The Silurian of
Ohio was a time of warm, shallow seas in which marine
invertebrate life blossomed. The diverse faunas of the
Ordovician expanded in the Silurian without significant
appearances of new groups. The Silurian Period is
sometimes referred to as the Age of Corals because these
organisms built extensive reefs, which were a haven for
other invertebrates. Late in the period, a momentous event
occurred when land plants first appeared. Although they
were small and probably had little immediate impact on the
landscape, they were the harbingers of a new way of life
for many groups of organisms that would conquer the land
in later geologic periods. Jawless fishes, which include
several groups collectively called agnathans, diversified
during the Silurian, thus setting the stage for the rise
of other vertebrates, including many varieties of jawed
fishes and the first amphibians, in the succeeding
Ohio's record of
Silurian life is not one that adequately reflects the real
abundance and diversity of this period. Many Silurian
units are dolomites in which recrystallization has
destroyed most fossils. Some units, particularly those in
the Salina Group, were deposited on tidal flats or in
hypersaline waters in which few organisms could prosper.
Despite these shortcomings, there are still many excellent
Silurian fossils known from the state. A number of these
species are described and illustrated in Division of
Geological Survey Bulletin 70, Fossils of Ohio.
Corals and echinoderms, particularly crinoids, are
probably the two most abundant fossil groups in Silurian
rocks in Ohio. Remains of these organisms were important
contributors to rock volume in some units. It is estimated
that the Brassfield Formation is made up mostly of
echinoderm remains, although it is rare to find whole
specimens. Another important reef-building group during
the Silurian was the stromatoporoids, organisms of rather
uncertain affinity that were shaped like heads of cabbage.
Some of the
best-known Silurian fossils from Ohio include a very large
pelecypod, Megalomoidea canadensis (formerly called
Megalomus), and the brachiopods Pentamerus
and Trimerella. These fossils are found mostly as
internal molds. Pentamerus specimens sometimes
occur in "nests," that is, clusters of specimens still in
life position, and are popularly referred to as "turtle
heads" or "fossil pig's feet."
Underside of a slab of Silurian dolomite from the
Dayton area containing multiple specimens of the
brachiopod Pentamerus oblongusin life position.
University of Dayton collection (from Feldmann and
The Brassfield Formation is one of the most productive
units for Silurian fossils in Ohio; however, many Silurian
units locally have produced excellent fossils. Although
Silurian dioramas typically are characterized by
illustrations or models of eurypterid arthropods, these
scorpionlike fossils are exceedingly rare in Ohio.
As with many human
observations, perspective is critical. The fossil
collector may perceive the Silurian of Ohio to be a
monotonous sequence of poorly exposed and poorly
fossiliferous rocks, completely eclipsed by the diversity,
abundance, and preservation of fossils in the underlying
Ordovician rocks and overlying Devonian rocks, but
occasionally providing an exquisite trilobite or crinoid.
The mineral collector perceives them as the host for
exquisite specimens that rival those from anywhere in the
world. From an economic perspective, these rocks rank as
one of the most important in the state, providing an
abundance of critical commodities that contribute to our
well-being. To the tourist or casual observer, the cliffs,
caves, and waterfalls developed in Silurian rocks present
great beauty. All would agree, however, that Ohio would be
diminished greatly if Silurian rocks were not present in
I thank Survey
geologists Mac Swinford and Glenn Larsen for their
perspectives on the Silurian and particularly Glenn's
guidance through the maze of Silurian stratigraphy.
George, and Stieglitz, R. D., 1978, The occurrence of
sulfide and associated minerals in Ohio: Ohio Division of
Geological Survey Report of Investigations 104, 11 p.
Carlson, E. H., 1991, Minerals of Ohio: Ohio
Division of Geological Survey Bulletin 69, 155 p.
Carman, J. E., 1946, The geologic interpretation of
scenic features in Ohio: Ohio Journal of Science, v. 46,
p. 241-283 (reprinted as Division of Geological Survey
Reprint Series 3).
Clifford, M. J., 1973, Silurian rock salt of Ohio:
Ohio Division of Geological Survey Report of
Investigations 90, 42 p.
Feldmann, R. M., and Hackathorn, Merrianne, eds., 1996,
Fossils of Ohio: Ohio Division of Geological Survey
Bulletin 70, 577 p.
Janssens, Adriaan, 1977, Silurian rocks in the
subsurface of northwestern Ohio: Ohio Division of
Geological Survey Report of Investigations 100, 96 p.
Larsen, G. E., 1991, Development of Silurian and
Devonian lithostratigraphic nomenclature, central-western
and northwestern Ohio: Ohio Division of Geological Survey
Open-File Report 91-1, chart.
Ohio Division of Geological Survey, Report on Ohio
mineral industries: published annually.
Verber, J. L., and Stansbery, D. H., 1953, Caves in
the Lake Erie islands: Ohio Journal of Science, v. 53, p.
White, G. W., 1926, The limestone caves and caverns
of Ohio: Ohio Journal of Science, v. 26, p. 73-116.
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