Current thinking among most evolutionary biologists is that Homo Sapiens did not descend from Cro-Magnons. Rather, Cro-Magnons were a collateral branch that died out. Even if we assume that humans descended from Cro-Magnons, your reasoning is not valid. It is not a measure of the Cro-Magnon's thinking capacity. They lacked our ability to visualize, to plan ahead, etc.
Fossils are actually quite rare.
When animals die, they are usually consumed by their internal bacteria and by external scavenging animals. Essentially all bodies will rot and disappear. Only under very unusual circumstances will a body be preserved long enough to be converted into a fossil. An index fossil is generally the fossil of a species that is believed to have emerged at a certain time, and which became extinct at a more recent, known time. Thus the rock that it is imbedded in can be roughly dated if the fossil is present.