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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."
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?
Sheesh, evil *and* a jerk. -- Jade
Does nobody read the summaries before posting them?
Yes, nobody reads everything.
Sheesh, evil *and* a jerk. -- Jade
I've read much of this before and it still seems pointless. People keep trying to explain how dinosaurs were such poor survivors yet they are by far the most successful large terrestrial species the planet has seen. The egg thickness theories have nothing to do with dinosaurs suddenly disappearing. The only thing it really explains is why dinosaurs had to have such a rapid growth rate. They ranged in size from around the size of a chicken to nearly the size of a Blue Whale with the largest eggs being not much larger than an Ostrich egg and the smallest on pare with a chicken egg. Those conditions existed for tens of millions of years before their extinction so egg size and shell thickness couldn't have been a factor in their extinction. Mammals also didn't suddenly change towards the end of their reign so it's unlikely that they suddenly found dinosaurs and their eggs tasty. The mammals driving dinosaurs into trees is silly since birds had been around for tens of millions of years before their extinction and T-Rexs didn't suddenly decide they had to climb trees. Birds were better at exploiting the nitch than the flying reptiles. Like most extinction events it's complicated and other than the meteor impact there aren't any smoking guns. Odds are it was climate change than was the death blow to the ones that survived the impact. The more interesting fact is the only species that survived were either small so they needed less food or they were able to go for long periods without eating like Alligators. Odds are most starved to death since some were even cold adapted and survived in higher latitudes than even alligators so the freezing theory wouldn't explain all the deaths. Ultimately the best explanation is starvation brought on by climate change caused by a meteor strike. Odds are it was that simple.
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.
UNIX? They're not even circumcised! Savages!
What the papers say is that dinosaurs went through different size classes throughout their lives that caused them to compete in different ecological niches. Because their whole life cycle was dependent upon two or more different ecosystem places (eg, tiny dinosaur stage A eats small plants, medium dinosaur stage B eats shrubs, and huge ass dinosaur stage C eats lots and lots of swamp ferns and tree leaves). So if any of these niches were disturbed due to the meteor event, the life cycle could not complete itself into adulthood, and thus the dinosaurs wouldn't be able to mate and repopulate the continents... So if the meteor event killed lots of large trees (that would take decades if not centuries to grow back) then adult dinosaur sized herbivores were screwed, repercussions echo up the food chain, etc. In modern times, large african and asian mammals are very vulnerable to habitat loss and climate change in ways small animals are not.