The Triumvirate

While during the Lentocene the dominant fauna comprised only one single group (ducktails), the Cambiocene hosts a richer and more "disputed" biotic association., composed by three main clades: ground tyrants, rostrids and rodents.
This bizarre triumvirate was able to maintain itself for millions of years thanks to a large niche partition: large ground tyrants occupy the northern portion of Antarctica, where warm temperatures favour the presence of this biodiverse group.
The colder southern portion, too harsh for most of the large ground tyrants, was instead monopolized by non-scansorial rostrids, which possess various primordial characteristics that enable them to better resist the local climate (a great example of exaptation).
Big rodents, on the other side, are equally widespread in the entire continent, filling the niche gaps left by the two groups of birds.
We don't know how much this condition will perdure in the future: after a mass extinction, an overabbundance of shorth-living clades try to fill vacant niches, eventually dying off when more efficient taxa emerge or because of climate change.
Time will be the only answer to this question.