In isolated pockets of the Cretaceous and Jurassic worlds, the evolutionary experiment of the dinosaurs sometimes seems to have run amok. The result: extremes of size, shape, and lifestyle that seem to defy the notion of survival of the fittest.

Supersaurus, Ultrasaurus, Seismosaurus - those were the biggest creatures to ever walk the earth. Now a new contender for the title of ""biggest of them all"" is coming out of the earth in Argentina.

With astonishing rapidity, tiny mammals stepped into the void left by the extinction of the dinosaurs. In the blink of an eye, evolution-wise, giant predators once again strode the earth: saber-toothed lions and tigers, dire wolves, and even saber-toothed marsupials.

The Sabre Tooth Cats, predators so ferocious they preyed on mammoths and mastodons. The last died out 11,000 years ago, but could they return from the stock of today's wild cats?

When the curtain rang down on the dinosaurs, the mammals took centre stage. But mammals actually had their start long before the first dinosaurs - and they were as bizarre and mysterious as the dinosaurs themselves.