A sundial notes the days that passed as the skull lived and died, and the phrase written on the cheek bones was taken directly from Masaccio's Holy Trinity, a fresco in Santa Maria Novella in Florence. Trinity is one of the first artworks to incorporate true, scientific one-point perspective (it was completed in the early 1400s). The latin inscription above the skeleton (and now on the bison) reads "What you are, I once was. What I am, you also will be." A good reminder, don't you think?