Egg-laying, Not Environment, May Explain the Size and Downfall of Dinosaurs

ananyo writes "Paleontologists have argued that dinosaurs were able to grow quickly and fuel large bodies when temperatures were warm, oxygen levels were high, and land masses such as the supercontinent Gondwana provided abundant living space. But two new studies contradict that idea and suggest the key to some dinosaurs' vast size lies in the limitations of egg laying. In the first study, researchers examined whether changes in body size followed changes in environmental factors and found no correlation. A second study argues that the reason dinosaurs grew so large was because they were forced to produce relatively tiny young (abstract only), as developing embryos would not be able to breathe through the thick shells of large eggs. When the young of large animals start out small, they must grow through a large size range before reaching adulthood. As a result there was intense competition between small and medium-sized dinosaurs, forcing adults to keep growing until they reached very large sizes to gain a competitive edge. But being big also had drawbacks. When an asteroid impact 65 million years ago wiped out most large-bodied animals, there were so few small dinosaur species that the group was almost obliterated, with only the birds surviving."

Comments

Circular reasoning?

[Black Parrot]

When the young of large animals start out small, they must grow through a large size range before reaching adulthood. As a result there was intense competition between small and medium-sized dinosaurs, forcing adults to keep growing until they reached very large sizes to gain a competitive edge.

IOW, dinosaur species had to be big, because young dinosaurs of big species had to become big?

Re:Circular reasoning?

[PrescriptionWarning]

The small dinosaurs probably got eaten, so the thought would be that the bigger dinosaurs live long enough to breed and they would beget bigger dino's as well else they would die, and so on. Circle of life would be the circular reasoning you're thinking of.

Re:Circular reasoning?

[SailorSpork]

I read another article that kind of explains this better. The gist is that as dinosaurs grow up, they need to develop through several different kinds of ecosystems, young occupying an ecosystem of smaller fauna, medium of slightly larger fauna, and so forth, competing for similar resources. Because the existing dinosaurs had established themselves and crossed all ecosystems at some life phase or another, that was the status quo. When the asteroid hit and changed the status quo, mammals (which didn't grow through different fauna-sized ecosystems and better adapted to their own niches) were better able to compete for the same resources in the smaller- and middle- ecosystems, thus crowding out the slow-growth dinosaurs. It took an asteroid hitting the reset button on the global population for this to happen... dinosaurs didn't die overnight, they just never re-established themselves afterwards as well as the smaller species like mammals, smaller lizards, birds etc did.

Re:Circular reasoning?

[Black Parrot]

Probably came about from pondering why the fuck they were born with such short arms.

Couldn't reach a conclusion?

Re:Circular reasoning?

[FrootLoops]

As near as I can tell, the argument is...

Premises:
  (1) Dinosaurs had some initial size diversity due to environmental factors
  (2) Egg sizes were limited because thick shells would be air tight
  (3) Egg-laying dinosaurs went through large size variances as they grew to adulthood (compared to mammal-scale)

Reasoning:
  * Because of (1), (2), and (3), a particular species would occupy a broader environmental niche, eg. with small juveniles going places adults couldn't reach
  * Increased niche breadth would cause species to interact and compete more with other species
  * Increased competition results in a size arms race since larger animals get food more, which incidentally increases niche breadth all the more
  * The process doesn't continue indefinitely since large sizes eventually hit environmental constraints, though "steady-state" sizes would be larger in egg-laying dinosaurs than eg. mammals. Birds have strong environmental reasons to stay small that tend to overcome increased competition.

[If you're a biologist, preferably one who has read the paper, please correct me if I'm wrong. The Nature article is pretty vague and I can only read the abstract of the journal article.]

Re:Few is not the same as none

[Zocalo]

The big misconception about the asteroid theory is that there was a big impact and all the dinosaurs died out pretty much immediately, but this does not actually seem to be the case. The actual extinction appears to have taken quite some time, with the larger land based lifeforms being the ones that were the most likely to die out.

That seems to make quite a bit of sense to me; an big asteroid impact would throw a lot of dust into the atmosphere, so a prolonged period of cooling would likely result. That could reasonably be expected to lead to a significant reduction in the available foliage for consumption by herbivores, leading to the larger herbivores being the first to starve to death. Fewer herbivores, means less meat for the carnivores, so the big predators are the next to find that the larder has suddenly gone dry, and down the chain it goes.

The most likely survivors in that scenario are those that can survive on meagre food supplies and digest more of what is available; if you can eat branches and the trees are bare, those of your competitors that require more succulent fare are going to have a harder time of things. Similarly, those species that relied more on stealth/cunning than just sheer numbers to survive would have have more of their preferred diet to go around and/or be more likely to avoid predation.