants to humans", then.

Given the floatation issue, it does make (superficial) sense for the whale's lung-to-body ratio to be relatively small.

The trick is, due to how the Square-Cube Law works, lungs would become increasingly inefficient with size: Their volume would go up, but the inner surface area, where gas exchange takes place, wouldn't increase as much, and would end up being inneficient.

However, large animals have complex lung shapes rather than mere air pouches, and mammals in particular have alveoli, which makes it so gas exchange surface area remains pretty much constant with lung volume (after all, alveoli are about the same size no matter the mammal). The issue with larger animals is VENTILATION: We lack a proper way of removing air from our lungs (we take air actively, but let it out passively), and there is always a "dead volume" of air that we can't vent out. That gets larger the larger the animal is.

Birds (and crocodiles to a degree) have that solved, as they have parabronchial lungs rather than alveolar ones: rather than tiny sacs, their lungs are made up of tiny ducts, and air doesn't come out the same way it came in. If you factor in the air sac system birds have, you will see they get fresh air in their lungs both when they take air in and when they let it out (as they push air out of their lungs by pushing it out with fresh air from the air sacks).

Ah, and most dinosaurs had an air sack system. If you add to them their probably slighty lower metabolic demands compared to mammals, you can understand why large sizes were common among them.

Did ornithischians have air sacks?

Probably Ornithischia had no air sacs. Take note that birds are descended from (and actually are) Saurischia despite the name.

Pterosaurs did have air sacs, and their origin may predate the bird-croc split. Ornithischians may have lost them, or at least the ones associated with their bones.

Birds are theropods. The actual nature of the base of the dinosaur tree is nowadays unstable, and we can be confident on Ornithischians, Theropods and Sauropodomorphs splitting from each other fairly quickly, but knowing which one was the first to split off is a different matter (and our lack of Triassic Ornithischians, as well as possible convergence with Silesaurs, are puzzling).

(Ornithischian means "bird hipped", but near-bird theropods convergently evolved that hip orientation; if you look at a Velociraptor skeleton, you'll see it is bird-hipped as well, but NOT an ornithischian. Conversely, ceratopsians, which are bonafide ornithischians, developed a hip configuration highly reminiscent of the Saurischian one. And, seeing Saurischian means "lizard hipped", MANY animals other than Theropods and Sauropodomorphs have that hip configuration, which is the ancestral condition for dinosaurs)

Edited by Eriorguez on Aug 24th 2018 at 7:27:51 PM

In that case, did giant ornithischians like Shantungosaurus retain air sacks?

Nothing in the bones. And evolution doesn't work that way; the lineage that led to Shantungosaurus would have to have retained air sacs.

Giant Ornithopods are in the ballpark of sauropods tho. Or of trully large land mammals, as the largest proboscideans and indricotheres are NO slouches.

Are any ground sloths in the same size range as those, or are they closer in size to giant theropods like Tyrannosaurus, Spinosaurus, giant allosauroids, etc.?

Edited by Spinosegnosaurus77 on Aug 24th 2018 at 2:08:23 PM

The largest megatheres were around the size of modern elephants. Deinotherium and the larger mammoths were a tad larger.

x8. Hence the 'superficial'.

I didn't know about the dinosaurs' air sac system, however, so thank you for that.

Edit: My spelling is atrocious today.

Edited by Wyldchyld on Aug 24th 2018 at 8:56:50 PM

@Eriorguez: That looks so superior in every important way to the alveolar-style respiratory system, I'm surprised that only birds and crocodilians ever evolved it. Is there catch? (Aside from "there was no evolutionary pressure to go that route", of course.)

Edited by MarqFJA on Aug 24th 2018 at 11:05:42 AM

To my (limited) knowledge, alveolar lungs allow you to have a more flexible trunk, which is useful for mammals since they evolved from burrowers.

Incidentally, archosaurs appear to be restricted to oviparity (unlike mammals & lepidosaurs), but I don’t know if there’s any evidence that this is correlated with the respiratory system.

Edited by Spinosegnosaurus77 on Aug 24th 2018 at 4:35:47 AM

Flexible trunks are still very useful for species who need to run and/or jump. I know there have been studies on the link between speed and the number and flexibility of trunk vertebrae, where natural selection may tolerate mutations in slow animals with stiffer spines but can prove fatal in animals that need a highly flexible spine for running. I don't know any on the relationship with lung types off the top of my head, however, but I've been out of the game for a little while.

Edited by Wyldchyld on Aug 24th 2018 at 9:32:45 AM

Alveolar lungs are a more obvious solution than parabronchial ones to a common problem, so it happened.

Remember that we share a cavity for both respiratory and digestive tracts, and that most animals have their esophagus surrounded by their brain.

Small theropods that presumably did a lot of running & jumping (Deinonychus, Struthiomimus, Ornitholestes, etc.) appear to have had rigid avian-style trunks, though.

Incidentally, some workers have suggested that the dinosaur respiratory system was more similar to crocodilians than birds, but this has been called into question.

Were they running on four legs or two? If I recall the studies I've seen, they were all regarding vertebrates that move (and jump) using four limbs. I don't think the studies had progressed to the stage where they were studying vertebrates that moved on two legs as standard (I'm not just thinking of dinosaurs there, but of extant species such as kangaroos and wallabies).

The point of the studies were to investigate the possibility that things like speed and agility might act as a limiting factor on the evolution of the vertebrate spine (rather than vice versa). I'll have to see if I can dig them out.

Edited by Wyldchyld on Aug 24th 2018 at 1:40:42 PM

All theropods are bipeds.

Spino isn', most of the time.

That’s assuming its proportions have been scaled correctly.

Spino being quadrupedal is an asspull. It is far from the most likely hypothesis with the data we have, and the data that would support it is quite unlikely to happen (theropod forelimbs evolving to bear weight).

Check out Andrea Cau's blog.

This Quora page does conclude that Spinosaurus was most likely bipedal, just one whose body was visibly lower to the ground than your average bipedal dinosaur.

Must've one goofy looking dinosaur when it was alive.

Deinocheirus: Hold my beer

Edited by Spinosegnosaurus77 on Aug 25th 2018 at 5:22:25 AM