I thought it was "australopithecusian". But that is just picking at nits.
Different critter, and wrong demonym. It's "australopithecine", I believe.
P.S.: Here you go: https://www.merriam-webster.com/dictionary/pithecanthropus
I was particularly leaning into the "extinct primates intermediate between man and the anthropoid apes" part of the definition.
