The earliest of the three is a winged death's head, with blank eyes and a grinning visage. Earlier versions are quite ornate, but as time passes, they become less elaborate. Sometime during the eighteenth century -- the time varies according to location -- the grim death's head designs are replaced, more or less quickly, by winged cherubs. This design also goes through a gradual simplification of form with time. By the late 1700's or early 1800's, again depending on where you are observing, the cherubs are replaced by stones decorated with a willow tree overhanging a pedestaled urn.
Pay special attention to the graph of the popularity of each motif and the slideshow of example gravestones. (thx, peterme)
Update: A reader writes in:
In regards to your post on Gravestone Motif Analysis, I think that the most important text on the subject is still Graven Images: New England Stonecarving and its Symbols, 1650-1815 by Allen Ludwig. It was originally published in 1966, before the article that you linked to. However, Wesleyan University Press published a new edition in 2000 to help meet the rising demands of Material Culture Studies courses. Lots of helpful images and histograms showing the changing patterns of gravestones over that time period.
I *love* that the collective readership of this site knows what the definitive text on New England gravestone carving is. (big thx, fletcher)