Marsupials and monotremes lack the corpus callosum that facilities communication between left and right brain in placental mammal, so it is as if they already h

Marsupials and monotremes lack the corpus callosum that facilities communication between left and right brain in placental mammal, so it is as if they already had this surgery. https://en.wikipedia.org/wiki/Corpus_callosum https://en.wikipedia.org/wiki/Corpus_callosum This seems to put into question their statement about "all mammals", but I haven't read the article, so maybe they actually mean placental mammals only. Monotreme mammals don't have nipples.