The thing I like about Holbein is that while paints real people, he tends to do so with a tongue in cheek while doing it. He will throw other items into the painting as if to take the focus away from the main subject of the painting and adds stuff for the viewer to make inferences about. In the case of this particular painting, while the main focus are the ambassadors, he inserts a skull that is distorted at the bottom as if to throw off the balance of the painting. Either that or he used it to add balance.

I saw this at the National Gallery in London, and seeing it in person is a rewarding experience. The skull corrects itself as the viewer goes to either extreme of the painting.