Well, hair came before modern earbones and live birth (as shown by the megaconus that I mentioned earlier.) Milk did come before live birth, but nipples are still up for grabs (so to speak.) Monotremes don’t have them, instead just leaking milk out pores which the puggles then lap up. But marsupials do have them, and it is hard to imagine the infant marsupial lifestyle without having a nipple to clamp onto, so you probably never had live birth without nipples. But it is possible that nipples could have come around while mammals were still egg-laying.
(As long as we are talking about naughty bits, monotremes and marsupials have bifurcated wangs, which is the primitive condition. If there were no monotremes or marsupials around today, we’d probably never have conciser that the earliest mammals were anything other than monowanged.)
So you have an issue with faux-Latiniatization of plurals but not the use of the term “wang”?
My rule of thumb: in non-formal writing, if any word sounds funny getting the “ii” suffix for pluralization, that justifies the words getting the “ii” suffix for pluralization.
True, but also consider that within mammals, there are traits that are not universal for example the way that the teeth are formed, and whether they continue to grow or not - so there’s a bit of circularity involved in the definition. If all mammals happened to have blue ears, the group might be called Blue-Ears rather than Milk-makers
There is no such thing as an -ii suffix for pluralization. The number of Latin words that use -ii as a plural marker is exactly zero. The general rule (to which, admittedly, “octopus” and “platypus” are exceptions) is that a word ending in -us has the -us changed to -i. The only reason I can see for the confusion is “radii”, but that still follows the same rule as everything else: Take “radius”, chop off the -us, and replace it with -i. “Penii”, if it existed, would be the plural of “penius”.
And while we’re at it on Greek-Latin confusion, I think my “tetrafurcated” should actually have been “quadrafurcated”.