Mazon Creek - Fossil Ecosystems of North America: A Guide to the Sites and their Extraordinary Biotas - page 111

Most fossils show very little decay and,
indeed, there are instances of bivalves
preserved on the edge of the nodule at the
end of their death trail ( 109 )! The
concretions do not normally extend far
beyond the fossil, so the size and shape of
the nodule correlates with the organism
inside. Few concretions exceed 300 mm
(12 in), so large animals are rare in the
biota. The evidence presented so far tends
to indicate that the concretions formed
very soon after death and burial of the
organisms, mollusks stopped in their
tracks, seed-fern pinnules at right-angles to
the bedding, and little decay ( 110 ). Large
animals such as big fish and amphibians, it
is presumed, could escape this
environment and not be preserved. That
the organisms are preserved three-
dimensionally, at least in the cores of the
nodules, while the surrounding matrix,
like all siltstones, is greatly compressed
implies that the concretions formed before
any appreciable compaction. Indeed, the
siltstone laminae can be seen ( 111 ) to
widen progressively from the matrix
towards the centre of the nodules,
suggesting that the concretions grew
during compaction. Also, cracks within the
nodules, commonly infilled with kaolinite,
can be related to dewatering of the sedi-
ment (syneresis) during their formation.
Because the fossil-bearing concretions
mirror the shape of the fossil, and the
fossils are generally situated fairly centrally
within the concretions, we can assume that
the organisms contributed significantly to
their formation. Moreover, barren
nodules can usually be explained as
containing rather flimsy fossils,
unrecognizable organic matter, trace
fossils, and the like. The nodules contain
about 80% siderite cement, which implies
that when the concretions formed there
was at least 80% water by volume in the
sediment before compaction. Iron would
normally react with sulfur under the
influence of anaerobic bacteria in the
presence of decaying organic matter to
form pyrite (FeS 2 ), as in the Hunsrück
Slate (Selden and Nudds, 2004, Chapter

4), in preference to siderite but once any
sulfate was used up by this process (some
pyrite does occur in the nodules), then
methanogenic bacteria would help to
generate siderite. Conditions at Mazon
Creek which helped this process would
have been an abundance of iron and a
weak supply of sulfate. The Mazon Creek
nodules are commonly asymmetrical, with
flatter bottoms and more pointed tops.
This is due to the effects of gravity: the
weight of the carcass presses into the
sediment beneath, the concretion can
grow more easily upwards (where there is
less compaction), and any light fluids
resulting from decay would also rise
preferentially.

D ESCRIPTION OF THE M AZON
C REEK BIOTA
The Mazon Creek biota actually consists of
two biotas: the Braidwood and the Essex;
the former occurs mainly in the north of
the area, the latter in the south. Eighty-
three percent of the Braidwood nodules
contain plants, with the next commonest
(7.8%) inclusions being coprolites.
Following these (in descending order)
can be found: freshwater bivalves
(1.8%), freshwater shrimps (0.5%), other
mollusks (0.4%), horseshoe crabs (0.3%),
millipedes (0.1%), and fish scales (0.1%);
insects, arachnids, fish, and centipedes
form the remainder (<0.1%). On the
other hand, only 29% of the Essex biota
consists of plants, the commonest animal
being the 'blob' Essexella (42%). Follow-
ing these (in descending order) are
burrows and trails (5.9%), the marine
solemyid bivalves (5.5%), coprolites
(4.8%), worms (2.8%), miscellaneous
mollusks (1.9%), the marine shrimp
Belotelson (1.8%), the marine bivalve
Myalinella (1.4%), miscellaneous shrimps
(0.5%), the crustacean Cyclus (0.5%), the
enigmatic Tully Monster (0.4%), the
scallop Pecten (0.3%), the jellyfish
Octomedusa (0.3%), and miscellaneous
fish (0.2%); insects, millipedes and
centipedes, hydroids, horseshoe crabs,
arachnids, and amphibians form the