This may be an easily answered question - but does anyone know off the top why land mammal megafauna never grew to the same size as the largest dinosaurs?
I know that in the sea the whales are the largest animals that ever existed - why no mamalian carnavores the size of a T-Rex or herbovores the size of the largest Brontosaurus?
(My apologies if the question has previously been addressed).
IIRC gigantic insects (e.g. dragonflies half a meter long) used to be viable because the atmospheric composition was far more oxygen-rich, as high as 35% at one point (now ~20%).
<speculation>
Insects and mammals respire in very different ways, but perhaps the high O2 content also made megafauna possible then which are simply not possible now.
</speculation>
Has the oxygen content changed much over the past 65 million years? I have no real idea.
It was 30% O2 about 65M years ago. Cite.
Now, that’s cool. 
Thanks!
I’d be very interested in knowing if the larger maximum size of dinosaurs is related to this … sounds very plausible.
There used to be some pretty large mammalian herbivores, not only mammoths and mastodons but also Paraceratherum, which was comparable to the sauropods in size.
I can only offer a wild-assed guess but for what it’s worth, the size of dinosaurs may have been driven by the body plan of the main carnivores, the various types of Theropods. Their bidedal body plan may have allowed very large predators to remain comparatively fast and agile, in turn creating a selection for herbivores gigantic enough to be resistant to attack. By contrast, it’s doubtful that any terrestrial mammalian carnivore exceeded one metric tonne average size, puny by Theropod standards.
The first issue would be the difference between warm and cold-blooded. To stay warm, a mammal needs to eat significantly more than a cold-blooded animal. In a hot climate, they also need to shed that heat. (Hence the elephant’s big ears). The evolution of exceptionally clever predators has guaranteed that extremely large meal tickets were begging to be extincted.
Before that - well, how big really were mammoths, for example? One site says up to 14 feet tall. I guess when every bit of body needs to be fed regularly, large tails seem to disappear. that’s probably one of the reasons dinosaurs seemed so big - they had long necks and tails.
I’d forgotten about one of Piers Anthony’s more obscure works for the last 19 years it seems:
Big enough that they should have gone ahead and called it an Oliphaunt 
Steppe mammoth - Wikipedia
Giant Ground Sloths where quite big as well -
Not something that you would want to find going through your garbage at 5am. 
Aren’t fires much more likely to happen at 30% O2? What types of adaptations did plants have back then in a fire-rich environment? Ditto for the dinos and other animals too.
And a bit of extenstion to John’s question. What would be the short and long term effects of the atmosphere returning to a 30%+ oxygen content?
I know we wouldn’t be seeing dragonflies with a two foot wingspan, but how much would it throw off things like internal combustion engines?
Oxygen levels is a good place to look; in would be interesting to look up estimated O2 levels during different eras.
Being huge is not easy. There has to be a sufficient selective pressure for a species to evolve a large size: an otherwise unobtainable food resource or, as mentioned above, protection from predation. The sauropods probably browsed in the high trees; maybe the tall tree-browsing mammals faced more competition for that resource from climbing mammals.
I wonder if there is any scientific consensus on the issue.
Probably not. At least, not all of them. Recent reconstructions (.pdf doc; not stated in the paper, but it is from 2005) indicate that most sauropods were likely medium to low browsers (or possibly even grazers), while a scant few (e.g., Brachiosaurus) could have been high browsers.
IC engines would enjoy faster combustion, better power density, and improved efficiency, assuming you didn’t give a rip about emissions. More O2 and higher combustion temps would result in greater NOx production; if you wanted to meet existing emission regs, you’d need to do what we do right now - that is, add EGR, in even greater quantities.
There would also be a greater tendency for knock/ping, so you’d either have to retard spark timing (on an SI engine), or use EGR as described above to reduce the in-cylinder O2 concentration.
I don’t think there is a scientific consensus. However, I think past levels of CO2 and global temperature are a more likely cause of gigantism during the middle and late Mesozoic than differences in O2. Levels of CO2 and global temperature were generally high during the Jurassic and Cretaceous. From here:
Increased global temperatures and higher CO2 levels would likely cause much higher primary production by green plants. Because of this, herbivores would find it easier to obtain enough food within a particular area. Because they could find more food with less effort, they would be able to grow larger. With larger herbivores, carnivores would also be able to get larger.
The KT mass extinction wiped out all almost all land animals larger than 5 kg. It took millions of years for mammals to become much larger. Climates during the Cenozoic have generally been cooler than in the Mesozoic, so land mammals have never reached the average size the dinosaurs did.
Interesting stuff. Thanks!
I’m going to go out on a limb and suggest another factor that the dinosaurs didn’t have to deal with: us. I can see humans having something to do with limiting the growth of mammalian megafauna in two ways: hunting them down and competing with them for resources.
I think that humans have played a role in the extinction of several species of mammalian megafauna, but of course that would only be in the last hundred thousand years or so. Plenty of time prior to that for giant mammals.
Edit: has there ever been a mammalian predator significantly larger than a polar bear? I know “cave bears” were pretty big …
