SKULL

While all artifacts were made by the zuish people, the skull artifact is the most substantial proof that the visitors were in contact with them, since neither Zu people or people from Gomez's village have a third eye (or such a big nose!).

When Gomez finds the skull, Dot says something that confirms many of our theories:

Unbelievable! ... So, the legends were true. They *did* come here...

I still don't know why the visitors would leave such a skull back, or if the Zu sculpted it (or if it is actually a Zu skull after all), but rumors between fans is that the skull may be used to solve the Black Monolith puzzle.

I will attach some images about it because it sure looks interesting, but there is no proof yet on how the skull might have been useful to either the Zu or to us players.

This theory was imagined by several players, but we found the image here.

Could it be an owl's skull maybe? The proportions are odd, but it is not impossible.

How about "memento mori"? It comes from latin and means "remember [that you have] to die". If it is the case, the skull is just a message to remember our own finitude (or to the Zuish people that eventually they were all going to pass), and no answer to any riddle in the game. Still, no definitive answer here.