In 1948, at Swartkrans Cave, in about the same vicinity as Kromdraai, Broom and South African palaeontologist John Talbot Robinson described "P. crassidens" based on a subadult jaw, SK 6. He believed later "Paranthropus" were morphologically distinct from earlier "Paranthropus" in the cave—that is, the Swartkrans "Paranthropus" were reproductively isolated from Kromdraai "Paranthropus" and the former eventually speciated. By 1988, several specimens from Swartkrans had been placed into "P. crassidens". However, this has since been synonymised with "P. robustus" as the two populations do not seem to be very distinct.