(http://www.histarch.illinois.edu/plymouth/deathshead.html)
After The Great Awakening took place in early America, citizen's philosophies began to change along with political ideals. Americans began to imitate Roman and Greek styles in their architecture and, surprisingly, on their gravestones.
The urn was an Imperial Roman device used to contain ashes. It symbolized the immortality of the soul. The willow was an ancient mourning symbol. The combination of these two symbols showed an increase in sentimentality towards death. The urn and willow symbol continued to be used on gravestones up until the 1930s (eyesofscyld).
During the late 1700s there was also a change in the entire shape of the gravestone. The earlier gravestones had rounded upper corners, while the later ones were square. This change was also because of the neoclassical era inspired by the ancient Romans.
The picture above is an example of the urn and willow symbol. Although this gravestone has rounded corners, it has a more intricate edge than the earlier stones.
Another change which occurs at the end of the 16th century is the statement carved on the gravestone. Earlier stones read "Here lies" or "Here lies the body of", and the later stones read "In memory of" or "Sacred to the memory of". This allows the stone to be placed without having to have the body buried beneath it. This would make sense if, say, someone died at sea, or if the body could not be salvaged.
Sources:
Deetz, Patricia Scott and Fennell, Christopher. Death's Head, Cherub, Urn and Willow. Dec 14 2007. Retrieved Sept 29 2013. Web.
eyesofscyld. New England Gravestone Symbolism. YouTube. Mar 3 2013. Web. Retrieved Sept 8, 2013.
Iconography of the Gravestones. City of Boston. Web. Retrieved Sept 22, 2013.