wich appear, grow and dessapear within a period, are excellent
for knowing the stratigrafic age of stratums where they
are. In fact, Geology is based upon fossils.
Biostratigrafy has a basic unit named "zone".
The "zone" is un group of rocky stratums characterized
by the presence of a fossil' species or a characteristic
group of species.
Fossils with short living-time and wide geografical distribution
are useful for a good biostratigrafical division, and
they are known as "guide-fossils".
Several fossil groups have been used for the biostratigrafical
subdivisions, coinciding with the periods where the group
is more successful, that is, when the species is represented
by a lot of individuals and besides they evolve fastly,
allowing us to make subdivisions and correlations at world
In subdivisions of Paleozoic, Trilobites and Graptolites
have been used; inthe Mesozoic, Ammonites; in the Cenozoic
and Quaternary, the Lamellibranchs.
Besides, from the Cretacic on the microfosils get big
Nevertheless, the Vertebrates are almost not used , because
their conservation is not good.