I believe the answer is: some of these fossilized remains can often be damaged and outdated, and can only provide insight as to the behaviors and activities that these fossils may have conducted.
Because of these limitation, it become really hard for researchers to point the exact time period of when the skeletons originally come from. Because of this, they use additional information from the depth of the ground or additional item that found near the skeleton to help them identify the time period.
