Just a symbol! It was a common motif for viewers to remember that all must die. Check out @perishthethought’s helpful link:)

A little context on the Mary Magdalene + skull pairing if anyone’s interested:

Another key part of the Mary Magdalene story was the development of a series of artworks now known as ‘Penitent Magdalens’. In this type of portrayal, the saint has seen the error of her ways and is depicted often as a hermit in a landscape or wilderness, lamenting her formerly sinful lifestyle.