About the Fossil Fern Site
In the United States, the Carboniferous period (360 -280 Million Years ago) is divided in
the Mississippian (360 - 320) and Pennsylvanian (320 - 280) periods.
During the middle Pennsylvanian period, the supercontinent Pangea, meaning "all land",
had formed along with the Appellation mountains, running north and south near the center
of Pangea. These new Appellation mountains were some of the largest mountains of all
time, and were probably as tall as the present day Himalayas. East of these Appellation
mountains, which ended in central Pennsylvania, were large flood deltas, or flood plains.
These large aluvial flood plains extended the entire way to the west coast of Pangea.
Around this time, Pennsylvania was approximately located 5 degrees south of the equator,
in a tropical
rain forest type climate with very little seasonal fluctuation. Also, at this time,
the Earth had much higher concentrations of carbon dioxide in the atmosphere. This was
very conducive to the growth of plants. Hence, in these flood deltas, large, swampy forests
grew. However, the plants in these forests were far different than plants of the present
day. Flowering plants, or deciduous plants of any kind had not yet evolved. Instead,
there were very large, simple, fernlike plants, such as Calamities (a giant horse tail),
Lycopods (which grew up
to 100 feet) seed ferns and herbaceous ferns (that grew up to 50 feet). However, at the end
of the Pennsylvanian, most of these strange plants became extinct, and deciduous plants
eventually evolved. Much of these forested areas became buried and now
create the carboniferous coal beds
that are mined today throughout the eastern United States and Europe. Because of this,
these swampy forests are often called coal swamps and coal forests.
In these forests, new insect life also flourished, such as dragonflies, mayflies, millipedes,
scorpions, and spiders. However, these were not normal insects, they were giant man eating
creatures. Well, the may not have been man eating, however some dragonflies had wingspans
of 2.5 feet, cockroaches were a whopping 4 inches, and flies needed extra large fly swatters.
Unfortunately, these fragile insects did not readily survive fossilization in the coal swamps,
and only rarely can be found as fossils.
A unique feature of this period was the development of the amniotic egg for reproduction.
This basically means animals could now lay eggs on land. So, very primitive reptiles, such
as Hylonomus and Anthracosaurs evolved. Also, around this time,
large amphibians existed. Fossilized jaw fragments from these amphibians can occasionally
be found in outcrops around the Ambridge area.
During this time, sea levels were in constant fluctuation. This resulted in
the Panthalassic ocean (which was almost like the pacific ocean, but on a larger scale)
on the west coast of Pangea to flood these deltas, creating a marine environment where the
tropical forest environment was.
During the times of these marine environments, many invertebrate creatures thrived.
However, Trilobites did not thrive, they were scarce and would become extinct by the
end of the Permian. Also, Armored Placeoderms became extinct, and were replaced by more
The roadcut next to the Ambridge Woodlawn bridge contains two levels. Each level contains
distinctly different fossils. The first level, the Mahoning formation, has the preserved
remains of one of these coal swamps. Above the remains of the coal swamp, is a marine
environment, the Brush Creek Marine Zone. This marine zone was formed from one of the
sea level fluctuations, when the sea invaded this place.
The Mahoning formation is composed of the very thin gray to green shale. This finely
grained, thin shale is packed full
of mainly carbon films of well-preserved middle Pennsylvanian plants,
such at Neuropteris, Pecopteris, and Calamites.
The Brush Creek Marine Zone is composed of black, oily shales and limestones. These
shales contain numerous gastropods, and straight shelled nautaloids. Also, ancient
forms of sharks lived here,
and occasionally their small teeth are found around this area (although I know of no one
actually finding one at this particular site).