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?

[Black Parrot]

I think the actual implication would be that big dinosaurs had to produce vast numbers of young, so that enough of them would survive to become full-sized adults.

Also, "the little ones get eaten" would apply to small species of dinosaurs - and mammals. (Unless most predators preferred the taste of chicken to the taste of beef.)

Re:Circular reasoning?

[virg_mattes]

No, the point is that bigger specimens of smaller dinosaurs had an advantage over the average, so there was environmental pressure driving larger animals to survive. Therefore, as the bigger dinosaurs bred more than their smaller siblings, the average size of their young went up, reinforcing their advantage until truly huge specimens became the norm.

Virg

Re:Circular reasoning?

[Luckyo]

Cannibalism is a distinct evolutionary advantage when there is too few resources to support population that has to birth a lot of young to ensure at least some of them survive to adulthood.

It's commonly practised among many species that fall within this umbrella to this day.

Economics

[The Evil Atheist]

Sounds like the ultimate free market.

Re:Few is not the same as none

[CrimsonAvenger]

Also, why didn't those small juvenile dinosaurs just grow up and repopulate the world?

It's not like there would have been a shortage of them post-asteroid. If anything, the juveniles would have had an unusual edge, since they were growing up into a vacuum where the big predators used to be.

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.