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Bone Beds of Middle Devonian Columbus and Delaware
Limestone in Ohio By Marc Behrendt
The Columbus and Delaware Limestones are marine formations exposed from central Ohio
north to the Lake Erie islands. The Columbus Limestone, the stratigraphically lower, thus
older of the two formations, is considered to be of Late Ulsterian Age. The formation is
over 100 feet thick, and has been subdivided into three members - the Bellepoint,
Eversole, and Delhi. The overlying Delaware Limestone, of early Erian age, is 36 feet
thick and has not been differentiated into members.
The Columbus Ls. is typically more fossiliferous than the Delaware Ls. Brachiopods and
corals abound, but gastropods, bryozoans, tentaculitids, ostracods, and occasionally
crinoids, blastoids, and trilobites can be found. Fossil concentration seems to peak at
certain horizons, forming the basis for further stratigraphic subdivision. "Zone
A" occurs at the bottom of the Columbus Ls., and sequentially lettered zones continue
to Zone M at the top of the Delaware Ls. (See chart 1)
Located within the Columbus and Delaware Limestones are four sequentially numbered
layers known as the bone beds (see Chart 1). Bone beds are concentrations of
disarticulated fish skeletons that include not only fragmented bones, but spines, scales,
armor plates, and teeth as well. (See photo 2a and 2b) Taxonomically the bone beds are
rather diverse, with heterostracan and osteostracan agnathids, arthrodires, ptyctodontids,
acanthodian, selachian and crossopterygian fish all contributing to the deposit. Along
with fish remains are other less abundant fossils, including conodonts, foraminifera,
ostracods, scolecodonts, sponge spicules and lycopod spore exines. Note that all these
constituents are microfossils - that is, under 5 mm in size. There are occasional crinoid
fragments, brachiopods and corals as well, but these are typically isolated in small
areas, commonly referred to as pockets.
The first and second bone beds occur in the Columbus Ls. and are of stratigraphic
importance - the second bed marks the boundary between the Columbus and Delaware
Limestones. The third and fourth bone beds are found in the Delaware Ls. The beds range in
thickness from a couple inches to 16 inches. Found along with fossil material are small
rounded limestone pebbles, rounded and angular quartz grains, chert residue, and grains of
zircon, hornblende, garnet, and tourmaline.
The origin of the bone beds has been a cause of debate for over a century. The first
hypothesis proposed to explain the unusually high concentration of fish remains suggested
a catastrophic mass death. However there are many arguments that weaken this theory. Among
them are 1) there are no articulated skeletons or complete bones, 2) the remains appear to
be worn to some degree, 3) there is no evidence of hydrogen sulfide in the rock, which
might be expected to occur under the hypothesized mass-death scenario. Each argument
against a catastrophic event has been refuted by contradictory evidence, but the
catastrophic hypothesis has not carried much weight in most modern interpretations.
But there is something a little fishy about the horizons in which these bone beds
occur. The Columbus and Delaware Limestones are marine in origin. The fish remains
primarily represent fresh water fish, as well as some that lived in brackish, estuarine,
and marine environments! Currently, the most accepted theory explaining this quandary is
that previously broken up fish material was periodically transported from freshwater lakes
and rivers into the near-shore marine environment. The carcasses decomposed, broke apart,
and were carried as fragments or decomposing pieces into shallow water flats and beaches.
They did not live in the same environment in which they were preserved.
The third bone bed offers an interesting insight to the mystery. The limestone under
the fish bed is covered by mudcracks, where the water briefly receded and the seabottom
dried up. The bone fragments and bed lie over and within the mud cracks. Obviously, the
fish were not living in areas exposed to air, so the most likely way this could occur is
to have fish remains wash or blow over dried mud. In the case of the third bone bed, very
few fine granule fragments are found, thus it is theorized wind, or perhaps water currents
carried the larger material into place, while the fine material was transported away.
Some localities do not have the mud cracks, but have sharply delineated layers. These
probably formed underwater as currents moved over the surface. Other bone beds gradually
phase in - these localities may have been under shallow water. The simple answer is that
these bone beds are not simple. There were probably a variety of processes at work to form
these beds, even if the products we see in the fossil record appear similar at first
glance.
Even if the formation of the bone beds remains somewhat of a mystery, the taxonomy of
the fossils they contain is pretty well understood. The specific appearance of these fish
is sometimes difficult to visualize when studying the tiny bone, spine, scale, plate, and
tooth fragments. An occasional piece is found exceeding 5 mm. (See photo 3) But most of
the material is fragmented and quite small. When walking in a quarry, with all the rock
exposed, the fragments cannot easily be seen. The rocks appear to have brown stains or
streaks running through them. On closer examination while on hand and knees, it is obvious
that the brown stains are all actually bone fragments, spines, and teeth. Dermal scales
are sometimes pyritized or rarely opalized, and will sparkle in direct sunlight. From my
observations, the scales and ossicles are too small to identify as such without a lens.
Chart 4 lists the species found in the limestone. Again, most of these fish have never
been found even partially articulated, our imagination must decide how these aquatic fauna
appeared during life. These bone beds will never displace such places as Green River as
the USA fish fossil Mecca. However, to sit and study the layers under a lens opens a brand
new realm to fossil fish collecting.
Acknowledgement: I thank Shanan Peters for his suggestions and excellent editing.
Bibliography:
Bentley, R. 1988. Fish Fauna from the Second Middle Devonian Bone Bed of Central Ohio,
Senior Thesis - Ohio State University, 27 pages.
Newberry, J.S. 1873. Ohio Devonian system. Ohio Geological Survey vol. 1, part II,
pages 263 - 268
Stauffer, C.R. 1909. The Middle Devonian of Ohio. Geological Survey of Ohio, Bulletin
10, 204 pages.
Wells, John W. 1944. Fish Remains from the Middle Devonian Bone Beds of the Cincinnati
Arch Region. Palaeontographica Americana, vol. III, no. 16, 62 pages.
Wells, J.W. 1944. Middle Devonian Bone Beds of Ohio. Geological Society of America
Bulletin, vol. 55, pages 273 - 302.
Wells, J.W. Date unknown. Provisional Paleoecological Analysis of the Devonian Rocks of
the Columbus Region. Geological Survey of Ohio file article.
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permissions to reproduce these articles, please contact:
Marc Behrendt
421 South Columbus Street
Somerset, Ohio, 43783 USA
(740) 743-2818
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