Mazon Creek - Fossil Ecosystems of North America: A Guide to the Sites and their Extraordinary Biotas - page 115

Bivalves
Bivalves are common fossils on account of
their hard, calcareous shells (valves), but
those in the Mazon Creek nodules are
important because they commonly
preserve soft-part morphology. There is a
high diversity of bivalves in the Mazon
Creek, with 12 superfamilies represented.
It is convenient to divide the fauna into
freshwater and marine forms: i.e. those
found in the Braidwood and Essex biotas
respectively. The commonest marine
bivalve occurs as articulated valves (called
'clam-clam' specimens by amateur col-
lectors), and is occasionally found at the
end of an unsuccessful escape trail ( 109 ).
Previously misidentified as Edmondia , it
has been shown recently to be an
undescribed solemyid; true Edmondia are
rare at Mazon Creek. These solemyids are
burrowers (infaunal benthos), but other
common bivalves in the Essex biota are the
thin-shelled swimmers (nekton)
Myalinella and Aviculopecten ( 117 ). The
family Myalinidae also includes freshwater
forms such as Anthraconaia , found in the
Braidwood biota.

Worms
Apart from their tiny jaws, called
scolecodonts, polychaete annelids are
rarely preserved as fossils because they are
soft-bodied. Therefore, the great diversity
of polychaetes found at Mazon Creek is an
important contribution to the fossil record
of this important group of marine animals.
One of the commonest Mazon Creek
polychaetes is Astreptoscolex ( 119 ), which
shows a segmented body and short
chaetae (spines) along each side.

Other mollusks
Three other classes of mollusk occur in the
Mazon Creek biota: Polyplacophora,
Gastropoda, and Cephalopoda. Though
there are many freshwater gastropods
(snails) today, the only gastropods at
Mazon Creek are from the marine Essex
biota. Polyplacophora (chitons) are
exclusively marine animals which are rare
in the fossil record because they usually
inhabit rocky shores. However, one genus,
Glaphurochiton ( 118 ), occurs in the Essex
biota. Cephalopods are also wholly marine
and, though normally common in fully
marine Pennsylvanian rocks, they are
rarer than chitons at Mazon Creek,
but quite diverse in the Essex biota. In
addition to the orthocone bactritoids,
coiled ammonoids and nautiloids with
external shells, coleoids (with internal
hard parts) are also present. Jeletzkya is a
small, squid-like coleoid with an internal
shell similar to a cuttlebone.