The researchers used a technique called laser ablation to examine teeth from four individuals collected from the Swartkrans site in South Africa which contain isotopes of carbon absorbed from food during each hominid's lifetime. Since trees, shrubs and bushes produce a different carbon isotope signal than grasses and sedges, the team was able to determine that Paranthropus was often dramatically altering its diet over periods ranging from months to years.
"This is the first study to paint a portrait of an early hominid eating its way across a varied landscape," Sponheimer said. "None of us involved in the study dreamed Paranthropus would have had such a variable diet over thousands of years, much less in just a few months time."
Which is interesting. The preferred hypothesis is that early hominids outcompeted the Australopithicenes because they had tools to more efficiently extract resources. I had thought meat played a role as well, which ties into the tool use (hunting, butchering).
This related story on the same research seems to indicate some C4 plants in the diet as well but it seems more plausible they were getting it from whatever C4 plants were around. So, eh.