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Trilobites of the Cincinnati Region
By Marc Behrendt
During the
late Ordovician Period 450 million years ago, central North America was fully
submerged under a warm tropical sea. The Cincinnati region hosted a vibrant and
well populated ecosystem. Sediment from the distant eastern landmasses
periodically supplied layer after layer of mud and carbonate, creating the
countless layers of sedimentary rock so easily seen in every area roadcut.
Storms added sea bottom debris in some layers that are commonly called “storm
hash.” Fragile fossils, such as trilobites and crinoids, tended to be
best preserved in the “cleaner” shale.
Later in
time, the earth’s crust
pushed upwards, creating a dome with the Cincinnati region at the top.
Erosion slowly removed the overlaying rocks, and what remains is the oldest
surface bedrock in the Ohio/Indiana/Kentucky region. This dome of rock is known
as the Cincinnati Arch.
The
accompanying chart illustrates two stratigraphy naming systems. Though separate
by intent, both systems are used to identify the matrix, formations and
associated fossils. The United States Geological
Survey (USGS) wanted to standardize the stratigraphy nomenclature, so they
created yet another naming system! Dr. Steven Holland PhD, at the University of
Georgia, has been effectively working to tie all the naming systems together and
better define the stratigraphy.
|
U.S.A. Series |
U.S.A. Stages |
Subdivisions based on lithology and
fossils |
Subdivisions based on Lithology |
|
Cincinnatian |
Richmondian |
Elkhorn |
Preacherville member of
Drakes Fm. |
Elkhorn Fm. |
Whitewater Fm. |
|
White-
water |
Upper Whitewater |
Upper Whitewater Member |
|
Saluda |
Bull Fork Formation |
Saluda Member |
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Lower Whitewater |
Lower Whitewater Member |
|
Liberty |
Tanner Creek Formation |
Dillsboro Formation |
|
Waynes-
ville |
Blanchester |
|
Clarksville |
|
Fort Ancient |
|
Arnheim |
Oregonia |
|
Sunset |
|
Maysvillian |
McMillan |
Mount Auburn |
Grant Lake Formation |
|
Corryville |
|
Bellevue |
|
Fair-
view |
Fairmount |
Fairview Formation |
|
Mount Hope |
|
Edenian |
Latonia |
McMicken |
Kope Formation |
Eden Shale |
|
Southgate |
|
Economy |
Stratigraphic names used in the Ohio,
Kentucky and Indiana Region
The total array of fossils found in the Cincinnati region
would require a full-sized book to list and describe them all. This chapter
will review the trilobites found in the region.
By far
the most common and well known of the Cincinnati trilobites is the
Flexicalymene. Flexicalymene granulosa is found in the Kope
Formation. It is the least common of the Flexi's. F. granulosa is
identified by the tiny bumpy pustules that cover the entire shell. The shell
feels very rough when prepared, in contrast to the “smooth” shell of the other
Flexicalymene species.
In the layers above the Kope,
Flexicalymene is represented by F. meeki and F. retrorsa.
The “butter-shale” layers are famous for the Flexicalymene beds, where
many specimens may be found in a small area. The butter shale is a common name
for the very smooth, clean, finely grained shale found in the Richmond. When
wet, this shale quicklyturns to clay and mud. At Caesar Creek State Park north
of Cincinnati, Ohio, the “melting” of this shale layer is responsible for
millions of fossils scattered along the surface of the emergency spillway.
Surface collecting is allowed, after signing in at the park office. However
where legal, intact butter shale must be dug out from the outer weathered,
crumbled layers. It is this unweathered material that yields the wonderful
prone, large 3D specimens we often see displayed or for sale.
F. meeki
and F. retrorsa are differentiated by the genal
area and the anterior “lip.” F. meeki possess small genal spines and
have a smaller lip, while F. retrorsa has no genal spines and exhibit a
larger lip. There has been much discussion about whether these differences are
significant enough to designate these as separate species. Added to the
argument is the subspecies F. retrorsa minuens, which have been
identified because of their consistanly small size. Presently, thistiny species
is being studied, the results may finally declare whether this is a separate
species or just a subgroup.
Often confused as a Flexicalymene, Platycoryphe
christyi is a rare gem not properly identified until the “Flexicalymene”
is examined closely or prepared. The cephalon is similar, but Platycoryphe’s
glabella is larger, smoother and less defined. By closely observing the
Flexicalymene specimens in other folks’ collections and for sale at shows, I
have found 2
Platycoryphe labeled as Flexi’s! Platycoryphe is found in the
Whitewater Formation.
The second most common trilobite species found in the Cincinnatian is
Isotelus. Though most specimens found are small, some have reached 18
inches in length! Tom Johnson of Peebles, Ohio is renowned for his skill
finding these Ordovician monsters. There are 3 species recognized from the
Cincinnati region: Isotelus maximus, I. brachycephalus, and
I. gigas.
Isotelus maximus, the Ohio State Fossil, is found throughout the
Cincinnati strata. This trilobite has a rounded cephalon and pygidium, and long
straight genal spines. I. brachycephalus appears very much like
I. maximus, however the genal spines are curved. There has been
debate whether these are the same species. I. gigas has a triangular
cephalon and pygidium, and has no or diminutive genal spines.
Another common trilobite, though often overlooked, is Meadowtownella
crosotus, previously known as Primaspis crosotus. Rarely exceeding 15
mm (.5 inch), this tiny trilobite is a spiny beauty. A member of
Odontopleuridae, each of M. crosotus’ thoracic segments tapers into a
spine. The pygidium also exhibits several sets of spines. Quite often, this
trilobite is found within an indentation on bryozoa. And from this author’s
experience, if one is found, there are usually others on the same matrix. Meadowtownella
is found in the Kope Formation.
Similar and related to Meadowtownella, but much rarer is Acidaspis
cincinnatiensis. Acidaspis is considerably larger, just as spiny, and is
easily identified by a large triangular occipital spine, pointing rearwards from
the back of the cephalon. Acidaspis is found in the Kope Formation and up
through the Maysville.
A very rare member of the Cinci trilobite fauna is Achatella carlyensis.
An early dalmanitid, Achatella is one of only two regionaltrilobites with
schizochroal eyes, the large faceted eyes possessed by the suborder Phacopina.
The pygidium is triangular and has no spines. Achatella is found in the
Fairview Fm.
Tricopelta breviceps
(previously known as Chasmops breviceps) is a beautiful dalmanitid
species, also with the “Phacops”-like eyes. Usually thickly shelled and
well preserved, the large eyes are the first feature one notices. The species
is not large, generally measuring an inch or less. It appears similar to
Achatella, but the eyes are larger and the specimen is usually 3
dimensional, whereas the Achatella is usually flattened with lower, less
elevated eyes. Tricopelta is found in the Whitewater and Liberty
Formations.
Cryptolithus tessellatus,
commonly called lace collar trilobites, are found in the Kope Formation.
Usually only the head shield is found; complete specimens are spectacular and
rare. Complete specimens will be found in shale, partials are common fossils in
the Kope limestone. The cephalon has 3-4 rows of pores arranged in rows along
the wide border that ends in 2 long, narrow genal spines. The glabella is
usually domed and highly 3-D. If the thorax and pygidium are present, they are
narrower than the cephalon. The thorax has 6 segments; the pygidium is small and
triangular.
There are 2 cheiurids found in the Cincinnati
region. Less rare is Ceraurus milleranus, a large and beautiful
trilobite. Most often found in fragments, a complete Ceraurus can
measure 2 inches or more in length. The shell is typically thin and is covered
with small pustules. The genal spines are long and robust; the pygidium ends in
2 strong spines. Ceraurus
fragments can be found from the Kope up into the Whitewater Formations.
Complete specimens are very rare and are found in shale.
Exceptionally rare is the ultimate
collector’s trophy - Ceraurinus icarus. This incredibly rare trilobite
tends to have a thick, 3-D shell that is usually caramel colored.
It can measure 2 or more inches long. Ceraurinus are found in the
Richmondian formations.
Amphilichas shideleri
is a highly sought after and very rare trilobite
found primarily in Grant Lake Fm. Amphilichas has the typical lichid trilobite
appearance, with the triangular, well pustulated cephalon, large eyes, and
notched pygidium.
Triathrus eatoni can be found in the lower layers of the
Kope Formation. Triathrus tended to live in an environment unsuitable
for other trilobites with depressed oxygen levels, so one must look in barren
shale at the base of the Kope formation. Recently a pocket of complete 1 inch
long Triathrus were found, but in my experience most often specimens
measure less than .5 inches. Usually the specimens are missing the cephalons,
but complete Triathrus can be found with perseverance. The collector
must examine the shale carefully, as the specimens tend to be faint, very flat,
and extremely thin-shelled.
Proetidella parviusculus
and Proetus spurlocki are very rare trilobites. They have
been found in association with Triathrus in the clean, black, oxygen
deficient shale of the Lower Kope. The proetids have a cephalon with long genal
spines and a large glabella, which together are shaped like a quarter moon. The
thorax and pygidium are oval shaped.
The Cincinnati area is so remarkably
fossiliferous it is difficult to find a piece of shale or limestone without some
sort of fossil on it. Yet ironically, public collecting sites are tough to come
by. Driving the interstate highways, one will see the most amazing roadcut
geology, tons and tons of unexamined rock loaded with fossils of all sorts - and
so it shall remain. It is understandably illegal to collect along interstate
highways.
However with some perseverance and a full tank
of gas, sites along state and local roadways can be found. As stated earlier,
Caesar Creek State Park north of Cincinnati is a popular collecting locality.
After signing in at the park office, collectors
head for the emergency spillway further down the road. On either side of
the road is an enormous open area that collectors may surface collect. Digging
is not allowed at this locality. Huston Woods State Park is another popular
collecting site. Some local collectors have gone so far as to purchase
property and create their very own private fossil
collecting exposures. For those less adventurous souls, the regional fossil
club “The Dry Dredgers” is a very active group that goes on monthly
collecting trips. You can learn more about The Dry Dredgers and obtain a
membership application at their website Drydredgers.com .
Davis, R.A.(Ed.) (1992). Cincinnati
Fossils. Cincinnati: Cincinnati Museum of Natural History.
Feldmann, R.M.(Ed.) (1996).Fossils of Ohio, Division of Geological
Survey, Bulletin 70, Columbus, Ohio.
Fuchs, R. (2006). Some Trilobites of the Cincinnatian Found
at
http://drydredgers.org/trilobit.htm.
Hanke, Brenda, PhD, personal communication, 2006.
Holland, S. (2005). The Stratigraphy and Fossils of the Upper Ordovician near
Cincinnati, Ohio. Found at
http://www.uga.edu/~strata/cincy/index.html.
Moore, R.C. (1959) Treatise on Invertebrate Paleontology, Part O, Arthropoda
1, Kansas: Geological Society of America.
Peters, Shanan, PhD, personal communication, 2006.
  
Flexicalymene granulosa
Flexicalymene meeki
Flexicalymene retrorsa
  
Achatella carlyi
Acidaspis cincinnatiensis Amphilicas
shideleri
  
Ceraurinus icarus
Ceraurinus marginatus-Canada
Tricopeltis breviceps
(for how complete specimen may appear)
Ceraurius milleranus
Ceraurus globulatus-Canada
Rusaphycus (trilobite "nest")
(for how complete specimen may appear)
  
Isotelsus gigas
Isotelus maximus Isotelus hypostomes-mouthparts
  
Meadowtownella crosotus
Proetus spurlocki
Platycoryphe christyi
 
Triathrus eatoni
Triathrus eatoni deathbed
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