Geoscience Reference
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early conifers. Animals, too, had diversified
and moved into the new niches provided by
the plants. Insects had appeared, evolved
wings, and some Pennsylvanian early
dragonflies had a wingspan of three-
quarters of a metre (30 in) (though those
found in the Mazon Creek beds were
smaller). Myriapods became giant in the
Pennsylvanian too, with large, armoured
millipedes and the enormous (>2 m [6.6 ft]
long) arthropleurids, the largest known
land arthropods. Vertebrates had not only
followed the arthropods onto land but
amphibious tetrapods had attained large
size (up to 1 m [3.3 ft] long), and there
were freshwater sharks with bizarre dorsal
spines.
to the sorting plant. The coal is dug in
long strips, so the overburden is used to
back-fill the strip where coal was previously
removed. It is the Francis Creek Shale
which is the source of the exceptional
fossils at Mazon Creek. Once news of the
coal company's generosity in allowing
access to its site spread, there was a regular
stream of amateur fossil collectors at the
site looking for the elusive special fossil.
The fossils at Mazon Creek occur in clay
ironstone nodules (concretions). These
usually require a winter or so of
weathering before they will split easily with
a single hammer-blow, usually along the
weakest line which is that of the fossil.
Some collectors artificially freeze and thaw
the nodules to accelerate this process.
Many of the concretions contain seed-fern
fronds, some contain indeterminate
shapes which were termed 'blobs' and
consequently thrown away. Later research
has shown that most of these blobs are
actually fossil jellyfish, attesting to the
unusual preservation of soft-bodied
animals at Mazon Creek. The scramble of
collectors at Pit 11 could have resulted in
the loss of the best fossils to personal
cabinets, never to be studied by experts
were it not for the efforts of Dr ES
('Gene') Richardson who encouraged the
collectors to meet at regular intervals at
the Field Museum in Chicago and show
their finds. In this way they learned from
both the scientists and each other about
what the animals and plants were and
the best ways of finding them. They could
be swapped, and the best specimens
presented to the museum for study. The
annual Mazon Creek Open House for
fossil collectors continues to this day.
Some 20 years ago the Peabody Coal
Company sold Pit 11 for the construction
of a nuclear power plant. While mining no
longer goes on there, the tips remain and
are still picked over. Moreover, during
construction of the power station,
boreholes were drilled which passed
through the Francis Creek Shale and
yielded some important information
about its environment of deposition.
H
ISTORY OF DISCOVERY OF THE
M
AZON
C
REEK BIOTA
Mazon Creek is a small tributary of the
Illinois River, situated some 150 km south-
west of Chicago (
106
), and it has given its
name to this Lagerstätte. The fossils come
principally from spoil heaps of the strip
coal mines which have operated in the
area over the last century. The importance
of the Mazon Creek biota is that it has
been so well collected, particularly by an
army of keen amateurs, that it has yielded
the most complete record of late Paleozoic
shallow marine, freshwater, and terrestrial
life. More than 200 species of plants and
300 animal species have been described,
including representatives of 11 animal
phyla.
Plant fossils were collected and
described from natural outcrops and small
mine tips in the Mazon Creek area for
many years before the large Pit 11 open
strip mine was opened in the 1950s. In the
late years of that decade, Peabody Coal
bought out the Northern Illinois Coal
Company and allowed local fossil
collectors to visit the pit and collect from
the waste material. In strip mining, the
overburden (in this case the Francis Creek
Shale) is stripped off by giant buckets on
drag lines to reveal the coal beneath,
which is then simply dug out by smaller
diggers and loaded into trucks for removal
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