If I were the one to first pluralize Zinjanthropus, I'd agree, it shouldn't be in Wordie. However, it was used by Austin Bukenya (1972) in The People's Bachelor, p. 144, "The SSP men were fully armed and dressed for action, and they looked as ugly as Zinjanthropuses...." Zinjanthropi has five bona fide G-hits (the rest are mostly compilations of words), so it probably is the preferred plural.
Not sure you can pluralize this just to make it panvocalic (and if you could, wouldn't it be zinjanthropi?), but australopithecine works. Same hominid, basically. Turns out neither name stuck--zinjanthropus boisei is now classified as paranthropus boisei.