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The Grande Coupure, at the end of the Eocene epoch, was one of the most drastic faunal turnovers of the Cenozoic era. During and (much more so) after this time the world started to drastically cool as tropical forests gave way to grasslands and the once diverse Paleogene megafauna started to decline. Many well-known beasts from brontotheres to creodonts started to decline. This was a new age. The age of the pecorans and carnivorans; and the age of grass, and ice, feeding on the loss of the great Eocene forests, and with them all of their flora and fauna.
Then the world started to become an icehouse. The ice caps started to develop. The ice age began. An unimportant primate from the African savannah became a skilled hunter and conquered the world. And it was driven by the savannahs where it evolved, a result of the sudden domination of the grasslands in the Miocene.
But this timeline is different. Grass never fully diversified, inhibiting the development of the ruminants, with the Pecora, one of the most successful recent groups of mammals, on the brink of extinction. The split happened in the Priabonian, 35 million years ago, meaning that more of the unfamiliar Eocene fauna was kept, and the Neogene fauna was almost nonexistent. The world stayed in greenhouse climate for the rest of the Cenozoic. And thus, an entire new Earth was created.
Note: These periods go by HE timeline, and do not reflect changes in the fauna and climate of this timeline. Geologic divisions are not necessarily different from HE in this scenario.
Speaking as the split happened in the Late Eocene, there wasn't much to Eocene aside from HE material. However, in HE, the Eocene–Oligocene boundary, known as the Grande Coupure, was by far the largest extinction event in the Cenozoic. However, here the Eocene went on smoothly on smoothly into the Oligocene, without any faunal turnovers. In fact, the lack of disruption is what sets this apart from HE, and as such there really isn't much notable in terms of faunal changes. Instead, it's more of an extension of HE Eocene.
During the Oligocene, the perissodactyls stayed in domination, and the tylopods did well. The Cetancodontamorpha (pan group Whippomorpha) were represented by the Anthracotheriidae; entelodonts; and the cetaceans, of which the archaeocetes were on the decline. Meanwhile, the Desmostylia (which evolved slightly earlier than shown in the fossil record) and Sirenia were doing well in the seas, but still kept some basal forms. All primate groups survived, with the exception of the sparse North American primates, which went extinct after being nearly decimated by the drastic climatic change of the Eocene-Oligocene boundary.
The climate of the late Oligocene was warmer than in HE's Oligocene, but was the coldest and driest time period after the split. The cause for this was directly astronomical, so no further cooling progressed. Instead, for other astronomical causes, the Eocene conditions were about to return, because the Earth was heading for a thermal maximum.
The first landmark of the Miocene was that the first sea monsters to originate since Basilosaurus had gone extinct. And these were no mammals. These were true sea monsters, the first marine reptiles to evolve since the time of the mosasaurs, and these were long gone. No sea monsters of the time, and in fact the days of large predatory whales were gone; rivaled the Carcharosuchia.
The Miocene reverted to a warmer climate, and although most groups survived some were wiped out entirely. Rainforests quickly spread throughout the world, and Eocene temperatures returned. The Miocene Thermal Maximum occurred 15 Ma before present, and it's effects were quite surprising.
The Xiphodontidae, always on the brink of extinction, quickly died out. Equidae was severly damaged, with only one species surviving. The Pyrotheria went extinct in South America, already on the decline and in competition with Astrapotheria, which had a nearly identical niche. Entelodonts, already in competition with so many other carnivores, were all wiped out except for an isolated pocket in the heart of North America, where the carnivorans and creodonts were few. And barely hanging on, the archaeocetes went extinct, with competition from Odontoceti, Mysticeti, and a whole other variety of sea creatures.
Several groups, such as the few Ruminantia, which hadn't been competing with other ungulates; Creodonta, which held firm carnivorous niches; Tylopoda, which were major herbivores; and Perissodactyla, which also held a similar herbivorous niche were not affected by the extinction. But three groups thrived in the climate, and they were to dominate for the rest of the timeline. These were the primates, because of the forests they do so well in; the carnivorans, because they could grab niches from the entelodonts; and the crocodilians, which not only took over the niches of large carnivores on every continent, but Carcharosuchia had a second burst of evolution.
During this time, the sea level rose suddenly, and the strange and endemic fauna of Zealandia was gone. The volcanic activity resulting in the creation of New Zealand from the once large continent of Zealandia did indeed happen, but the islands sunk nevertheless, and with them the sphenodonts, and the ratites of the archipelago which apparently escaped the fossil record.
Now, the niches were set for the rest of the timeline, and evolution could continue uninterrupted – well, with the exception of the rise of the now fierce crocodilians, and the resulting possible extinction of many mammalian carnivores.
After the Miocene Thermal Maximum, which brought the extinction of several groups, and the decline of many others, albeit helping at least three groups; the temperature began to cool again. By the early Pliocene, the temperatures were back to those of the time of the split. It was still a hothouse world, but not like the Miocene or Eocene.
By the Pliocene, all groups had their modern niches, with only a few changes from after the Miocene Thermal Maximum. During this period, the main changes in niche were the spreading tylopods, especially the Anoplotheriidae; the increasing prominence of the Carnivora, namely the saber-toothed Nimravidae; and the increase in sebecosuchian dominance in South America. Certain groups, mostly the Equidae, severely declined during this period, and had been declining since the Miocene. Still, Equidae managed to survive.
As for other reptiles, by this time, Crocodylomorpha was probably the most successful group of reptiles, and continued to be; but Squamata and Choristodera were doing well in the hot climate, which especially helped the Choristodera, which went extinct due to cold temperatures in our timeline. The Sphenodontia had already gone extinct, as Zealandia sank completely.
Then, during the late Pliocene, the temperature began to rise again, perhaps due to increased volcanic activity. This was only a relatively minor fluctuation, though, and did not lead to any sort of thermal maximum.
The Quaternary was little more than an extension of the Pliocene in this timeline, with previous trends continuing. In South America, the sebecosuchians, phorusrhacids, and sparassodonts filled different carnivorous niches, making this a land of very fierce carnivores. With a higher sea level, the Isthmus of Panama never formed, and South America stayed as isolated as Australia, with their unique sparassodonts and xenarthrans. Although cooler than the late Pliocene, the Quaternary was no glaciation, or anything near a glaciation. A short temperature dip, lasting about 50,000 years, occurred during the mid-Pleistocene, about 1.5 million years ago.
The creodonts started to decline, and the saber-toothed nimravids showed they had a clear advantage over almost all other mammalian carnivores. Although carnivorans encompassed everything from non-canine canids to bear dogs, the creodonts still held most niches not held by the carnivorans. Despite a carnivoran domination on most continents, South America had few placentals, only xenarthrans, rodents, Meridiungulata, and Platyrrhini, with the carnivorans and creodonts being absent; and Africa was relatively conservative, with the creodonts doing very well here. As for primates, strange forms like adapids remained dominant, and the Sivaladapidae gave rise to the sivalads, which have the intelligence of apes.
And there it is – what happened to the Earth in a timeline where a small fluctuation in temperature stopped the grasslands from dominating, and ultimately changed the course of evolution.
|Mammalia||Eutheria||Laurasiatheria||Artiodactyla||Cetancodontamorpha • Dichobunoidea • Ruminantia • Tylopoda|
|Ferae||Carnivora • Creodonta • Pholidota|
|Perissodactyla||Chalicotherioidea • Hippomorpha • Rhinocerotoidea • Tapiridae|
|Euarchonta||Dermoptera • Haplorhini • Scandentia • Strepsirrhini|
|Afrotheria • Leptictida • Xenarthra|
|Sauropsida||Aves||Neoaves||Passeriformes • Phorusrhacidae|
|Choristodera • Crocodylomorpha • Lepidosauria • Testudines|
|Habitats||India • South America|