Tuesday, May 19, 2009

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.


  1. I can't wait to be able to use that. Probably have to wait for thirty years or so.

  2. What about placentals and marsupials with tear duct placement in a nebulous area?

  3. I'm not aware of any placental with the tear duct outside of the eye socket. The marsupial skull should have the duct visible outside of the socket or on its margin. The placental should have the duct inside the socket.

    There are other characters that can be used, including dentition and cranial size, presence of a cranial crest, but these require a knowledge of teeth, or other skulls for comparison.

  4. That is, close to the "edge". Just inside or just outside the orbit.

  5. If it is inside the orbit, even just inside, it's a placental. If it's on the outside (visible when looking face to face with the skull, even on the edge, it's a marsupial.

  6. What about monotremes?

  7. wow - i have yet to take mammalogy (and im not a paleontologist, just a groupie biologist with metatherian leanings) so i think this is quick hint when i dont want to refer to dental fomulas and all the psychotic obsessiveness therein.


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