(note: this was not actually written by me; a friend of mine on another board wrote the original version of this. it was for the same basic concept, though)
(btw, i AM actually working on this project myself, though very gradually)
Paleocene & Eocene Epochs (61.7 – 33.9 million years ago)
Because there was no asteroid collision with the Earth at in the Maastrichtian stage, the Cretaceous period continued for another 3.8 million years. The Cenozoic Era is instead marked at the beginning of the Selandian stage, and the Paleocene epoch began. Tetrapod fauna remained mostly unchanged during this Paleocene, since there were no major events for another 6 million years or so.
The end of the Paleocene and the first major event of the Cenozoic began approximately 55.8 million years ago, called the Paleocene-Eocene Thermal Maximum. During this period of about twenty thousand years, global temperatures rose dramatically, thus producing a tropical climate all over the world. As rainforests expanded, dinosaurs were forced to adapt to the new conditions, couple with the collision of India with Asia, causing a faunal interchange.
Consequently, some clades suffered while others died out. Pachycephalosaurs and stegosaurs are unknown in the fossil record any younger than the Paleocene. Sauropods, despite being browsers, were somehow unable to adapt well to rainforest habitats due to their toeless feet, and as a result were restricted to polar forests in the Eocene. Ceratopsids were thrown into decline and would eventually die out in the Oligocene, though the hornless ceratopsians became more diverse. The only large herbivores to experience total success were the hadrosaurs, which were present on every continent except for Africa. Hypsilophodonts seem to have been more common in the southern continents, while thescelosaurs expanded shortly into Asia. Ankylosaurs were hardly affected by the thermal maximum.
The collision of Europe with Asia allowed the large descendants of Pyroraptor to invade the other northern landmasses. This, coupled with the invasion of abelisaurs from India, cast the dominant tyrannosaurs, which were less adapted to rainforests than their competitors, into decline. Only smaller, Dilong-like tyrannosaurs survived. Africa came to be dominated by noasaurs, while Australia was inhabited by late living allosaurs. Ornithomimids, troodonts, and alvarezsaurs diversified while therizinosaurs came to be restricted to somewhat small tropical forms and huge polar ones. Aside from the larger dromaeosaurs, there were also smaller ones descended from Rahonavis that diversified as tree climbers. Birds initially suffered from the thermal maximum, but recovered in massive amounts of enantiornithes, apsaraviformes, paleognaths, and fowl. Ichthyornithes and pseudodontorns diversified following the extinction of pteranodonts, while hesperornithes were reduced to smaller freshwater forms, with many still capable of flight.
Aside from the aforementioned extinct pteranodonts, the larger azhdarchids were restricted to polar forests and coastal zones as a result of the grasslands they needed to take off being replaced by dense forests. In these regions, they produced a new lineage of pterosaurs, whose ancestor was Eunemicolopterus, a surprisingly small animal. Anurognathids and ctenochasmatoids also reappeared in the fossil record.
Mammals didn’t change much, aside from the appearance of primates, cimolestans, mesonychians, and hyaenodonts. Volaticotheres reappeared in the fossil record as bat-like flying mammals, while the first members of Archaeoceti appeared in Asia. Champsosaurs remained common in America, though they were reduced to small, lizard-like forms in Eurasia. Crocodiles were common all over the world in both salt and freshwater forms. Mosasaurs were common in all oceans while plesiosaurs were restricted to a few marine forms and many freshwater ones.
Oligocene Epoch (33.9 – 23.03 million years ago)
Following the climatic chaos at the end of the Eocene, the global rainforests began to evacuate into the tropics, though the climate was still fairly warmer than it would be in the present day and grasslands had yet to significantly expand. Temperate and mixed forests dominated the landscape in lieu of the rainforests. The climate of the Oligocene most closely resembled that of the Cretaceous.
With the end of the rainforests came the return of sauropods and therizinosaurs to their role as dominant herbivores in the northern continents. Hadrosaurs remained reasonably common, with lambeosaurines dominating in Asia while hadrosaurines ruled in the Americas. Hadrosaurs of uncertain origin appeared in South America and Australia. Protoceratopsids began to produce large forms as the ceratopsids died out completely, though they rarely reached the same scale as their horned cousins. Hypsilophodonts experienced a downfall as competition with ornithomimids, avimimids, protoceratopsids, and even mammals strained their presence in the north and even wiped them out in Laurasia, and only continued their existence in the south of the planet. As during the thermal maximum, ankylosaurs were barely affected. In Africa, a lineage of ornithischians descended from heterodontosaurs and destined for greatness achieved dominance.
The Eurasian abelisaurs began to lose their dominance to dromaeosaurs and tyrannosaurs, with the latter regaining their large sizes as the rainforests that they couldn’t adapt to disappeared. Oviraptorosaurs as a whole remained mostly unchanged since the Cretaceous, though they reached Africa, Australia, and South America by swimming due to semi-aquatic forms that fed on mollusks evolving. Troodontids remained mostly unchanged. In South America, the fauna became fairly strange: because of a brief land bridge in the Cretaceous, troodonts, dromaeosaurs, microraptors, and oviraptorosaurs of North American stock were all present on the island continent, much to the dismay of the native abelisaurs and unenlagiines. Africa had its own native populations of unenlagiines and noasaurs.
The dawn of the Oligocene marked the return of the azhdarchids to success. In Africa, flightless forms evolved and took over niches that were occupied by small theropods elsewhere in the world. Smaller pterosaurs, including the eunemilopterids, continued to rule the skies, doing nothing new.
Afrothere mammals experienced a radiation of new forms, ranging from the small tenrecs to large, capybara-like mammals, and cat-sized hyaenodont predators appeared in Eurasia and Africa. Multituberculates occurred in beaver-like forms, and mesonychians became fox-like opportunists. Metatherians took on mustelid- and opossum-like forms practically everywhere, while cimolestan diversity diminished somewhat. Volaticotheres stayed virtually the same, and aquatic mammals, better known as whales, occurred in marine habitats for the first time.
The plesiosaurs return to prominence while the mosasaurs fall into decline. Hesperornithes experience a magnificent comeback, while auk- and loon-like penguins have the southern hermisphere as their stronghold. Dyrosaurid crocodiles die out, but champsosaurs come to the sea for the first time, and have potential to evolve into filter feeding animals in the future. Sphenodonts are a prominent group of reptiles, present in both Australia and in South America.
Miocene Epoch (23.03 – 5.332 million years ago)
When Africa collided with Eurasia, total chaos ensued. The last of the hadrosaurs and many ceratopsians in Eurasia were replaced by the African heterodontosaurs. Therizinosaurs and sauroods moved into Africa while flightless pterosaurs invaded Eurasia, taking on the niches of ornithomimids and large troodontids, which by now are all but gone from the continent. However, some smaller troodonts remained while others lost their niches to unenlagiines. Abelisaurs came to be represented by one or two gigantic genera, with their smaller niches having gone to dromaeosaurs, noasaurs, and tyrannosaurs. The large protoceratopsids produced a lineage of grazing herbivores, a remarkable development because of their browsing ancestry. Ankylosaurs came to be restricted to a single genus in Eurasia.
However, hadrosaurs still dominated South America and Australia, which by now have become vast grassland. The last abelisaurs died out in South America, allowing unenlagiines, dromaeosaurines, and oviraptorosaurs to become the dominant predators while troodontids diversified over omnivore niches. Microraptorines thrived in North American forests, but succumbed to competition from arboreal mammals elsewhere in the world. Meridiungulates took over the ecological roles of small ornithischians, and sauropods died out in South America. Australia hardly changed during this time period.
Pliocene Epoch (5.332 million years ago to present day)
As the global climate got colder, Antarctica froze over for the first time and tundras appeared in the Arctic, severely affecting the fauna of Eurasia: choristoderes, monitor lizards, several bird clades, small pterosaurs, sauropods, ankylosaurs, and primates vanished from Europe. In contrast, eastern Asia wasn’t as affected, and many of those kinds of animals continued to exist in such regions. Metatherian mammals occurred in shrew- and mustelid-like forms while cimolestans came to be represented by treeshew-like analogues to squirrels. Primates all over the world are lemur-like, except, surprisingly, in Madagascar, which is instead home to diverse plesiadaptiformes. Hyaenodonts and mesonychians became the foxes, cats, and civets of the world.
The last abelisaurs finally died out around this time, unable to adapt to the changing climate, while noasaurs became restricted to the tropics and a few exceptionally large genera in Central Asia. Dromaeosaurs and tyrannosaurs reigned supreme in colder regions, and continue to do so, while some of them ventured into Africa as the equivalents of jackal- or hyena-like generalists and a leopard-like form. Sauropods and ceratopsians exist mostly in Africa and southern Eurasia, though the occasional dryings of the Mediterranean Sea caused some of these animals to become stranded on small islands, thus producing dwarf forms of them. Heterodontosaurs, however, have been able to establish themselves in the colder zones. Avimimids completely took over the omnivorous niches previously occupied by ornithomimids and flightless pterosaurs over the older oviraptorosaurs.
The flying pterosaurs, when not gigantic azhdarchids soaring over all other landmasses, are smaller eunemicolopterids occuring in the world's rainforests as small frugivores/omnivores (perhaps some ground hornbill like form as well?) or even smaller anurognathids flying around at dusk or dawn as our world's nightjars do (should ctenochasmatoids survive the world's cooling or not?).
The seas see the demise of mosasaurs, now restricted to the tropics, gharials and choristoderes (the later two reduced to freshwater forms in Asia and America respectively), while sea birds and polycotylids managed to adapt just fine to colder waters; so did aquatic mammals, now bigger than ever. South America is hitten by an asteroid, as its fauna was still recovering when the Isthmus of Panama was formed. Some clades like troodontids and meridiungulates not only survive but achieve success (partially due to the cold climate, at least the later have less competion on the colder zones of Laurasia), but others decline; native dromeosaurs and hadrosaurs are examples of that, both reduced so smaller species or to large elephantine ones respectively. In Australia fauna hasn't changed as much except for the extinction of its native top predator (some sort of allosaurid theropod), which was replaced by unenlagiines and crocodilians. Hadrosaurs, basal ceratopsians and large mammals (not to mention flightless birds) are among its denizens.
