Understanding the ages of related fossil species helps scientists piece together the evolutionary history of a group of organisms.
For example, based on the primate fossil record, scientists know that living primates evolved from fossil primates and that this evolutionary history took tens of millions of years.
See also index fossil, microfossil, trace fossil.fossilize verb The evidence in rock of the presence of a plant or an animal from an earlier geological period.
Fossils are formed when minerals in groundwater replace materials in bones and tissue, creating a replica in stone of the original organism or of their tracks.
Without a doubt, we see sudden changes in dominant fossil shapes as we ascend the geologic column, but this is not macroevolution.
The species changes touted by punctuated equilibrium that we do see are either common variation of individual offspring, or adaptation of a population to differing conditions.
Punctuated equilibrium doesn't even address the larger changes needed for meaningful evolution.
The primate fossil record for this crucial transitional period leading to australopithecines is still scanty and somewhat confusing.
However, by about 4.2 million years ago, unquestionable australopithecines were present.