Post your wild theories about why dinosaurs were so big:
January 29, 2014 8:19 PM   Subscribe

To ask the question another way, why are modern land vertebrates so small? If it's to do with environmental factors, which ones?

Convergent evolution suggests that a body shape that "worked" from c.200mya to c.65mya should still "work" now. But that's not the case: the biggest modern land mammal ever, paraceratherium, was about the same size in mass terms than triceratops, which wasn't one of the bigger dinosaurs. The biggest bird, aepyornis, was half a tonne. So something has changed -- what?

I've seen arguments about the type of vegetation, atmospheric oxygen, but nothing that really explains it. Are mammals and birds in some way genetically incapable of attaining those sizes -- and if so, why were birds' close relatives, the huge therapsids and sauropods, capable of it?

I'd really like to hear good or speculative ideas on this. What about parasites? Plate tectonics and gigantism? Warmbloodedness seems to have fallen from favour and there's the whole debate about whether dinosaurs were warmblooded, but could that explain it?
posted by 8k to Science & Nature (18 answers total) 6 users marked this as a favorite
Best answer: I don't know about dinosaurs, but I highly suggest reading this essay by J.B.S. Haldane, On Being the Right Size, which will at least give you some interesting ideas to think about.
posted by number9dream at 8:33 PM on January 29, 2014 [4 favorites]

One theory is that there is a constant ratchet towards larger size (due to the evolutionary advantages it often confers), but this ratchet is reset by major disturbance / extinction events. Large animals are disproportionally affected by these extinction events because they reproduce, adapt and evolve more slowly. The last major extinction event was only 12,000 years ago, so the theory is that in that time animals haven't had the chance to evolve back to large sizes.

This is just one theory. It's far from a settled question.
posted by Jimbob at 8:46 PM on January 29, 2014 [4 favorites]

Maybe being too big equates to being more vulnerable to certain environmental events. If you are too big, if there's freak weather or airborne contaminants you can't hide in the dirt or under a rock, you can't shelter under leaves and downed trees and in hollow logs, and you can't take refuge in a cave.
posted by Dansaman at 9:01 PM on January 29, 2014 [1 favorite]

Mamoths are no longer around because we small humans found ways to kill them to eat. I suspect they are not the only large animal to do so at our hands.
posted by 101cats at 9:28 PM on January 29, 2014

in (the book) jurassic park, they talked about how there was more oxygen in the air at that time.
posted by cupcake1337 at 9:35 PM on January 29, 2014

One of the more unusual theories is that the earth was smaller then, with correspondingly weaker gravity.
posted by coleboptera at 9:45 PM on January 29, 2014 [2 favorites]

Best answer: During the Cretaceous, as Cupcake says, there was more oxygen in the air than now. That's why, for instance, a lot of arthropods became much larger than anything we can see today. (There were some truly monstrous millipedes, for example.) There is indeed a general tendency for a species to increase in size until it runs into something that stops that progression.

Inability to breath enough is definitely one of the limits. If a creature the size of the largest dinosaurs existed today it would be immobilized by inadequate oxygen intake, kind of like someone with emphysema. And if any of those giant arthropods was around, it would suffocate.
posted by Chocolate Pickle at 10:11 PM on January 29, 2014

Giant versions of standard species exist today in Antarctic waters, presumed to be because of more oxygen, cooler temperatures and lack of predators.
posted by freya_lamb at 3:00 AM on January 30, 2014

Best answer: I think the best explanation is that they expended less energy than modern mammals and resources were more abundant. Blue whales, which are larger than any known dinosaurs, apparently reach the size they do because a filter-feeder in the sea has more food available to be consumed per day than an elephant on land.

I'm not sure oxygen is really the limiting factor here, although it might explain why food was more abundant. Dinosaurs likely had an adaptation called "continuous oxygen uptake", allowing them to pull more oxygen from the air—a feature seen in modern crocodiles.

There's some good literature on this subject; the (free) paper Resources and energetics determined dinosaur maximal size lays out the thinking on how metabolism affects size, and the PLoS collection Sauropod Gigantism (a suitably gigantic PDF) has a number of papers on why and how sauropods were so big.

Alternate theory: Mesozoic theme parks all had novelty oversized "must be this tall to ride" signs out front, and dinosaurs embarked on a program of selective breeding in order to learn what roller coasters were like.
posted by xkcd at 3:12 AM on January 30, 2014 [21 favorites]

Best answer: My sister did some research for her dissertation that suggested that the different atmosphere allowed plants to become more nutritious. IIRC, her pet theory was that the higher nutritive value could helped dinosaurs grow larger.
posted by sleepinglion at 9:10 AM on January 30, 2014

An article that discusses her early research.
posted by sleepinglion at 9:12 AM on January 30, 2014

Yep, I came in to say I also read/heard a theory that gravity was weaker.
posted by jbenben at 10:15 AM on January 30, 2014

I'll throw something out there that occurred to me - and you asked for wild theories. The smaller dinosaurs had small bones which dragged off or chewed up. The fossil records favor the big bones. This would suggest there are fewer baby dinosaur bones than adults. It would bias to thinking of dinosaurs are universally big, rather than explaining why some grew large.
posted by dances_with_sneetches at 3:22 PM on January 30, 2014

My wild theory is that the earth's gravitational pull was less back then, lower-gravity = bigger animals.

The reason it's higher now is because of hundreds of millions of years of dust accretion.
posted by Dag Maggot at 5:56 PM on January 30, 2014

Response by poster: Thanks for all the interesting responses -- what I love about this question is it's so seemingly basic but brings up a lot of major issues that we still don't understand very well!

dances_with_sneetches, I agree the fossil record may favour bigger bones, but that's why it's interesting that in 65 million years of mammal-avian dominance we've never found bones of any creature approaching the size of the bigger dinosaurs. They really were exceptional by current standards.

Jimbob and dansaman I like the vulnerability/ratchet idea but from what we know of major extinction events there wasn't anything that unique about the Jurassic-Cretaceous -- see here.

Minor extinction events are pretty common, and in fact it's interesting that these large body shapes were remarkably persistent throughout the period -- it suggests they survived a lot of smaller extinction events, meaning it was a pretty successful design that wasn't especially vulnerable to environmental changes. That does seem different to what we see in modern giant vertebrates -- giant sloths and wombats and pigeons (dodos) didn't last long after the introduction of competitor species.

I think the direction that xkcd and sleepinglion are going in seems convincing, but it throws up a bunch of interesting contradictions -- particularly when you ask why the dinosaurs weren't out-competed by the mammals that were around at the time.

If nutrition was abundant, as sleepinglion's sister's research suggests, why didn't that favour high-metabolism endothermic mammals, as it does in the modern world? The first paper xkcd mentions makes some interesting points modeling the metabolisms of dinosaurs on those of varanids -- komodo dragons and the like. But how come modern varanids are so small? I live in Australia, where modern varanids have been more successful than anywhere else, in the forms of goannas and perenties -- but though you find them in varied environments from rainforest to savannah and arid semi-desert (there's a goanna that lives in my mother-in-law's roof...) there's nowhere that they're the largest species (of course, there was megalania but even that wasn't dinosaur-big).

The oxygen theory mentioned by Cupcake1337 and Chocolate Pickle does seem more convincing the more I think about it, and it links in nicely with the nutritional theory, and perhaps with the metabolic one. I can't think of another global factor that's changed so much over the right time frame.

Still, it continues to beg the question of why oxygen concentration was such a decisive advantage to gigantic creatures vs. mid sized ones. Why didn't that mid sized form factor outcompete the giants, which must have taken so much more time and nutrition to reach sexual maturity?

Anyway, all interesting! My wild theory about parasitism is that soft-bodied internal parasites and fungal infections wouldn't show up much in the fossil record. We assume that they must be ancient because they're 'primitive' but that doesn't necessarily follow -- so maybe internal parasites have evolved that tipped the scales away from very large, slow-growing creatures and towards ones that killed their parasites regularly by dying relatively fast.

The wild tectonics theory is that the Cretaceous saw a lot of small continents and islands so maybe there was an element of island-style gigantism, but that doesn't really work because the Jurassic saw a couple of big continents like the present day. And neither of these wild theories squares well with a sudden KT-style extinction event.

number9dream, that lecture is awesome and made me want to know much more about Haldane -- his thought seems astonishingly up-to-date for 1928. And xkcd, I love the fairground ride image. Someone should make a comic out of that...
posted by 8k at 6:14 PM on January 30, 2014

Response by poster: Oh, and that gravity thing - c'mon! Where did the extra mass come from to make gravity stronger? Unless of course the chicxulub meteorite was really, really big!
posted by 8k at 6:20 PM on January 30, 2014

Response by poster: One huge problem with the oxygen theory: where did it go? The theories I've seen involve studies of bubbles trapped in amber and suggest that oxygen concentration was 30% vs. the current 20%.

That's something like 500 trillion tons of oxygen that's vanished, if we believe the high concentrations in amber bubbles reflects that in the ancient atmosphere. Where is it?

You would think there's be some glaring geological evidence of this -- unprecedented quantities of charcoal from global firestorms, for instance. The only other place it could plausibly have gone is the ocean, but I would have thought the fossil evidence of that transformation would have shown up too -- surely something pretty weird would happen to marine life if oxygenation suddenly increased massively?

This isn't very well thought out ... But like I say, wild speculation... What am I missing?
posted by 8k at 8:19 PM on January 30, 2014

A lot of oxygen is trapped in lime and chalk deposits -- calcium carbonate. See the White Cliffs of Dover, for example.
posted by Chocolate Pickle at 8:46 PM on January 30, 2014

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