In a previous post I talked about the position of the tear duct in placental mammals verses marsupials, and how the duct was outside the orbit in marsupials, but inside the orbit in placentals. Someone asked what was the condition in monotremes, which was a good question. I didn't know the answer and had to wait until a recent trip to Adelaide to be able to access a monotreme skull to find out.
The answer is that, in monotremes, the duct is inside the orbit, just like placentals.
The skull of an Echidna (above) shows the duct to be inside the orbit. In an actual specimen, the duct is placed in the bone surface sloping into the orbit (not clear on a two-dimensional photo). In a marsupial the duct would be clearly visible outside the orbit.
So there you go, only marsupials have the duct visible outside the orbit.
So the question now is, how do you tell a placental anteater skull from an Echidna skull?
The answer is that the Echidna is far more bird-like than the anteater, and the cranium tends to slope sharply downward at the front in the Echidna, but tends to be much flatter in the anteater (see below).
Echidna - University of Washington
Anteater - Natural History Museum, London