"Don't Cry For Me, Anteater . . . "

In the previous post, TamanduaGirl, commented that it looked like the tear duct in the anteater skulls figured was outside the orbit, unlike other placental mamals.

Actually the tear duct is on the inner part of the orbit (where the orbit bone slopes towards the eye). It's difficult to see on a two-dimensional image, but I've enhanced the image below.


In each image, the tear duct is actually the smaller hole (ringed in the lower image) - the larger hole is a nerve opening. In each instance, the duct is positioned just slightly on the inner side of the orbit. The limit of the orbit is indicated with a dotted line in the lower image.

In the image linked to appears to be a cast, and, as such, only shows the larger nerve opening and not the tear duct. Small openings are often missing from casts.

More on monotreme-marsupial-placental evolution soon.

And now you don't (again)

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).



Photo credit
Echidna - University of Washington
Anteater - Natural History Museum, London

Now you see it, now you don't

While we are talking about marsupials (see previous post), it's a good time to show how to tell the difference between a marsupial and placental mammal skull - you know for those life or death, save the world, situations when you need to be able to tell the difference. There's nothing like being prepared.

There are a few differences between the marsupial and placental skulls, however the easiest by far is the nasolacrimal duct - that's the tear duct to you and me.















Above are the skulls of a Thylacoleo (left) and a Thylacene (right). The tear duct is clearly visible outside of the eye socket, sitting on the cheek.


Now the skull on the right is a dog skull. See the tear duct?

Oh no, that's right, you can't, 'cos it's not visible. In placentals, the tear duct sits inside the eye socket, and so you would have to view the skull from above and look down into the eye socket to see it. Whereas in marsupials, the duct sits outside the eye socket and is clearly visible when viewing the skull from the front or side.

So the next time you have to make a life or death identification, you can spot the marsupial with confidence.