There happens to be a ceramic skull in the drawing studio which appears in many of the recent poses. I decided to post a few examples of paintings which include the skull as part of the composition:
Frans Hals, Youth with a skull
St. Jerome in his Study, Durer, 1514
Thomas Smith's Self-Portrait
Philippe de Champaigne's
Vanitas (c.
1671) is reduced to three essentials:
Life,
Death, and
Time.
Cezanne, Paul, Pyramid of Skulls, c. 1901
The Ambassadors Hans Holbein the Younger,
1533, The skull, rendered in anamorphic perspective:

Vanitas, by Pieter Claesz, 1630
Fra Angelico, 1435
Pompeiian mosaic. I especially like the plumb bob in the lower left corner. Often, figure drawing students will make use of a plumb line to find the vertical axis through the
center of gravity of their subject and lay it down on paper as a
point of reference. The device used may be purpose-made plumb lines, or simply makeshift devices made from a piece of string and a weighted object, such as a
metal washer. This plumb line is important for lining up anatomical geometries and visualizing the subject's
center of balance.