Architectural cow skulls in Boston and Virginia

Adam at Universal Hub wrote about some cool architectural grotesques around Boston today. One that caught my eye was the cow skull on the Boston 5 Cents Savings Bank. As I wrote in the comments, this isn’t a gruesome ornament but a classical one, which also appears in Jefferson’s architecture at the University of Virginia:

The drawing that Jefferson prepared for Pavilion II carried the notation that the order was based on the Ionic of the Temple of the Fortuna Virilis in Rome, which was illustrated in plates in The Architecture of A. Palladio; in Four Books. One of the distinguishing features of this order is that the volutes of the capitals at the outer corners of the columns are positioned diagonally at the ends of the portico. The Temple of Fortuna Virilis has a frieze of ox skulls, putti, ribbons, and garlands festooned with fruit motifs, which were also used in Pavilion II.

I wonder what the thematic significance of the ox skull was originally.