Megafauna

[brix11]

@Malthus

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.

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

[Malthus]

@brix11

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 …

[Colibri]

@brix11

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.

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.

[Lumpy]

@Malthus

Edit: has there ever been a mammalian predator significantly larger than a polar bear? I know “cave bears” were pretty big …

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:

[Darwin_s_Finch]

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:

1.,
Within a trophic level, body mass of the top species will increase with land area, with a slope predictable from the slope of the relation between body mass and home range area.;
2.,
For a given land area, the top herbivore will be larger than the top carnivore by a factor predictable from the greater amounts of food available to herbivores than to carnivores.;
3.,
Within a trophic level and for a given area of landmass, top species that are ectotherms will be larger than ones that are endotherms, by a factor predictable from ectotherms’ lower food requirements.

Also:

The largest terrestrial vertebrates of all time were dinosaurs, of which Table 2 lists the largest known herbivores and carnivores for three continents. (Those listed top herbivores and top carnivores were not necessarily contemporaneous.) Their body masses all fall closer to the ectotherm lines than to the endotherm lines of Fig. 1 A, suggesting that dinosaurs were “cold-blooded.” However, the top carnivorous dinosaurs were still 12 times heavier, and the herbivorous dinosaurs 1.5–3 times heavier, than predicted from the regression lines from Fig. 1 A for extant ectotherms; therefore, the never-since-surpassed size of the largest dinosaurs remains unexplained. Perhaps the answer has to do with high net primary productivity during the dinosaur era: atmospheric CO2 levels then were up to 10 times the present levels and elevated CO2 stimulates productivity of some plants.

[Polycarp]

@Lumpy

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:
http://upload.wikimedia.org/wikipedia/commons/b/bc/Giantpredatorsscale1.png

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”.

[Malthus]

@Lumpy

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:
http://upload.wikimedia.org/wikipedia/commons/b/bc/Giantpredatorsscale1.png

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.

[Malthus]

@Darwin_s_Finch

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:

Also:

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.