Earth's Eocene Epoch

The Eocene Epoch followed the Paleocene and saw the emergence of the first modern families of mammals. It lasted from around 55 million years ago until 34 million years ago.

At the beginning of the Eocene Epoch, the continents of Australia and Antarctica were still connected, causing ocean currents to mix warm and cold water into a more homogenous temperate ocean temperature. However, as the period wore on, the split between these two continents caused ocean currents to change and create cold water channels alongside Antarctica which would help lead to its eventual glaciation. Mountain building was common during the Eocene and it is during this period of the Cenozoic that India collided with Asia, pushing up the Himalayas. At the beginning of the epoch, Eocene climate is marked by a very rapid spike in global temperatures in an event known as the Paleocene-Eocene Thermal Maximum. This event caused many forests to become more tropical as Earth's climate became more homogenously warm and wet. This warming and change of flora had an effect on many groups of specialized mammals that had thrived in certain types of trees, leading to their extinction. However, more adaptable mammals were able to seize upon the opportunity the new climate presented and began to diversify into a number of new shapes, sizes, and niches.

The moist, warm conditions caused forests to proliferate across the planet. Tropical and temperate rainforests dominated Earth and created entirely new and diverse ecosystems. As the Eocene Epoch wore on, global cooling took place and thinned out many of the forests that had sprung up in the first ten million years of the period. This allowed newly evolved grasses to spread in within the gaps and form vast prairies for grazing animals. In addition, as the Earth's climate became more seasonal toward the end of the Eocene, deciduous trees that shed their foliage in cold conditions began to thrive over many of the trees suited for warm climes.

It is during the Eocene that mammals became much more specialized for their environments. Teeth and limbs evolved to cope with the rigors of ecological niches and foodstuffs upon which the mammals fed. It is during the Eocene that the first ancestors of many modern mammalian groups emerged, such as the first primitive horses, rodents, and bats. Mammalian brain-to-body ratio was still quite small, as it had been with most dinosaurs during the early Mesozoic Era, especially in the large herbivores.

Large herbivorous mammals began to proliferate, especially on the open grasslands of the later Eocene Epoch. Without the tight spaces of forests to restrict overall growth, mammals were able to balloon to sizes that even the largest carnivores could not touch. Gigantic, rhino-like brontotheres were among the largest of these grazers, reaching eight feet in height (two or three times the size of modern rhinos). It is also during the Eocene that the first marine mammals appear. These mammals evolved from land-dwellers and soon their legs began to evolve into specialized flippers for better adaptation in water which contained plentiful caches of food. The first proto-whales called Archeocetes became the top predators of the Eocene oceans, replacing the giant marine reptiles of the Mesozoic as the titans of the seas. The largest of these early whales, basilosaurus, reached lengths of up to 25 meters.

Birds continued to diversify and the flightless diatriyaformes continued to be the top predators in many portions of the world. However, as carnivorous mammals became more efficient and widespread, the heyday of these six hundred pound birds effectively drew to a close. Unique flora and fauna began to develop across South America and African because the two continents had not yet come into contact with others and were surrounded by ocean on all sides.

The Eocene Epoch came to an end with an extinction event which is estimated to have affected as many as 50-90% of certain groups of invertebrates, mammals, and plants. Marine ecosystems were especially affected by this event which might have included a global cooling trend. As the planet began to cool down, ocean currents would have been severely affected and so would the distribution of plankton and fish, forming the basis for food chains. On land, the disappearance of many tropical rainforests with the cooling could have accounted for the extinction of many groups of primates, hoofed mammals, and rodents specializing for life in the dense canopy.