South America is a land of isolation, a land of biodiversity, a land where evolution runs wild. The rainforests such as the Amazon which we are familiar with today cover the entire continent. The Pampas which cover much of the continent today are entirely gone, with grass having gone extinct in all parts of the world but an isolated pocket in the Himalayas. But it wasn't always this way. During the Oligocene, temperatures were just barely low enough to support a desert plain habitat in the southern tip of Patagonia. This was at latitudes of up to 65° South, just barely disconnected to Antarctica, but in this hothouse world, even a desert plain was a characteristic of a cold spell.
The Miocene Thermal Maximum wiped out all animals of the plains. These included running terror birds and desert sebecians. While grass had already been wiped out on the continent, even desert shrub couldn't exist. A small set of islands off the coast of Antarctica was all that remained of this habitat, but the few animals that managed to get there soon diversified. The Patagonian–Antarctic archipelago soon became a hot spot for endemism.
Meanwhile, in mainland South America, things got a whole lot weirder. The major herbivores of this continent were pampatheres and ground sloths. Insectivores included a whole variety of ameridelphians, not just opossum that we are familiar with, but also rodent-like forms, and even small carnivores. Rodents themselves became giant, including almost ungulate-like capybara relatives, at least in terms of niche.
But the true ungulates here are in fact the endemic Meridiungulata, three surviving orders of large ungulates, including tapir-like forms (tapirs are nonexistent due to a land bridge to North America never forming) of the order Astrapotheria; large-bodied forms which could be described as South American pachyderms, in the order Notoungulata; and weird creatures of the order Litopterna which cannot be described as anything other than similar to their iconic member, Macrauchenia.
Anteaters diversified into far more than four species, as in our timeline; they are composed of at least fifteen species in this timeline. Basal platyrrhines, almost prosimian in appearance, dominate the dense rainforest trees, but their niche is shared with convergently evolved tree sloths.
Meanwhile, the large predators are made up of some of the strangest and notorious creatures endemic to South America. Of the three amniote classes, each one has a representative group. The reptilian predators here are Sebecia, entirely terrestrial crocodilian relatives which first evolved during the Late Cretaceous, 90 million years ago. The avian predators are the Phorusrhacidae, better known as terror birds. Here, though, the cursorial forms went extinct, and the survivors are 1.5 meter tall robust forms. The mammalians are the sparassodonts, and although the saber-toothed Thylacosmilidae are absent; huge 130 kilogram predators of the family Proborhyaenidae are extant, analogous to bears, if you will.
And thus, this stable yet strange ecosystem still exists, and without contact from the rest of the world, South America might as well be a unique planet.
|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|