David Campbell wrote,
> In the eastern U.S., most of the late Pleistocene (ice age) fossil shells
are of living species.  Earlier in the Pleistocene, and especially before
the mid-Pliocene (ca. 3.5 million), the proportion of extinct species is
much higher.

Ah yes. In 1833, Charles Lyell divided the Tertiary Period into "Newer
Pliocene, Older Pliocene, Miocene, and Eocene" epochs based on percentage of
living species among the fossil mollusks. The Newer Pliocene is now called
the Pleistocene and is reassigned to the Quaternary Period. Lyell's Older
Pliocene is our Pliocene. His Miocene and Eocene are now our Miocene,
Oligocene, Eocene, and Paleocene.

And the latest twist is to substitute Neogene and Paleogene Periods for the
familiar Tertiary Period, but that hasn't caught on universally. Neogene =
Pliocene + Miocene, and Paleogene = Oligocene + Eocene + Paleocene. These
days, the epoch and period boundaries are all carved in stone: Instead of
calculating molluscan percentages, committees of stratigraphers select exact
levels at particular outcrops of strata ("stratotypes"). The outcrops are
chosen for their excellence of fossil diversity and preservation; usually
marine sections are preferred. For the Tertiary, microfossils are more
important to stratigraphers than mollusks these days. Sic transit
gloriamundi!

Andrew K. Rindsberg
Geological Survey of Alabama

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