|SK 48 - P. robustus from Swartkrans, South Africa|
The researchers looked at the inside of traits (the anterior pillars on either side of the nose) that look similar on the outside in two South African species (A. africanus and its neighbor robustus) and determined that they're different. They conclude that they're different because they develop differently due to genetic differences and so they are not the same structure in the two homs.
Without that link to africanus anymore, robustus does not need to be reconstructed as part of a South African lineage of Australopithecus. Instead, it can be appreciated for all its obvious, massively toothy similarities to another well-known robust species, boisei from East Africa.
Before this study, based heavily on the anterior pillar link, it looked like robustus and boisei may have evolved in parallel or convergently from local africanus (South) and afarensis (East) respectively. This is why they were kept in Australopithecus but were always labeled as "robust australopiths." This study indicates that they did not evolve in parallel and supports the lumping of all robusts into one genus with a single (monophyletic) origin: Paranthropus.
But to even more strongly support that robustus and boisei evolved from a common ancestor--to support one genus for the "robust australopiths"--they needed to look at the internal structure of that same region of the face in some boiseis and girlseis and show us that the similarities there with robustus beat out the differences between africanus and robustus.
And they did that...how satisfying!
And guess what they found? The boiseis look like the robustuses on the inside even though the outside is different.
Teeth and jaws are no longer parallels between East and South, instead anterior pillars are a parallel within the South. The case is strengthened for lumping australopiths with the huge teeth and jaws into their own genus with a single origin. Where that origin took place is another question for another time...