Following the great KT extinction event, the great winners were the mammals. Without the dominant dinosaurs, and thus with the size and night constraint lifted, they started becoming larger and filling all niches.
In Laurasia, the condylarth-like mammals diversified:
hooved runnersdigging omnivores
Oxygen level reached 23%. Europe was now separate from N America.
Primates insectivores
Leptictids
Other mammals survived the extinction, but they were outcompeted by the rodents.
Similarly, the deep-sea ray-finned fishes survived the extinction and radiated out to fill in the niches left open, especially the perch-like fish with their prominent dorsal fins:
The only surviving dinosaurs were the (mostly flightless) ornithine birds, with three main lines.
Geese (e.g. swans), migratory Ducks
The most successful of these early fowl radiated out:
50-25Mya Eocene and Oligocene
When India made first contact with Asia, the perissodactyls diversified:
The artiodactyls diversified even more:
agile forest herbivores, able to ferment, ruminate, then digest leaves.
Protocetus
Whales permanently at sea
The African mammals grew in size:
The South American mammals grew in isolation:
Notoungulates
The Australian (+New Guinea) marsupials also diversified in isolation:
The birds continued to diversify:
The mesonychids fluorished, but in the end, the creodonts replaced them and the flightless carnivore birds in Laurasia. They lived in Africa, some as big as bears hunting arsinoitheres.
Rodents diversified and spread, replacing the multituberculates.
Similarly looking mammals are unrelated:
With time, mammals evolved to their largest sizes.
25-5Mya Miocene
The leaf eating monkeys diversified, especially in Africa; many like to sit.
Grass savannas replaced forest as the climate started to become drier. Herbivores evolved into new grazers.
In Asia
In N America, the horse and the llama evolved.
The miacids evolved into felids and canids, displacing the creodonts. The cunning canids diversified, first in Asia, replacing the creodonts.
The whales split into two main lineages:
In S. America, the ant-eaters and litopterns diversified.
Late Miocene
The climate became much drier and colder (av. temp. ), as the Himalayan erosion removed CO2 from the atmosphere. The advanced grasses discovered a novel C4 synthesis pathway, more efficient at capturing CO2, so were able to survive a dry season; they proliferated, forming savannas.
In N. America, the marsupials, taeniodonts, pantodonts, dinocerates and titanotheres did not survive the change in habitat; only a few species remained. Similarly their predators, the creodonts were replaced by new canids and felids.
New herbivores, such as the horses, rhinos, llamas, antelopes, oreodonts, and hippos, had to have specialized digestive systems, with symbiotic cellulose-digesting bacteria, to deal with the nutrient-poor grasses.
Africa joined Eurasia, allowing many animals to cross over and adapt from Eurasia to Africa:
Gazelles
...or the other way round:
... but , the Mediterranean filled up and isolated Africa. The African elephants and apes diverged from their Asian counterparts.
social omnivores
5Mya Ice Ages — Pleistocene
Around , the climate worldwide became cooler as the Antarctic, then N. America and Europe/Siberia, accumulated ice; several species adapted, some by growing larger.
N. America joined Eurasia, as sea-levels fell: many animals passed from America to Eurasia...
or vice versa, from Eurasia to America:
Similarly the waters around Australia were sufficiently low that bats, birds and rodents were able to pass from Asia.
N. America joined S. America: various animals diffused from either continent, tapirs, llamas, cougars, leopards became jaguars, peccaries; but in the end, the large S. American animals (giant ground sloth, glyptodon, terror birds) became extinct.
The Ice Age, and interglacials, proved too much for many animals: sabre-toothed cats, most elephants (deinotheres, gomphotheres, mammoths, and mastodons), mid-range whales, several fresh-water fish; but they provided opportunities for others e.g., big cats, pigs, rats and mice, the gigantic whales.