The Eocene was an epoch in Earth's history. During that point in time, the birds had become the top predators on Earth since the absence of the dinosaurs. The mammal, though small during the early Eocene, eventually rose above the birds and diversified.
During the early Eocene, most of the Earth was surrounded in dense forests. In these large areas of vegetation, mammals were unable to grow large due to the limited space. However, the descendants of the dinosaurs - the birds - were able to take full advantage of the small inhabitants and quickly became the top predators.
However, as time progressed, the forests began to disappear, allowing the mammals to get larger. As a result, by the late Eocene, mammals had diversified enough to dominate the globe. The predatory birds were eventually out-competed by the larger, mammalian predators that had just evolved.
Even in the waters, the mammals reigned supreme. The large whales had evolved and became the brutes of the ocean. Even the bit in-between - the mangrove swamps - were populated by incredibly bizarre mammals like Moeritherium and Arsinoitherium.
However, at the end of the Eocene, the world's climate went into turmoil. As the continents shifted and closed the ancient Tethys ocean, the polar icecaps began to freeze over. The resulting extinction event was the largest since the Cretaceous extinction.
Nigel Marven traveled to the Eocene and encountered an Arsinoitherium in the Egyptian mangrove swamps. Later, he dove with a Basilosaurus and subsequently traveled forward in time to the Pliocene epoch.
Edit