1983] 
Burnham — Geraridae 
47 
Mazon Creek, in contrast, was once part of a major delta on the 
edge of the Illinois Basin Sea. Periodic floodings in this area 
resulted in the burial and preservation of a wide diversity of organ- 
isms, both plant and animal. Two assemblages are recognized: the 
Essex fauna (mostly marine organisms); and the Braidwood assem- 
blage (freshwater to brackish flora and fauna). According to 
Richardson (1956:1 1-12) the Braidwood fossils “represent the fauna 
that lived on an aggrading plain, [just] above sea-level” and con- 
sisted of “more than 200 species of small animals, including insects, 
arachnids, mussels, and amphibians.” Over 140 species of insects 
have so far been described from this locality (Richardson, pers. 
comm.) and many of the specimens show exceptionally fine detail. 
Unlike the Commentry fossils, which are preserved in shale, 
Mazon Creek fossils are found primarily in iron carbonate or side- 
rite concretions. These concretions (also called nodules) form due to 
decay of the organism contained within them, but will do so only 
under the right conditions (iron-rich sediments, high pH, rapid bur- 
ial). They are characteristic of certain Upper Carboniferous coal- 
bearing strata and have been recorded from localities in the United 
States, France, England, and Germany. Nodules are shaped roughly 
according to the dimensions of the organism they contain and can 
be split along the bedding plane to reveal their fossilized contents. 
Preservation is generally good, although appendages (particularly 
legs) are frequently lost due to insufficient chemical reaction in the 
extremities. For a more detailed account of concretion formation 
see Woodland and Stenstrom (1979). 
The Mazon Creek biota has been known since the middle of the 
nineteenth century (Nitecki, 1979), but their initial discovery (unlike 
Commentry) was due to the erosion of fossil-bearing strata by 
stream action rather than by mining exploration. Concretions 
washed out by the stream (Mazon Creek) accumulated along its 
banks, and were found there by local collectors. Eventually the area 
became the focus of extensive mining exploration and several pit 
mines were dug in an effort to obtain coal. This was enormously 
beneficial to paleontologists because it exposed great numbers of 
concretions that then became available for study. Although most of 
the mining has now ceased, at least one mine remains open (pit 
eleven) from which fossils are still being collected, primarily by an 
