Mazon Creek - Fossil Ecosystems of North America: A Guide to the Sites and their Extraordinary Biotas - page 122

very reminiscent of a group of shell-less
gastropods known as heteropodids. The
fame of the Tully Monster was assured a
few years ago when it was voted as State
Fossil of Illinois.

branchiosaurid. A single specimen
(4 vertebrae) of an anthracosaurid is
known. Aïstopods, limbless, snake-like
amphibians, are represented by two species
and numerous specimens, and orders
Nectridea, Lysorophia, and Microsauria are
represented by single specimens. A single,
immature specimen of a lizard-like,
captorhinomorph reptile is known.

Fish
More than 30 species of fish are known
from Mazon Creek, but their
identification is hampered by the fact that
many of the fossils are small juveniles or
isolated scales. Agnathans are represented
by a hagfish and a lamprey, as well as
two agnathans which cannot be assigned
to known groups. Cartilaginous fish
(chondrichthyans) are represented at
Mazon Creek by a rare but diverse fauna
of mainly juveniles. Interestingly, they do
not appear to be the juveniles of the
better known Mecca Quarry Shale sharks
described by Zangerl and Richardson
(1963), which were approximately coeval
with the Francis Creek Shale forms, but
seem to be the young of sharks which
lived in a different habitat from the Mecca
Quarry. Palaeoxyris , which is believed to be
a shark egg-case, is a common fossil in
the Braidwood biota and, to a lesser
extent, the Essex biota. About 15 genera
of bony fish (osteichthyans) occur at
Mazon Creek. Most specimens are
small and not easy to identify. However,
there is a great variety of types
from different habitats, from fresh
through brackish to marine waters.
Palaeoniscids, including deep-bodied
forms as well as fusiform species generally
referred to ' Elonichthys ' are common in
both Essex and BraiJdwood biotas.
Among sarcopterygians, rhipidistians
(which gave rise to tetrapods),
coelacanths, and lungfish are all present
at Mazon Creek.

Coprolites
These are fossil faeces, which can occur in
both Essex and Braidwood biotas. Though
not so aesthetically pleasing as plants or
animals, coprolites can tell us a great deal
about what animals were eating. For
example, spiral coprolites containing fish
remains indicate that there were probably
quite large sharks swimming in the Mazon
Creek area, for which we have no body
fossil evidence.

P ALEOECOLOGY OF THE M AZON
C REEK BIOTA
It is obvious from the evidence presented
that the Mazon Creek area represents a
variety of habitats: terrestrial,
freshwater, brackish, and restricted marine,
associated with a deltaic environment. The
Colchester Coal represents an environment
of swamp forest dominated by tree-
sized club-mosses and horsetails with an
understorey of seed-ferns, among other
plants. The Francis Creek flora is
dominated by fern, seed-fern and horsetail
debris, which suggests it came from a more
upland setting. The terrestrial animal
fauna, such as myriapods, arachnids, and
insects, presumably lived among these
plants.
The Francis Creek Shale coarsens
upwards, so the initial inundation of the
swamp forest was rapid, and later delta-
derived sediments filled the marine
embayment. Many sedimentological and
paleontological features suggest rapid
deposition: failed bivalve escape structures,
Lingula buried in life position, and edgewise
seed-fern pinnules, as well as the
preservational features associated with rapid
burial mentioned under Taphonomy, above.

Tetrapods
Tetrapods are rare at Mazon Creek,
but are diverse and include 23 specimens of
amphibian and one reptile. Temnospondyl
amphibians are represented by larval
Saurerpeton , both adult and larval
Amphibamus , and a possible