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."

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.

Few is not the same as none

[satuon]

> there were so few small dinosaur species that the group was almost obliterated, with only the birds surviving

Yes, but why didn't those few non-bird species survive? Or did they mean that birds were the only small dinosaur species?

Re:Circular reasoning?

[The Evil Atheist]

No, it's called a positive feedback loop. You know, evolutionary arms race. You know, evolution. Evolution has the advantage of being dependent on time and space, making mathematical logic completely irrelevant to how nature actually works.

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?

[ananyo]

Right! Essentially, as the source story says, dinosaurs ended up competing with their own young in a way that mammals didn't. Mammals were able to occupy all niches - ie niches appropriate for small and medium sized animals, while adult dinos had to keep getting larger and larger to keep their competitive edge. The two papers are pretty neat and work well together - one shows the traditional hypothesis isn't right (environment doesn't correlate to dino size), the other suggests a credible reason why.

Doesn't explain anything

[Grayhand]

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.

Re:Circular reasoning?

[Anonymous Coward]

A simpler explanation would be that post-KT there wasn't an ecosystem to support the huge adults, and when the environment won't support adults the whole species dies.

I think the idea is, when you are born small (category A ecosystem), than grow mid-size (category B ecosystem) than very-large (category C), you need all these different ecosystems intact in order to achieve a full circle. And you need those 3 categories to coexist in relative close proximity, if not at the same location.

When you are born category A and remain category A, you only need a category A ecosystem to survive and category B/C ecosystems don't matter at all.

So let's say a big event, destroyed parts of A, B and C ecosystems, chances of finding a location where all 3 remain intact are much lower than chances of finding a location where only 1 ecosystem remains intact.

Re:Circular reasoning?

[gnick]

It seems to me that cannibalism would be an evolutionary disadvantage...

In order for a species to survive, an animal only needs to survive long enough to produce children who can survive long enough to produce their own. Once you're old enough to survive to the point of reproduction, how does snacking on a parent hurt anything? In fact, if the parent isn't providing anything that helps you survive, you're just having a meal and cutting down on competition.

"It's people! You're eating PEOPLE!"

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

[samoanbiscuit]

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.

Re:Circular reasoning?

[Artifakt]

If the size of the eggs themselves has a limit, then all dinos start out small, regardless of how big they end up

There are risks in being a growing animal: Until the creature settles down to its adult size, it has to adapt to different food sources, learn over and over again how to move efficiently at each new size, and expose itself to predators foraging for food. Growing animals are awkward at some stages, and need more food, more often than when they finish growing.

Triple your size in a year and get it over with, those risks are proportionately small. Double over and over again every six weeks, and those risks are much larger.

So, there needs to be some advantages once you get big, to offset the disadvantages of the growing years. If a species has more disadvantages than competitors, and doesn't have advantages, it dies out from the competition. But the advantages of growing bigger than a competitor species accumulate with very large sizes:

For example, there's not much advantage to being just a little larger than a pack hunter such as Deinonychus, but if, like Apatosaurus, you're so large your hide is thicker than the packs 6" killing claws and so tall the pack can't even reach your vital spots, the advantage is your adult species members are practically totally immune to Deinonychus attacks. To eat you, Deinonychus doesn't just have to evolve to be a little taller, it has to evolve in the direction of T-Rex.

There are other trends in dino evolution: By the time smaller, early fast predators actually get to T-Rex size descendants, all the Apatosaurus like dinos are gone, and horned and armored herbivores take their places. Bulk can only do so much, and it's hard to see how anything could simply get big enough to ignore a pair of T-Rexes attacking it. But these biologists aren't saying that the trend towards bigness overwhelmed all other factors, just that it was a more major cause of more effects than is immediately obvious.

You can call all this circular reasoning. The biologists are in effect arguing that the advantages and disadvantages must have pretty well balanced in each stage of evolutionary history, because natural selection must work as the theory. But there are other, non-circular, lines of thought which support this. Reducing Darwin to "Survival of the Fittest" is tautological, but when you use actual math on the actual fossils, and look at how many different species in different size groups there were, over the millions of years leading up to the extinction event, you get non-circular predictions as well, like that number of different species would taper off for the last few million years before the extinction, and that it would be lower by far than for most typical dinosaur eras.

.

Re:Circular reasoning?

[Anonymous Coward]

There are multiple problems with this rationalization:

• If it takes a dinosaur 30 years to reach breeding maturity, and it can have a brood every 2 years after that, for the next 30 years, the mother that is devoured by her young will have a single successful brood. The other mother wil have 15 broods (likely a mix of successful and unsuccessful). Which mother has a likelier chance of producing more, healthy offspring?
• Modern-day saurians are not eaten by their young.
• Dinosaurs would need to lay many, many more eggs if their offspring relied on devouring the mother at their size. They didn't have broods of millions or even thousands. They had broods of 2 dozen or less. Mix in with the fact that egg thieves were somewhat prevalent among dinosaurs and the likely size of the average brood is somewhere around a dozen?
• Parents DO provide things to their offspring, other than food, in hostile environments: Protection, warmth, shelter, etc.
• Animals that live in family groups may practice cannibalism (chimps, for example), but they rarely make cannibalism their practice.

Your Dawkins-grade rationalizations are contradictive to these observable points (and their accompanying Dawkins-Grade rationalizations.)

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