I was watching a presentation by Dr. Jessica Theodor [1] who discusses (at 38:10), among other things, the skeletal structure of dinosaurs vs. mammals. "Dinosaurs have way lighter skeletons, which means that for the amount of material they can be bigger. If we start to look at sauropods, they have this really interesting set of structures, these big holes in their vertabrae... it allows you to build a lighter, stronger skeleton. Mammals can't do that. It's not how we build our bones."

Why don't mammals do this? Are there fundamental trade-offs that make our (apparently) weaker, heavier bones better, or is it just chance that we haven't matched this development?

(Of course I'm open to the possibility that I misunderstood the talk, or that it was simplified to the point of inaccuracy on this point.)

[1] Jessica Theodor, Being Giant: Why are mammals not as big as dinosaurs? (2015)

  • $\begingroup$ If one wonder about why mammals in general are so much smaller than dinosaurs, I think the answer lies somewhere in that we simply have not gone down the competitional path of pure size. Just in the same way as we generally don't have evolved to use flight (except for bats) or live an aquatic life (except for whales and dolphins). One could say that we don't have bone structure suited for extreme growth and size just like we don't have bone structure suited for flight. $\endgroup$ – Alex Mar 27 '16 at 7:33
  • $\begingroup$ That is because, they have twenty one percent of body mass than minimum skeletal volume. In Dr.Bill Sellers's journal "Biology Letters", it was said that they laser scanned Brachiosaur as they were estimating it to weigh about 80 tons. But, it actually weighed 23 tons which is relatively light. $\endgroup$ – Komal stavita Mar 27 '16 at 16:15
  • $\begingroup$ @Alex: It's more like "why don't mammals have lighter skeletons, following dinosaur's plans". $\endgroup$ – Charles Mar 27 '16 at 16:44
  • $\begingroup$ @Alex: But also remember that what we know of dinosaurs (and extinct mammals) comes only from fossil remains that have been found. Until relatively recently, big bones sticking out of the ground were a lot more likely to have been discovered than little tiny ones. I suspect, though I don't know for sure, that larger bodies were more likely to have become fossils in the first place. $\endgroup$ – jamesqf Mar 27 '16 at 18:05
  • $\begingroup$ you link to a private video. It's not possible to check your source... found this video which is probably it: youtube.com/watch?v=whxn9Cc1_Ho $\endgroup$ – Frieke Jun 7 '20 at 12:07

I have also heard the claim that dinosaur bones were lighter than mammal bones of equivalent size, and at the moment I do not believe it. I will not believe it until somebody explains it to me how that is possible.

If you have a structure that you want to add more weight to, to support that weight, you want to ADD structural elements, not subtract them. If the structure could support itself before adding more weight, then adding structural elements will make it stronger (unless it's already at the absolute limit). Otherwise, the structure does not stand at all.

If you google about the weight of bird bones, you can find recent research saying that bird bones are actually not lighter than mammal bones. Their weight is just distributed differently (more densely on the outside of the bone, as opposed to more evenly spread throughout the bone as in mammals) to resist bending and twisting better at the cost of being weaker to impacts. Birds arguably have a much stronger reason to lose weight than dinosaurs, but their bones are still just as heavy as ours. Since birds are dinosaurs, I find it very unlikely that ancient terrestrial dinosaurs had magic bones lighter than both mammals and birds.

Also, I'm aware that the internal structure of our bones is very efficient. As I understand it, the bone on the inside of a bone grows thicker where it is more stressed. If you train a computer to calculate the ideal lattice structure to support the stresses of a given bone shape, the computer calculates pretty much the exact same lattice as what actually exists in our bones. So, if the internal structure of our bones adapts to better handle the stresses we experience in life, and if the bones of vertebrates can evolve into any external shape whatever, I cannot imagine how dinosaur bones could be stronger than ours unless they were made of a different material. I am not aware that this is the case.

  • 1
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