Mazon Creek - Fossil Ecosystems of North America: A Guide to the Sites and their Extraordinary Biotas - page 117

Shrimps
A wide variety of Crustacea occur at
Mazon Creek, many of which have a
shrimp-like body shape. There are both
freshwater and marine shrimps, found in
the Braidwood and Essex biotas,
respectively. Belotelson magister , a robust
species, is by far the most common shrimp
in the marine Essex biota. The second
commonest marine shrimp is
Kallidecthes . Acanthotelson ( 120 ) and
Palaeocaris are freshwater shrimps which
occur in the Braidwood biota and are also
found rarely in the Essex nodules, where
they presumably were washed down by
currents.

Eurypterids were discussed in Chapter 5
in connection with the Bertie Waterlime.
By the Pennsylvanian they were mostly
amphibious and represented largely by the
genus Adelophthalmus , with numerous
specimens in Mazon Creek nodules.
Terrestrial arachnids are well
represented in the Braidwood biota,
and the extinct group Phalangiotarbida
has the most numerous representatives;
trigonotarbids are also well represented.
The latter are close to spiders in
morphology but lack poison glands and
silk and, as arachnids go, are relatively
common in late Paleozoic terrestrial
ecosystems. Two living orders of
arachnids, Uropygi (whip scorpions) and
Amblypygi (whip spiders), have some well-
preserved examples in the Braidwood
biota ( 122 ). An interesting order of living
arachnids, Ricinulei, is represented at
Mazon Creek by three genera. Ricinulei
are rarely encountered animals, even
today, and are restricted to tropical forests
and caves. They are only known from the
Pennsylvanian and Recent, but seem to
have changed little in between. Three
other arachnid orders occur at
Mazon Creek: Opilionida (harvestmen),
Solpugida (camel spiders) and
Scorpionida. The scorpions are the most
ancient of arachnids and, though
exclusively terrestrial today, are found in
aquatic environments in the Silurian.
Scorpions are found only in terrestrial
environments by the Pennsylvanian, and
are the second most abundant arachnid in
the Braidwood biota.

Other crustaceans
Some squat, crayfish-like forms also occur
at Mazon Creek: Anthracaris in the
Braidwood biota and Mamayocaris in the
Essex biota. There are also phyllocarids
( Dithyrocaris ) in the Essex biota,
conchostracans (crustaceans almost
completely enclosed in a bivalved
carapace) which were probably fresh- or
brackish-water forms, some ostracodes and
barnacles. A crustacean which commonly
occurs in Pennsylvanian nodules, Cyclus , is
also found in the Essex biota. As its name
suggests, Cyclus has a round, dish-like
carapace, and has been thought of as a
fish parasite, though it may have been
free-living.

Chelicerates
The arthropod subphylum Chelicerata
includes horseshoe crabs (Xiphosura),
scorpions, eurypterids, spiders, mites,
and other arachnids. Mazon Creek has
yielded some exceptionally fine fossils
of these animals, which have provided a
great deal of information on the
evolutionary history of the chelicerates.
Euproops danae ( 121 ) is one of the best
known horseshoe crabs in the fossil
record. Work by Dan Fisher (1979) of
the University of Michigan revealed the
amphibious mode of life of these animals,
which are found predominantly in the
Braidwood biota.