Re: Life's scale reduction since the Dinosaurs

["Nicholas R. Longrich" <longrich@phoenix.Princeton.EDU>]

On Wed, 24 Jan 1996, JCMcL wrote:

> But why no mammals as large as large sauropods?
> 
> How long after the first broad adaptive radiation of archosaurs did really 
> large sauropods (bigger than elephants) arise?

        Not just sauropods, either. Carnivorous dinosaurs get huge, too, 
compared to the modern mammals. 
Eight tons is the most recent maximum known, with a five ton allosaur, 
megalosaur, and ceratosaur from the Jurassic. Plenty of "medium" weight 
theropods at two or three tons. A "puny" tyrannosaur is the size of a 
bear, more or less, and it's debatable whether bears can be considered 
terrestrial carnivores (polar bears like to swim, browns are omnivores) 
so that still exceeds something like a Siberian tiger. And haven't 
we found some big two or three ton hererrasaur from Africa? Whatever 
Deinocheirus was, it was big. 
        Ceratopians and duckbills get up to elephantine dimensions, but 
here I guess the question is: we know dinosaurs were often bigger than 
mammals, did they also reach elephantine sizes more often than modern 
mammals do? Granted, megafauna extinctions make them look bigger in 
comparision, but they were still big.
        One: the environment MUST have selected for larger size in herbivores. 
There must have been a reason that dinosaurs evolved to grow bigger. This is
 at worst half the explanation for sauropods. It might be that these 
conditions no longer exist.
        Two: Sauropods may have been able to evolve bigger size than 
mammals because of certain features of the dinosauria. Bakker claims that 
air sacs filled the pleurocoels, allowing cooling of big son-of-a-guns 
like Brachiosaurus. If that is true, maybe dinosaurs are better at 
shedding their heat. Maybe they have something else that lets them 
respond to the environmental pressures better.
        Three:environment must have selected for large carnivores to eat 
the large herbivores. But here, one would think that the modern 
environmental pressures would be similar to those of the mesozoic I.e.,
 you have a bunch of Chasmosaurs, so you have some 
albertosaurs to eat them. But while we have rhinos and had mammoths, you 
never saw anything big enough to actually take them on. Maybe there 
aren't enough mammoths and rhinos, maybe the carnivores can't get that 
big. 
        Again, either we don't have the right environmental conditions, we 
don't have animals that can respond to those environmental conditions. 
I'm sorry if this is too painfully obvious for anyone, but it at least 
helps me clear my head a little.
        -Nick L.