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So this is different than the question of "why are megafaunal mammals smaller than dinosaurs", which I understand to be a combination of:

  • live birth limiting size (even sauropods had miniscule egg sizes, on the order of ostrich egg sizes).

  • Breathing efficiency due to not having to rebreathe the same air breathed out.

  • Lighter bones

  • Mesothermy in large bodied animals.

  • Different life stages filled different niches.

  • Dinosaur mass estimates have historically been overblown

  • Dinosaurs weren't that much bigger excluding sauropods (Many members of the paleoloxidon genus were significantly heavier than modern T-rex estimates, and the estimates of most non sauropod dinosaurs, even African Elephants can get 2x the mass of an edmontosaurus and larger than a T-rex, lots of animals and the ranges of megafauna today are also smaller specifically because of human intervention, perverting our perception of animal sizes).

The only thing I can think of is the tendency for mammalian predators to hunt in packs (Even within the same biome, Hyenas, Painted Dogs, Lions, heck even Humans!), replicated in virtually every landscape on every continent except Antarctica and some what to a lesser extent in Australia (cave hyenas, wolves, cave lions plus sabertooth cats, and again, members of the homo genus), and afaik, the evidence for dinosaurs seems to show even those that hunted in groups weren't necessarily doing it to the same extent as mammals are/have been. This still doesn't explain why mammalian predators seem to cap out at around 1000kg -> 1500kg in mass through out virtually all of prehistory, even the "solo" hunters, such as short faced bear, when T rex itself seemed to be similar in mass to much of it's prey, as well as T rex relatives. The largest mammalian land predator (Andrewsarchus) has had it's mass estimate swinging downwards (back towards 1000kg, possibly no longer making it the largest) but even if it had been 2000kg, it still wouldn't be the mass I would be expecting.

So what are the hypothesis on why mammalian predators haven't gotten to the same relative sizes as prey as theropod dinosaurs did, or vice versa why didn't theropod dinosaurs stay smaller relative to their prey?

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