Will not contest that the initial energy carried by the projectile at the
moment of impact is sufficient to pierce steel or bone. However, the more
relevant question (and I genuinely don't know the answer to this), is how
rapidly that energy is dissipated as the bolt travels through the skull, and
whether it would lose enough energy to not pierce the bone on the other
side. A projectile fired from a firearm, on the other hand, isn't dragging
the extra weight and friction of the shaft and fletching that a crossbow
bolt would, and would (it would seem to me) be more likely to pass through
the skull rather than lodging in it.

I will end this sanguinary comment with the notation that there is a wealth
of forensic literature that illustrates that even un-jacketed, soft lead
firearms ammunition can and routinely does make clean entry wounds in human
crania, which the limited amount of information and presented in the article
and accompanying picture could be considered consistent with. Science
Daily's article on the find is more expansive, and indicates that the defect
show in the image is in fact the entry wound.
http://www.sciencedaily.com/releases/2007/06/070620081734.htm

But, more to the point, our colleagues in Peru and specialists in forensic
science classed it as a gunshot wound.

CGCD
-- 
Department of Anthropology
College of William and Mary
Williamsburg, VA 23187