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?

[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?

[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?

[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?

[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?

[mcgrew]

I RTFA and it looks like hogwash to me. It said that the adults had to be big to keep from being eaten by their own young. It seems to me that cannibalism would be an evolutionary disadvantage, plus the biggest dinos were herbivores. And I notice that most large animals today are herbivores -- elephants, cows, rhinos.

Perhaps the article was poorly written, but it doesn't seem logical. The only logical part was that the larger animals became extinct when the asteroid hit.

Re:Circular reasoning?

[ByOhTek]

The explanation was extremely poor. Hopefully this is less poor.

Start with this factor: eggs have to be small. If they are too large, the short of it is, the oxygen/volume ratio in the egg will get too low.
Now, consider that all of the large species started as eggs. Therefore their young start small.

Now, they have to compete with the smaller and medium sized creatures to become large. To reduce the time-frame of this competition, they have to grow fast, to grow fast they have to use a lot of resources. Result - a brief period of extreme competition instead of a longer period of light competition.

This could also increase the final adult size if growth 'ramps down' rather than just 'shuts off'.

Re:Circular reasoning?

[Black Parrot]

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.

Now that actually makes sense, though as others have pointed out, why didn't the same apply to mammals?

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?

[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.

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.

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:Circular reasoning?

[SuricouRaven]

With their higher body temp, the energy requirements for supporting a large mammal can become quite impractical. There's a reason for the expression 'eat like a horse.'

Re:Circular reasoning?

[SandorZoo]

In Niven and Pournelle's The Legacy of Heorot they claim there is a species of frog that survives by eating only its own tadpoles. The continually lay frogspawn, which grows in tadpoles that eat algae and the like, and the parent frogs eat (most of) the tadpoles. I have no idea if this is true or not (it's only a novel), but I always suspected it was. Anyone know the species?

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?

[s_p_oneil]

"I RTFA and it looks like hogwash to me."

Well, what do you expect from a science where no direct observations can be made, no experiments can be performed, and all of your theories are based on fossils millions of years old? While I wouldn't say the study of subjects like this are a waste (things can still be learned from them), these theories have to be taken with more than the usual number of grains of salt.

IMO every set of theories on a subject like this is built up from the bottom like a house of cards. Commonly accepted scientific principles that relate to it are the foundation (biology in this case). Still, if the theories aren't built up logically around accepted scientific principles, they won't be able to stand up at all. Even if they are built logically, they're full of holes, and they're pretty easy to knock down when you take a poke at them. What's not so easy is to build your own house of cards that is clearly superior to all the others and that can withstand new discoveries (which would be the equivalent of someone shaking the foundation/table your house of cards was built on). If you can do that, others will try to fill in the holes, giving you a house of blocks instead of a house of cards. It can still be knocked down (e.g. if someone removes a block like a game of Jenga), but it's more stable.

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.)

Re:Circular reasoning?

[dylsexia]

In Canada, we refer to them as the Bloc Quebecois.

Re:Circular reasoning?

[squiggleslash]

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.

Re:Circular reasoning?

[dylsexia]

Spadefoot

http://www.centre.edu/web/news/2009/brian_storz.html

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: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.

Re:Few is not the same as none

[Timtimes]

I got this one. The larger dinosaurs were dependent on larger trees and vegetation that takes longer to repopulate after the asteroid hit than their life cycles would allow. The growth cycle of a tree is many orders of magnitude slower then reptiles. So at first there would still be a lot of small babies of big (dead) dinosaurs to compete with the smaller dinosaurs (that evolved into birds?) from the short grasses that would emerge post fireball. The large bodied dinos would ultimately die when they got too big to consume enough of the smaller vegetation to allow them to exist. This is as good as example as I can think of off my head: You could, for example, try to reintroduce a particular species of extinct locust that used to exist until the late 1800's and it wouldn't do any good. They only bred in the deepest ruts of the prairies before the vast buffalo herds got slaughtered (Arizona history books mention that were all wearing hoodies). You could theoretically release the first "batch" of artificially created/cloned buffalo locusts into the wild, but when it came to the point in their life cycle to breed, they wouldn't have anyplace to lay their eggs. No herds of buffalo. No eight foot deep ruts in the prairie worn down over eons for them to lay their eggs. Enjoy.

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:Economics

[Black Parrot]

Sounds like the ultimate free market.

Yes, every meal was "all you can eat".

Re:God is an idiot.

[Black Parrot]

If he really exists, then he's an idiot.

Clearly, the dinosaur god couldn't compete. It's down to the human god vs. the beetle god now.

Re:Summary == Gibberish

[Black Parrot]

Does nobody read the summaries before posting them?

Yes, nobody reads everything.

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.

This myth dies hard

[Squidlips]

The gowns (usually from Ivy League Geo departments) who bitterly fought the Alvarez asteroid theory failed but next proposed that the dinos were already stressed out and headed for extinction when the asteroid hit. Now this...ugh, will they never give up?

Re:God is an idiot.

[cockroach2]

only 22% of respondents with postgraduate degrees believed

"Only"!?

"May explain"

[mapkinase]

I like this phrase. As long as people understand a difference between "explanation", "fact" and "possible explanation", the science is in a good shape

Maybe it is because they could

[hemo_jr]

Dinosaurs probably had a dual air sac respiratory system like birds do today. This respiratory system allows continuous oxygenation of the lungs, unlike mammals who breath in and out. This is a very robust system and may be the reason that early dinosaurs out-competed early mammals et al and established a dominance in the Triassic that continued until Chicxulub.

We already know that terrestrial arthropods, like insects, are limited in size by a combination of the O2 concentration in the air and the tracheal respiratory system (a network of tubes...). So it would not be surprising that a highly effective dual air sac respiratory system could be efficient enough to make the trade-offs for increased size more advantageous for dinosaurs than mammals.

Therefore, for dinosaurs, increasing size to compete may have been more evolutionary advantageous than for mammals. And that is why dinosaurs grew so big.

Re:God is an idiot.

[mcgrew]

I'd say to the GP "You're calling an entitty who is intelligent enough to design and build math, physics, time, and the entire universe an idiot? Only an idiot woud say something so stupid. To think that a mere human could understand the motives of a being that powerful is the very height of idiocy".