Mazon Creek - Fossil Ecosystems of North America: A Guide to the Sites and their Extraordinary Biotas - page 106

Mazon Creek

B ACKGROUND : THE C OAL
M EASURES
Once life became established on land it
quickly developed a more complex habitat
structure; as plants developed tree forms,
so forests evolved (by the Late Devonian),
and alongside this there was a great
diversification of animal life. The late
Devonian also saw the evolution of
tetrapods (four-legged vertebrates) which
emerged onto land in Mississipian times
and started preying on the abundant
invertebrate life already established there.
By the Pennsylvanian there were extensive
forests across the equatorial areas of the
globe, which included the present areas
occupied by north-west and central Europe,
eastern and central USA, and elsewhere
in the world such as southern China
and South America. These forests are
represented in the fossil record by coal
seams, which are preserved because in
these places the forests developed on mires,
permanently waterlogged ground; the
anoxic conditions prevented complete
decay of the forest litter and thus prompted
the formation of peat which, when
compressed under a great thickness of later
sediment, turned to coal. It was these vast
beds of coal, and associated ironstones,
clays, and other natural resources which, in
Britain in particular, provided the raw
materials for the Industrial Revolution.
A Coal Measure sequence of rocks
presents a more complex and interesting

suite of environments than a simple swamp
forest; most Pennsylvanian Coal Measure
sequences represent deltas with a range of
environments from marine bays through
brackish lagoons to sand bars, freshwater
lakes, levees, and swamp forests. Delta lobes
are geologically short-lived. If their
sediment supply is cut off, they rapidly
sink and sea water transgresses the land
surface, swamping the forests. Thus, in
many sequences there is a band of mud-
bearing marine fossils immediately above a
coal seam. The sea may persist in the area
for many tens or hundreds of years before a
new delta lobe builds out into the area and
relatively quickly establishes new silt, then
mud substrates upon which new forests
can develop. Even in established swamp
forests, floods are commonplace. As a
result of these changing environments,
Coal Measure sequences show a distinctive
pattern of thin mud or shale layers,
siltstones (often regularly laminated),
coarse sandstones, and coal seams.
Some of the vascular plant groups
discussed in Chapter 6 continued with
little morphological change into
the Mississippian and beyond, the
bryophytes for example, while others, such
as the psilophytes, gave rise to the
horsetails, club-mosses, and ferns that
attained gigantic proportions in the
Pennsylvanian. Many of these groups
formed the understorey vegetation too,
together with seed-ferns, cordaites, and