Nutcracker Man was a grazer?

This week's PNAS has a paper by Cerling et al. on the putative diet of Paranthropus boisei, previously known as Australopithecus boisei, previously known as Zinjanthropus the "Nutcracker Man." The name came from the skull's robust maxilla, which was suggested to be used for, well, cracking nuts. Cerling et al.'s stable isotope analysis shows that boisei instead fed on C4 plants (grasses and sedges). From the abstract:
Its diet included more C4 biomass than any other hominin studied to date, including its congener Paranthropus robustus from South Africa. These results, coupled with recent evidence from dental microwear, may indicate that the remarkable craniodental morphology of this taxon represents an adaptation for processing large quantities of low-quality vegetation rather than hard objects.
Cerling et al. 2011. Diet of Paranthropus boisei in the early Pleistocene of East Africa. PNAS 108:9337-9341.

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