Final answer:
The temporalis muscle is the muscle of mastication that attaches to the sagittal crest in P. robustus and P. boisei, necessary for their ability to chew tough plant materials.
Step-by-step explanation:
The muscle of mastication that attaches to the sagittal crest in P. robustus and P. boisei is the temporalis muscle. In these robust australopithecines, the sagittal crest serves as an anchor point for these large jaw muscles, which were essential for chewing tough plant materials. The masseter muscles, another group of mastication muscles, are responsible for elevating the mandible to close the mouth. However, it is specifically the temporalis muscle that is associated with the sagittal crest in these hominid species.