If C. megalodon was indeed somewhere around 16m and 40+ tonnes, it would apparently exceed the basking and whale shark as largest elasmobranchian - although since it it hypothesized to have also preyed on warm blooded prey presumably it too had a relatively fast metabolism. But since oxygen content in water is considerably lower than air, how would a shark this massive be able to function? Gill area for body mass would also be considerably less than even the great white, unless there were significant anatomical changes. How this species managed to out-compete whales to become a top predator is also an enigma to me...

There is no reason to think that megatooth sharks would have suffered from physiological problems because of their size, but I'm not sure that we can be confident that these sharks (which might have been more closely related to sand tigers and kin rather than to the warm-blooded lamnids like the great white) were endothermic. Indeed I can't recall seeing this issue considered in print.. but then, I'm not a shark expert (we do have a shark expert round these parts but she's not around right now!).

Furthermore, there is no evidence that megatooth sharks out-competed any cetaceans as you say. While megatooth sharks did live alongside an assortment of predatory cetaceans (including macropredatory sperm whales and ancestors of killer whales), I think we can conclude that there was enough ecological space for these species to exploit their own niches. Note that the diversity of large whales was higher when megatooth sharks were around (this was the case in the Miocene in particular), so there was probably much more prey to go round. The eventual decline and extinction of megatooth sharks has sometimes been blamed on the rise of modern killer whales, but it's more likely that global cooling and the decline of large whales were more important.

Last edited by Darren Naish (11th Dec 2007 10:56:15)