This is Mazon Monday post #194. What's your favorite Mazon Creek fossil? Tell us at email:[email protected].
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The September 1978 edition of the Field Museum Bulletin had a article by Gordon Baird about his project to survey Mazon Creek fossil localities across Will, Grundy, Livingstone, and Kankakee Counties in Illinois. Almost 300,000 concretions were collected from many, many localities (100+), consisting of 20 strip mines and 90 underground shaft mines. Gordon talks about how he could collect 500 - 1,000 concretions in a day of collecting.
The results of the study were published in 1985 in "Biota of a Pennsylvanian muddy coast: habitats within the Mazonian Delta Complex, northeast Illinois", which was authored by Gordon Baird, Charles Shabica, John Anderson, and Eugene Richardson, Jr. The paper was part of a larger study to determine the boundary line between the terrestrial and marine environments back in the Pennsylvanian. We posted the results of this survey as they were published 18 years later in 1996 in the "Richardson's Guide to the Fossil Fauna of Mazon Creek" (see Mazon Monday #126). Here is the original paper's abstract.
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.
The bulletin article is interesting because it explains the methods and purpose of the study. The full PDF of all 1978 bulletins can be found in the Biodiversity Heritage Library.
Strip mine spoil heaps at Pit 11 - Gordon C. Baird
MAZON CREEK CENSUS
By Gordon C. Baird (Gordon C. Baird is assistant curator of fossil invertebrates.)
Looming above the Cornfields, an hour's drive southwest of Chicago, are gray, cone-shaped hills which may catch the traveler's eye as he motors along Interstate Highway 55. These hills, notable for their drabness and symmetry, are evidence of a system of abandoned coal mines that extends both across and under 40 square miles of Will, Grundy, Livingstone, and Kankakee Counties. From the mid-1860s until the mid-1970s, when the last mine closed down, these shaft (underground) and open pit (strip) mines yielded millions of tons of bituminous coal. Today only these desolate hills of waste rock and the furrowed moonscapes of the depleted strip mines bear witness to that bygone industry.
To the paleontologist, however, these mined-out areas are neither desolate nor depleted, for they contain abundant fossils, including forms that are both unusual and spectacular. This region of fossiliferous mines, known to fossil collectors and paleontologists as the Mazon Creek area, is one of the greatest fossil localities in the world.
Sedimentary rock (one of the three basic kinds of rock) commonly yields fossils, but what makes the sedimentary deposits of the Mazon Creek area extraordinary is the superb preservation of the fossil specimens and the richness of species. More than 350 plant species have been found there, including ferns and a variety of ancestral fernlike plants. Parts of plants including leaves, bark, roots, seeds, and wood are abundant at many Mazon Creek localities. Animal specimens found in the area total more than 320 species, including numerous soft-bodied forms such as worms, sea cucumbers, larval fish with yolk sac still attached, and even jellyfish, which are rarely preserved as fossils.
In addition to Mazon Creek's preservational wonders, more than a score of important new fossil species have been described from there, including the earliest known isopod (a crustacean), the earliest known squid, and the earliest known chaetognath and nemertine (both marine worms). Other prize discoveries have been the only known fossil lamprey, the only fossil hydra, and the only fossil priapulid (a marine worm). Several species have been discovered whose placement in the animal kingdom is still unknown; these include the bizarre, wormlike "Tully monster."
Field Museum scientists have played a significant role in the study of Mazon Creek fossils, notably Eugene S. Richardson, curator of fossil invertebrates. Much of Richardson's work has dealt with the systematics (classification) of major groups such as clams, cephalopods, cartilaginous fishes, crustaceans, and worms. The late Ralph G. Johnson, a Field Museum research associate and University of Chicago faculty member, worked with Richardson in describing major animal communities in the Mazon Creek area. Richardson and Johnson recognized two major animal associations: the Braidwood and Essex faunas. The Braidwood fauna, representing fossils from strip mines near Morris and Braidwood, Illinois, contains freshwater animals including insects, amphibians, horseshoe crabs, and freshwater shrimp; the Braidwood fauna is also associated with abundant plant fossils. The Essex fauna is composed of marine or brackish water animals including jellyfish, polychaete worms, marine shrimps, sea cucumbers, and several forms that do not fall conveniently into existing classifications.
Mazon Creek fossils occur within ironstone concretions (characteristically lima bean-sized to football-sized, ovoid, slightly flattened nodules of iron carbonate) which formed around the organisms immediately after they were covered with mud, silt, or sand. The fossiliferous concretions occur in stratified mudstone or silty shale formally known as the Francis Creek Shale Member, found above the coal whichwas mined. The conical gray hills seen by the passing motorist are piles of this shale that has been excavated from underground mine shafts or been shoveled aside in strip mining. It is on the surface of these waste piles that loose, weathered-out concretions are typically found.
Animal and plant fossils in the Mazon Creek area are nearly 300 million years old, belonging to that segment of geological time known as the Pennsylvanian Period, or the"Coal Age." During this period extensive coastal swamps developed and peat deposits formed; later these would gradually turn to coal.
Fossil of "Tully monster" (Tullimonstrum gregarium)
The Francis Creek Shale was deposited along the northeastern shores of a broad, shallow inland sea which once covered much of the continent, including most of Illinois. One or more rivers emptying into this sea created a delta of sediment which, in time, turned to rock.
Spider fossil discovered at Mazon Creek by Walter Lietz
As the delta was gradually being formed and fanning outward, the sea level was also apparently rising, and various short-lived, transitory environments were produced in the shallow shore waters. From time to time flooding from channels on the delta dumped silt and mud into the adjacent bays. Millions of organisms were buried alive in this mud and, fortunately for the present-day investigator, were beautifully preserved. In the course of time a variety of environments were inundated in this fashion — sometimes the communities involved were marine, sometimes freshwater; sometimes they were in relatively quiet bays, sometimes in channels of constantly flowing water. The result, for the paleontologist, is a crazy-quilt distribution of numerous ancient environments across the Mazon Creek area.
Since my undergraduate days I had known of Mazon Creek and Pit 11 and the fossil treasures they had yielded. However, I knew little about the Mazon Creek area in general and was astonished, upon arriving at the Field Museum, to learn of the great number of other fossil localities in that region. In studying the work that Richardson and Johnson had done, I became intrigued by the idea of investigating these localities, including many that had not yet been examined.
I pored over state mining survey maps and aerial photos, eventually locating more than 20 strip mines and 90 underground shaft mines that had been worked in the area. Since most of these localities were little known and some had never been visited by fossil collectors, it seemed reasonable to assume that they, too, could yield new fossil organisms and even fossil assemblages. My plan now was to attempt a comprehensive survey of the faunal and floral associations over the entire area, gaining a better understanding of their distribution and composition.
Students collecting fossiliferous concretions from shale bank along Mazon Creek. - Gordon C. Baird
An initial, but time-consuming step was to sample fossil occurrences in all these localities —in effect, to take a census. Begun about a year and a half ago, this phase is now half finished, with 80,000 concretions collected from 120 localities.
The census data being sought includes taxonomic composition of samples, relative abundance of species, and the preservation quality of fossils at different localities. Generally, if a large number of concretions are present at a particular locality, I collect 500 to 1,000 specimens to get as representative a sampling as possible. At any given locality, all concretions with visible fossils and many whole concretions suspected of containing fossils are collected. Whole concretions are then stored at a Pit 11 field station provided by the Commonwealth Edison Company for the Museum's research activities. Other concretions brought to the Museum are being sorted, washed, and studied by several dedicated Field Museum volunteers.
Concretions to be split open are placed on the Museum's roof in water-filled buckets. The alternate freezing and thawing of the water during the cold months of the year has proven to be an effective means of splitting them open—a technique devised by amateur collector Larry Osterburger. Data from the samples I have collected in the field will be supplemented with census data on collections built up by Osterburger and other amateur collectors. The combined data will be synthesized to yield abundance and location information on the various kinds of fossils collected from the Mazon Creek area. This synthesis is also expected to give us a better understanding of the region's paleoecology and paleogeography.
The census has already revealed that the marine Essex fauna, which occurs over a large part of the Mazon Creek area, appears to be quite variable from locality to locality. In several areas around the towns of Morris and Braidwood it abruptly gives way to the Braidwood fauna. The latter seems to be confined to a narrow belt along the northeast margin of the area underlain by coal age sediments. This distribution pattern, together with the fact that many Braidwood organisms are typically freshwater species, suggests that they lived in ponds, marshes, or streams just above the northeast shoreline of the great inland sea. As additional data is processed, it is hoped that more light will be shed on the nature of the communities and of the ecological conditions that prevailed in northeastern Illinois nearly 300 million years ago.
The "census-taker" is the collector —usually myself—who must return to the coal field again and again each time loading the car with sackfuls of concretions, these to be taken back to the Museum for processing, examination, and storage. On a typical collecting day, last March, I found myself searching for abandoned mine dumps near Coal City, one of the towns in the Mazon Creek area. It was a bleak, gray morning, and underfoot there was snow and mud. But I knew that before the day was over my car would be loaded with new specimens. With a little luck I might even encounter a species new to science or in some other way remarkable. Every collecting day began with this feeling of excitement and anticipation.
I scanned the horizon, peered down the town's side streets, and scouted behind buildings for the piles of typically gray waste rock from the mines. There were several on my right, but these I had sampled on a previous visit. A quarter mile to the south, behind some sheds, I spotted several other piles. The next step was to secure permission to enter the property and do my collecting. At a nearby farmhouse I was told that the land with the rock piles was someone else's property. The farmhouse I was then directed to produced nothing but an unfriendly dog. I had no other choice, for now at least, to forget about collecting here. Further south, toward Braceville, I came across more dumps. After getting clearance from the owner I was finally able to set to work gathering specimens.
Representative Mazon Creek fossils, clockwise from top right: marine shrimp, cephalopod, hydra, jellyfish.
At this point, collectors typically gather up whole concretions, and carry them to a so-called "anvil," or flat rock, where they break them open. For census work, however, this procedure is too time-consuming as well as incompatible with efficient census-taking procedures. Therefore, I collected broken concretions that revealed fossils as well as whole concretions- these to be taken directly to my car. Later, at the Museum, they would be split open and their contents examined.
On this particular day I lucked out. I had been picking for about an hour and had nearly a full load to haul back to the car.. Although fossils were plentiful here, they were unspectacular; most were "blobs" (possible jellyfish), clams, and plants. Then, suddenly, I found myself holding a "Tully monster" (Tullimonstrum), a peculiar wormlike animal that showed its original color banding. Excited, I stashed it in my shoulder bag. Lifting the bag, I saw under it a fossil egg case of Paleoexyris, a sharklike fish. At that moment my back pack toppled over; out fell a concretion, which broke open; inside was a rare shrimp!
The weather had turned colder and a sleet storm had begun, but none of this mattered to me; the day had already been one of singular success. It was this same feeling of reward that had brought other Mazon Creek collectors back there time and time again; and in another day or so —sleet, snow, or otherwise—I knew I would be back.
The result of 15 minutes' collecting at Pit 11: whole concretions which must now be split open to reveal their contents—perhaps a leaf, perhaps a shrimp, perhaps a species new to science.
This post was suggested by Ralph Jewell. He ran across the article when scanning documents given to him by Bob Ulaszek. Thanks for passing this on Ralph!
