Slashdot Mirror
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."
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.
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.
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.
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.]
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.
Not so much circular reasoning as a description of an unsustainable feedback loop.
It's kind of like the American car market. As one soccer mom replaces her station wagon with a larger one, followed by an SUV, followed by a larger SUV, she forces other soccer moms to do the same thing, as those left with the slightly smaller model of SUV or station wagon finds themselves at a competitive disadvantage on the same roads, being unable to see past the rival Canyonero in front of them.
Eventually the group of soccer moms is unable to diversify by having multiple sizes of vehicles, which means that when a megadisaster, such as a massive rise in oil prices, occurs, the group suffers considerably more than they would have done. The species dies out, as one by one their homes are foreclosed upon, and groups that previously lived in the shadows of these groups - cyclists, Democrats, Prius drivers, etc, gain the upper hand.
That's how it works.
You are not alone. This is not normal. None of this is normal.