Mazon Creek - Fossil Ecosystems of North America: A Guide to the Sites and their Extraordinary Biotas - page 113

112 remainder (< 0.1%). From this list it can

be seen that the Braidwood biota consists
of terrestrial and freshwater organisms
while the Essex biota consists of
predominantly marine organisms, with
some drifted plants etc. No marine
organisms can drift into freshwater but
freshwater and terrestrial organisms can
be washed down into the sea. Notice, too,
that the marine Essex biota is not a typical
marine biota (there are no brachiopods,
corals, crinoids), so it must represent a
reduced-salinity, and perhaps muddy,
environment which fully marine animals
cannot tolerate.

Plants
The Mazon Creek nodules preserve a
typical Coal Measure flora, exceptional
only in that its preservation is better in the
concretions than in other Pennsylvanian
clayrocks. Large fossils are not preserved
in the Francis Creek Shale but the
presence of tree-sized club-mosses and
horsetails can be inferred from pieces of
bark ( Lepidodendron and Calamites ,
respectively) and foliage ( Lepido-
phylloides and Annularia , respectively).
Seed-fern pinnules are the commonest
fossils in all Mazon Creek nodules;
examples include Neuropteris ( 112 ),
Pecopteris ( 113 ), and Alethopteris ( 114 ).
Comparison with modern coastal swamp
forests suggests that much of the plant
debris found in the Mazon Creek nodules
originated not from the forest itself but
was carried down by streams from far
inland, so possibly represents a mixture of
coastal and upland forest. Plant debris is
allochthonous (i.e. drifted from its
original life position) in both Braidwood
and Essex biotas, but is commoner in the
former.

112 The seed-fern Neuropteris. MU. Pinnule
is 2.4 in 6 cm long.

parts. Most 'blobs' in Mazon Creek
nodules are jellyfish remains, and some
show distinct tentacles and other
structures. Essexella , for example ( 115 ),
shows a bell with a cylindrical sheet
hanging below it. Essexella belongs in the
Scyphozoa, while Anthracomedusa ( 116 ),
with four bunches of numerous tentacles,
is a cubozoan.

Cnidarians
This phylum includes the corals and
anemones (Anthozoa), hydroids
(Hydrozoa), Scyphozoa and Cubozoa
(jellyfish), and a few other groups. Apart
from the corals, they have no mineralized
skeletons but some jellyfish have stiffened