After the dinosaurs, many niches were left in the ecosystems. These niches would soon be filled by mammals. The dinosaurs were somewhat gone because they had evolved into birds. Mammals evolved into the largest animals on earth since the dinosaurs.
Large carnivores returned to the forests about 5 million years after the K-T Extinction. These large carnivores were Terror Birds, which were massive flightless birds that pursued prey through the jungles that covered the earth. These birds were about as tall as a man and weighed about the same. Gastornis and Diatryma preyed on horses like Propalaeotherium and rodents like Leptictidium. By about 45mya, a new group of mammalian carnivores had evolved: the Creodonts and Mesonychids. Creodonts were dog-like or cat-like carnivores with hooves and they were very unspecialized. They became the largest group of carnivores in Africa, Europe, Asia, and North America. In jungles, large Entelodons, Sarkastodons, Andrewsarchus, and Hyaenodons flourished. By 35mya, the Creodonts had vanished because massive climate changes killed most of them, and the others were beaten out of the food chain by the new carnivores: dogs and cats. Miacis and Amphicyon were small carnivores that had sat in the shadow of the large Mesonychids and Creodonts for 5 million years. Then, large fields replaced the dense jungles that covered the world. Dogs were quicker than Creodonts and the dogs worked together to bring down prey like deer, camels, and horses. Some dogs, like Borophagus and Epicyon, became massive enough to hunt the megafauna, such as elephants, brontotheres, and rhinos. Cats evolved into Sabertooths and other large cats and used their large teeth to crush an animal's trachea. In South America, Smilodon became the largest carnivore and eventually beat out the Terror Birds in the food chain. Marsupial carnivores evolved on Australia and South America and took over. Not all carnivores were large though. Mustelids like weasels and otters became scavengers. Potamotherium was a primitive Mustelid that scavenged in the fields and forests. Eventually, bears like Chapalmalania and Agriotherium evolved and became large carnivores in Europe and North America. Large carnivores were rulers of the earth until they were replaced by an even bigger carnivore: humans.
Since the earth was covered in lush jungles, herbivores were small creatures that did not graze; instead, they scurried through the undergrowth and usually fed at night (when the predators were asleep). Horses were one of the earliest mammals to evolve during the Tertiary. Like all other mammals, they had toes not hooves. They were about three feet long and scurried around with the early rodents. At about 35mya, the forests were disappearing and being replaced by open fields which allowed the horses to grow hooves and become taller. Pliohippus was a grazer instead of a rooter like Hyracotherium and Palaeotherium. Also, elephants like Eritherium, hippos like Prorastomus, rhinos like Indricotherium, brontotheres like Brontotherium, pantodonts like Barylambda, chalicotheres like Ancylotherium, giraffes like Sivatherium, gazelles like Hoplitomeryx, and cows like Tsaidamotherium dominated the plains. By the Miocene epoch, massive mammalian herbivores rose. At the end of the Tertiary, mammals had to adapt to the continually cooling climates. Xenarthrans, anteaters and sloths, evolved into human-sized herbivores. Monotremes, egg-laying mammals, set up on Australia. Elephants, rhinos, horses, bears, dogs, cats, etc. had to evolve heavier coats to cope with the new temperatures. Gomphotheres evolved into Mastodons, and Mammoths had evolved as the largest herbivores on land. Eventually, most of the magnificent animals of the Quaternary were wiped out by humans.
From 65mya to 45mya, the sharks had ruled the oceans again as the largest carnivores. They were the undisputed masters until Basilosaurus came along. Basilosaurus was a carnivorous whale that had evolved from dog-like semi-aquatic animals. Pakicetus and Ambulocetus spent some of their time on land and some of their time in water about 55mya. Then they evolved flippers and moved entirely to the water. In the water seals, like Ontocetus, and dolphins, Aetiocetus, were also evolving. Massive manatees like Steller's Sea Cow lived in the coastal waters around continents. Many of these animals were hunted to extinction by humans. Interestingly the large brains of land mammals carried over to sea creatures, particularly dolphins.
Humans evolved from small monkeys and aye-ayes from the early Tertiary (ex. Eosimias and Godinotia). By the late Tertiary, apes like Oreopithecus and Apidium had evolved. Very early forms of humans, Sahelanthropus, arose in Asia and Africa. Australopithecus, one of the first humans, evolved in Africa. Some species learned how to use tools and Homo erectus stood upright. Human's forehead began to protrude as they evolved. Homo sapiens appeared around 200,000 years ago and are still around today. This species hunted many of the large prehistoric animals to extinction.