The most recent part of the paleogene (33.9 million - 23 million years ago). During this period the Earth starts to become far more similar to the world we know. It's still.... a little... alien, but it's getting there.
During this period, South America split from Antarctica and moved up towards North America; Antarctica got it's ice cap; global temperatures start to drop and deciduous trees start to proliferate; baleen whales evolve; there is grass and there has been since the cretaceous, but modern grasslands and savannas and their associated ecosystems are still someway off.