Filling the gap

While in the northern forests the first large niches were rapidly monopolized by ground tyrants and big ragos, the large niches of the cold and snowy forests of the south remained mostly vacant for at least half million years before being exploited by the survivors of the END.
Armadrails spread like a wildfire in almost the entire continent, but without changing their overall body structure, mostly because of the rise of a new large rostrid species.
Twice the size of its ancestor, the neodungeater (Invictus novirostrida) is the true ruler ot the southern portion of Antarctica. While not as large as the singer of the Resurgence, neodungeaters counterbalances with several significant adaptations: without a size limitation imposed by an arboreal lifestyle, neodungeaters have evolved a larger and wider wingpouch, where eggs and chicks can be transported and protected from predators and cold temperatures (probably one of the key of success of this species in the southern forests, an example of exaptation); the feet orientation has also changed, with now three digits pointing forward, less useful to move on branches but surely great at walking and running on the ground.
With few or no herbivores to deal with, neodungeaters will flourish alongside ground tyrants and another group of vertebrates, forming the so-called antarctic triumvirate.