Complex sentence: The boy alerted his father to the find, who could not believe what he saw--a hominid clavicle, and upon turning the block over: sticking out of the back of the rock was a mandible with a tooth, a canine, sticking out.
Split into simpler sentences: The boy alerted his father to the find, who could not believe what he saw--a hominid clavicle. Upon turning the block over, sticking out of the back of the rock was a mandible with a tooth, a canine, sticking out.