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RE: Time Scale Divisions & Geology 101

Just to add to what Jerry said:

Something to appreciate about the Time Scale is that it wasn't assembled all
as a unit.  The original core of the Geologic Column were the Systems,
originally based on successions of strata in Europe (primarily the U.K.) and
correlated to the rest of the world by means of index fossils.  No one tried
to say that the fossils at the base of any particular unit represented some
major new adaptation in the history of life: indeed, the various terms like
Jurassic and Cambrian were developed in a pre-evolutionary scientific time.
Fossils were markers.  Incidentally, a System (and its subunits Series and
Stages) are units of rock, not units of time.

Later it was recognized that time could be divided up using the Geologic
Column as a basis.  So the Periods were recognized as the time in which the
various Systems were deposited, Epochs as the time in which each Series was
deposited, and Ages for Stages.

Later still the Periods were brigaded into formal Eras, and after that eras
into Eons.  Only these latter two large units were formally recognized on
the *importance* of major faunal changes (that is, that we were dealing with
important shifts in ecologies rather than simply replacing one arbitrary set
of markers with another set).

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-405-0796