You should have called the police because it could have been a crime scene. The skull could have belonged to a missing person.
I am sure the Mexican family that was left behind by now has a pretty good idea that the missing person is not coming back. The skull was found at least 20 years ago and had been there for many years from the look of it. There were only a few teeth left and no dental work on the remaining teeth. There was no bullet hole or crack in the skull. It was not on a road just a trail through the brush.
The person could have died in the summer from the 110 degree heat. Rattlesnakes are deadly. Another wetback could have killed him to steal his food.
This is pretty common but still they come because Mexico offers them nothing.