South America used to basically be a giant version of Australia--a land of strange animals that had been going their own way since the Cretaceous, more or less. But weirder--a land of grazing placental mammals and large predatory marsupials(?) and land crocodiles.
Even in the Late Cretaceous, southern South America is weird compared to northern South America (which is a lot more like Africa, India, and Madagascar in terms of its vertebrates).
Where's southern South America start for paleontology?
In this case, central Argentina on south, at least for the later Late Cretaceous. It shows up as a distinct pollen zone, too (see figure).
That would kind of lines up with the Salta Rift and related structures.
> a land of grazing placental mammals and large predatory marsupials(?) and land crocodiles ... and in the Cenozoic, the Phorusrhacids, "terror birds", huge flightless biped predators
Those are a bit less unusual, Laurasia had its own large terror birds that may or may not have been related to South America.
Titanis actually colonizing North America and refilling the niche vacated by things like Bathornis is interesting though. And predates other "Great Faunal Interchange" species fairly significantly....
Then something happened that the geologists and the paleontologists/biologists don't remotely agree on. 🙂