This is Mazon Monday post #126. What's your favorite Mazon Creek fossil? Tell us at email:[email protected].
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There are good shapes and bad shapes, but at the end of the day - it's what's in the concretion that counts. Everyone wants to know what might be in the concretion they just found. It's pretty well known that the northern Braidwood biota yields concretions that are more likely to have plant matter, while the southern Essex biota, being marine, is more likely to have animal fossils. Chapter 4B in the "Richardson's Guide to the Fossil Fauna of Mazon Creek" shows the relative scarcity of plants and animals found in the Mazon Creek fossil deposit.
Perseverance does not always pay off for every collector who stalks the coal company dumps or wades Mazon Creek. Although probabilities remain constant over time, chance sometimes matches a great fossil with the right collector on a day well remembered. Indeed, one hunter may find two lampreys in one day, while an other collecting just as intensely finds none in a year. Gene Richardson recalled the first time that his Field Museum colleague Bill Turnbull was taken to the strip mines. Turnbull promptly secured a splendid specimen of the snakelike amphibian Aornerpeton (=Phlegethontia), a prize for which some veterans have searched in vain for decades.
But what controls the probability? Unless you are so foolish as to walk in the fresh boot prints of one or more zealous amateurs, your chances are influenced by the original absolute and relative abundances of organisms fossilized in concretions within the exposed and weathered rock. An approximate measure of the relative rarity or abundance of Mazon Creek organisms at various localities can be established through census collecting. However, the quality of such data depends on the amount of material collected, identified, and recorded. Roy L. Moodie tentatively determined that on average at least 100,000 concretions from Mazon Creek would have to be split to yield an amphibian fossil (Moodie, 1912, p. 279); this figure is little more than an educated guess based on extrapolation, however, because no one has ever collected half a million concretions from along the creek to firmly establish the odds.
The chapter is a summary of the 1985 paper by Gordon Baird, Charles Shabica, John Anderson, and Eugene Richardson, Jr. called "Biota of a Pennsylvanian muddy coast: habitats within the Mazonian Delta Complex, northeast Illinois". The paper was part of a larger study to determine the boundary line between the terrestrial and marine environments back in the Pennsylvanian.
Baird, G. C., C. W. Shabica, J. L. Anderson, and E. S. Richardson, Jr. 1985a. Biota of a Pennsylvanian muddy coast: habitats within the Mazonian Delta Complex, northeast Illinois. Jour. Paleo., 59:253-281.
Abstract
The Mazon Creek biota (Westphalian D) is composed of plants and animals from terrestrial, fresh water and marginal marine habitats. Fossil animals, including jellyfish, worms, crustaceans, holothurians, insects, chordates, and problematica occur in sideritic concretions on spoil piles of more than 100 abandoned coal mines in a five-county region (Mazon Creek area) of northeast Illinois. These fossils record rapid burial and early diagenesis in a muddy, delta-influenced coastal setting submerged during marine transgression. Fossils are associated with laminar mudstone in the Francis Creek Shale Member of the Carbondale Formation, which records moderate to very rapid sedimentation in quiet water. Rapid engulfment of organisms is indicated by upright-standing trees, edgewise-buried plant leaves, and escape burrows with associated animals. Very regular sedimentary rhythms in the lower Francis Creek record daily sediment accumulation in a tide-influenced shallow bay or estuary. Major fossil associations include: 1) terrestrial plants and animals from coastal swamp, levee, and floodplain settings; 2) fresh water animals; and 3) euryhaline animals inhabiting waters near distributaries. Extensive fossil sampling shows a distinct regional boundary between marine (Essex) and nonmarine (Braidwood) faunal assemblages which parallels the ancient coastline. As this boundary is crossed, a low diversity nonmarine fauna is replaced by a diverse association of eurytopic marine benthonic and pelagic taxa. The faunal gradient between these associations as well as spatial changes in species dominance within the Essex fauna are attributed to salinity, turbidity, and oxygen depletion stress between localities. The Essex fauna is compared to coeval normal marine shelf faunas in western Illinois. Intermediate fossil groupings are absent in the Francis Creek, but one assemblage (Danville Fauna) in a younger Pennsylvanian unit clearly shows intermediate faunal composition and taphonomic characteristics; this delta margin biota is described and contrasted with Essex and normal marine faunas.
56,974 Braidwood concretions were collected, of which 22,802 (40%) were duds. For the Essex, 229,979 concretions yielded 144,799 duds (63%). As, many have noticed, there are far more duds found in Pit 11 and Braceville than the terrestrial localities. So yes, you will find a bunch of duds at Pit 11!
Table 4B.1
Relative Rarity of Selected Braidwood Fossils
Fossil Number Percentage Pecopteris 8971 26 Plant stem 4572 13 Epidophyllums 4476 13 Macerated plant 4233 12 Coprolite 2670 7.8 Neuropteris 2496 7.3 Annularia 1430 4.2 Misc. plants 885 2.6 Bark 839 2.5 Calamites 770 2.3 Clam 632 1.8 Shrimp 162 0.5 Misc. molluscs 149 0.4 Horseshoe crab 98 0.3 Millipede 21 0.1 Fish scale 20 0.1 Insect 11 0.03 Fish 6 0.02 Spider 4 0.01 Centipede 1 0.003 Table 4B.2
Relative Rarity of Selected Essex Fossils
Fossil Number Percentage Blob (Essexella) 35420 42 Plants 25072 29 Burrow / trail 5062 5.9 Clam ("Edmondia") 4673 5.5 Coprolite 4070 4.8 Worms 2409 2.8 Misc. molluscs 1656 1.9 Shrimp (Belotelson) 1497 1.8 Myaline 1190 1.4 Misc. shrimp 446 0.5 Cyclus 412 0.5 Tully 318 0.4 Pecten 292 0.3 Octomedusa 262 0.3 Blade (Esconichthys) 95 0.11 Misc. fish 60 0.07 Insect 25 0.03 Millipede 12 0.014 Hydroid (Drevotella) 12 0.014 Horseshoe crab 9 0.011 Spider 9 0.011 Amphibian 3 0.004 Kottixerxes 2 0.002 Centipede 2 0.002
The odds estimated at the end of the article are somewhat encouraging, although 250 is a massive number of concretions today. And, 1000 concretions in a day is just crazy. However, the odds hold even if they aren't collected in a single day. So, get out there and get collecting, and please.... send us photos for Fossil Friday!
The sheer number of different categories of relatively rare taxa significantly in creases the odds of really scoring on any particular day. Thus, the chance of finding a rare fossil is better than even at 250 fossils collected, and near certainty at 1000 fossils. This latter figure is what one person can collect in one day, assuming that he or she is the first to go to known choice areas in the early spring or after great storms. Seasoned and persevering veterans at "the Pit" know that, like a slot machine with numerous winning combinations, the area will continue to yield treasures. Moreover, unlike at a casino, there is more treasure because more untapped winning combinations surely await discovery. Let's all keep searching, collecting, and sharing our finds!
References
Baird, G. C., C. W. Shabica, J. L. Anderson, and E. S. Richardson, Jr. 1985a. Biota of a Pennsylvanian muddy coast: habitats within the Mazonian Delta Complex, northeast Illinois. Jour. Paleo., 59:253-281.
Baird, G. C., S. D. Sroka, C. W. Shabica, and G. J. 1 Kuecher. 1985b. Taphonomy of Middle Pennsylvanian Mazon Creek area fossil localities, northeast Illinois: significance of exceptional fossil preservation in syngenetic concretions. Palaios, 1:271-285.
Moodie, R. L. 1912. The Pennsylvanian amphibia of the Mazon Creek Shale, Illinois. Kansas University Sci. Bull., 6:323-359.
