Dinosaurs gigantism paradox; what is your favorite theory

[Mini Minmi]

Because one of the things I love the most about science is how things are only true until proven otherwise, I enjoy learning about new theories and suggested concepts that had never crossed my mind before, as silly as some of them may be.

Recently I was looking into attempts at explaining how dinosaurs managed to grow so large without collapsing under their own weight considering that even today's elephants and giraffes are pretty much pushing the limits of the laws of physics as it is. (Can't really compare mammals to dinosaurs but I mean in terms of mass vs what their bones and organs can handle)

I fell upon the expanding earth theory suggesting that gravity may have been different in the Mesozoic.

I also found someone suggesting that the atmosphere during the Mesozoic had a density closer to water, so while being terrestrial, dinosaurs may have benefited from the same advantages as marine animals to handle their weight.

While they don't all convince me, I'm having a great time reading about them.

What is your favorite theory to explain how dinosaurs managed to grow so large?

[Papi-Anon]

Something related to that, it's been shown that even non- avian dinosaurs had more light-weight bones compared to modern reptiles, but is it possible that non-avian dino muscle and organs had different densities than modern day megafauna? For instance, do crocodilians' and birds' flesh have varying densities pound-for-pound? If so then that MIGHT explain the gigantism partly, right?

[Halichoeres]

Muscle is mostly water, so you can't really vary its density much. Certainly not to the extent that you can vary bone density.

[Mini Minmi]

I'm not knowledgeable enough in physics and biology to know, would having a lighter bone density really be enough to allow sauropods to grow to those ridiculous sizes without bone structure or muscle issues? I understand that everything in them was designed to allow easiest access to the most food with the lowest expense of energy (long neck, air sacs, no chewing so smaller lighter head, honeycomb bones that are both light and strong,...) but I'm still floored that things this huge could not only exist but also walk and somehow mate! 

[Lanthanotus]

[...]

Recently I was looking into attempts at explaining how dinosaurs managed to grow so large without collapsing under their own weight considering that even today's elephants and giraffes are pretty much pushing the limits of the laws of physics as it is. (Can't really compare mammals to dinosaurs but I mean in terms of mass vs what their bones and organs can handle)

[...]

I disagree on this point..... mammals have shown that they can grow a great deal bigger than they do today, though admitted, they only did so once (as far as we know) with the Paraceratherium (or however you want to name it). So the sheer physics of bone and muscle etc. allow much more than a 5to 7 ton elephant can muster.

A limiting factor in terms of size for mammals may be another..... mammals have a considerable lower body temperature and lesser tolerance for varying body temperatures than reptiles and birds. A great many of recent reptiles' and crocodiles' metabolism works at an optimum level at a temperature that is very close to a lethal body temperature (around 42 °C). Most birds aswell sustain a very high body temperature of 40 °C and more. For some (genetic/evolutionary/...) reason such temperatures cannot be sustained by mammals....

With an increase in body mass, heat balancing can become a major threat to a terrestrial animal (because of the different thermal capacity of air and water, the same problem does not occur for whales). Gigantic dinosaurs grew so big because they could.... sauropods needed to grow big to reach the highest plants, theropods grew so big to hunt the big vegetarians. Equipped with a great tolerance for a high core temperature, they could go further than any mammal.

This however does not explain why there are no elephant eating carnivores today, carnivores to size of a rhino. The reason lies probably in the reproduction strategy of mammals. Mammals invest long time and lot energy in raising their young, the bigger the mammal, the longer gestation and upbrining of the young. Big mammals pay this with a very low number of young, often just one every other year. Unlike for example sauropods, elephants are naturally rare. To sustain a population of rhino sized carnivores, a way greater area would be required that to feed an Allosaurus. In addition and possibly more important: Because big mammals get so few young, they defend them vigorously and if living in a herd, all members join in the defence (at least in the case of elephants, but also buffaloes and others), increasing the risk of being seriously wounded or killed enormously. So we will never see lions, tigers or bears growing double their recent size.

[Newt]

I'm not knowledgeable enough in physics and biology to know, would having a lighter bone density really be enough to allow sauropods to grow to those ridiculous sizes without bone structure or muscle issues? I understand that everything in them was designed to allow easiest access to the most food with the lowest expense of energy (long neck, air sacs, no chewing so smaller lighter head, honeycomb bones that are both light and strong,...) but I'm still floored that things this huge could not only exist but also walk and somehow mate! 

The lighter bone density is not so much about lighter weight as more efficient use of material. Sauropod bones are more efficiently "engineered" than big mammal bones - they can span greater distances and so provide better angles for attachments of ligaments and muscles without using more energetically expensive bony material. This is the same principle that, in human construction, leads to I-beams, trusses, etc.


That lighter, smaller head is a key point. Animals that chew their food (mammals, ornithischians) have to have a head capable of handling the quantities of food they need. As the animal gets bigger, its head also needs to get proportionately bigger so it can process more food at a time, which creates some physical problems, or the animal needs to spend more time eating, which creates some lifestyle problems (especially for folivores, whose food is abundant but not very nutritious). This may be a major cap on these animals' sizes. Sauropods on the other hand processed their food post-orally, so a small simple head was adequate to maintain food intake even at enormous sizes.


Lanthanotus - don't forget that many extinct elephants were considerably bigger than modern elephants, more in Paraceratherium's weight class. And weren't sabertooth cats basically elephant hunters? Or is that notion outdated? I'm afraid I haven't kept up with the Neogene.

[Halichoeres]

On a couple of other points, the expanding earth hypothesis is a pretty fringe idea that contradicts everything we know about physics. It was originally floated to explain the arrangement of the continents before we figured out plate tectonics.

There is no support for the idea that the air has had a significant variation in density or viscosity during the Phanerozoic. Even relatively minor variations in atmospheric composition (like elevated CO2) have dramatic effects on the biota, let alone making the entire atmosphere "swimmable." Part of the reason pterosaurs could get so large is 1) bone pneumaticity and 2) membranous wings. Birds also obviously have bone pneumaticity, but the average birds' feathers collectively outweigh its skeleton. Making your airfoil out of skin probably costs you less in terms of ballast.

[Mini Minmi]

OH no, those theories did not convince me they were reality. These two cases specifically eventually lost their credibility in my eyes the more reading I did. I was fascinated and entertained by the fact these ideas had never crossed my mind. It was interesting to see how people came to these conclusions, how they argumented for or against it, all the intricate details of the explanations, etc. I wonder what else is out there.   

[Neosodon]

My favorite theory is that dinosaurs had more efficient metabolisms. If dinosaurs had more efficient metabolisms they could reach a larger size without having to eat as much as a mammal of the same weight. Lower metabolisms would also mean heat loss would be less of an issue so body thickness would be less of a problem.