But didn’t dinosaurs have much more time (from 230MYA-65MYA) at the top of the food chain to develop into such large forms?

Maybe lack of time to evolve into large forms is a factor, but on the other hand I understand that mammals did evolve from basically small squirrel-like things into mammoths in the 65 million year time-frame between the end of the dinosaurs and now …

Mammals didn’t have to deal with us either (or our ancestors with hunting capability) for about 63 million years. Human ancestors were confined to Africa for most of their history, and humans probably didn’t reach the Americas until about 15,000 years ago. Humans can’t account for the failure of mammals to reach dinosaurian sizes during most of the Cenozoic.

Not really. Polar bears average up to ~1500 lbs with exceptional specimens pushing 2000 lbs. Some extinct bears might have averaged larger within this range but not significantly exceeding it. The size of the Eocene predator/scavenger Andrewsarchus can only be estimated from a skull and comparisons with smaller related species, but is in this general range.

Size comparison: human, polar bear, Andrewsarchus, and large theropods:

Some answers to the general question can be found in the paper Dinosaurs, dragons, and dwarfs: The evolution of maximal body size. In it, the authors observe the following patterns:


Nice summary. I’ll add only that I immediately thought of Andrewsarchus as well, and had researched out the cite for a similar post, including both links you used; and that I’ve seen it said more than once that Andrewsarchus was “the largest extinct mammalian predator”.

Very interesting! So it would not be inaccurate to say that, on best available evidence, the largest mammalian carnavores were not significantly larger than those existing today.

That’s what I was after - thanks! I see there is some speculative support for the CO2 theory, though I’m a bit disappointed at the “remains unexplained” conclusion.