More from AICN:
Numerous rumors have indicated that the Skulls of the title are extraterrestrial in origin; I've pointedly been told that this was absolutely the case. Some online reports have said the "alien" creators of the Crystal Skulls would be suggested by temple statues that were pointedly designed to evoke the aliens in CLOSE ENCOUNTERS (i.e. a deliberate cross-connect between the two films); I know nothing about whether this particular conceit is in any way accurate.