On the dumps of coal strip mines of northestern Illinois - some of them not far from downtown Chicago - collectors find curious rounded nodules of rock. These nodules are concretions of siderite, or iron carbonate, and naturally split along the middle. With a careful tap from a rock hammer, a split nodule may reveal a beautifully preserved seed-fern leaf, a shrimp or millipede, or even a Tully monster. This is the Carboniferous-age Mazon Creek locality (or more accurately a set of localities called the Francis Creek Shale, of which Mazon Creek itself is one) whose uniquely preserved fossils provide a glimpse of the history of many organisms that would otherwise have left little trace. Similar concretions are found over a wide area of northern and eastern Illinois, at several sites. However, all of these are collectively known as "Mazon Creek" fossils.
Some "Mazon Creek" localities are known for terrestrial fossils, including beautifully preserved plants and, rarely, insects, centipedes and millipedes, scorpions and other arachnids, and even small amphibians. Others, which formed under marine conditions, include jellyfish and other cnidarians, crustaceans, molluscs, worms, and rare fish. Shown here is a polychaete annelid, Fossundecima, preserved in a concretion that has been split open.
The area of the Mazon Creek fossils is riddled with coal seams. The coal seams are composed of alternating layers of coal and shale. Each coal layer represents the ancient forest, fossilized and compressed into coal. The shale beds above and below the coal seams range in depth from twenty to sixty feet and represent a period when the sea advanced and flooded the swamplands in which the forests grew. The different strata piled on top of each other indicate that the sea level rose and fell a corresponding number of times in order to create the alternation between coal and shale. In some areas, there are sixteen coal seams, recordig the advance and retreat of the sea sixteen times. The shale bed immediately below each coal layer represent the former soil from which the plants in the coal layer grew; fossils of roots are commonly found in this layer.
Because of the abundance of coal in the area, coal strip mining is common, in which the top layer of shale is cut away in order to expose the coal. The shale that is cut away is deposited in spoil heaps close to the strip mines, and it is from these deposits that most of the fossils belonging to Mazon Creek are found in nodules of siderite, or iron carbonate. Because the nodules are found in dumps, it is not possible to know the relation of the fossils to each other in terms of position in the shale and therefore it is not possible to precisley know the relative time span.
The nodules are made from sediment carried down by water, burying the organisms and hardening around them. The sediment was cemented with iron carbonate to form a hard nodule distinct from the shale around it. The nodules range in size from a fraction of an inch to more than twelve inches long. The average size of a nodule is five inches in diameter lengthwise and two inches in diameter.
Because of the mode in which the fossils were formed, the fossils are mainly impressions or incrustations of organisms. An impression is formed when, for example, a plant part falls into water and the air spaces in the plant are gradually filled with water. The organism thus becomes waterlogged and sinks to the bottom where it is buried by sediment. The sediment then hardens around the organism, forming a nodule. The organism may then rot away, leaving an impression of itself in the sediment that hardened around it. This cavity may then be filled with coal, calcite, siderite, or any other form of sediment. By the nature of the formation of impressions, the fossils usually lie parallel to the plane of sedimentation.
The nodule in which the fossil is preserved can be split cleanly in half with a hammer, exposing the fossil that forms a natural plane of weakness in the nodule. This technique to expose the fossil does not yield as high a quality of fossils as those nodules that are split through natural weathering and weakening along the plane of the fossil. Nodules can also be split through the freeze thaw technique where the nodule is frozen with water and the ice is then melted in hot water, splitting the nodule. Many of the specimens occur individually within the nodule, but there are occasions in which animal fossils are found in association with plant debris. It is also sometimes possible to determine the nature of the gut content in soft-bodied organisms through analysis of the fossil.
The presence of upright stems, and bivalves that are not parallel to the plane of sedimentation indicates that the fossils were formed by the quick burial of organisms during some great catastrophe. Other clues leading to this hypothesis are the presence of fossilized upright trees that can reach nine feet in height. There is also no evidence of perturbations by animals in some of the fossils. That is, dead organisms are often scavenged and their remains would therefore be slightly disturbed and not preserved in the formation that it would have existed while alive. Also, if the environment were aqueous, clams and other bivalves would cut through sediment while making their burrows, and while such clam trails are evident, it is sometimes completely absent from a certain layer of shale, suggesting that the layer of shale was laid down quickly and to a great depth.
Other evidence for the quick deposition of sediment on organisms as seen in a great catastrophe is provided by the presence of soft-bodied organisms in the fossil record. In order for such organisms to be preserved in the fossil record, they must be buried rapidly with the inhibition of anaerobic decomposers, and the development of concretions must also be rapid. These conditions can only be met if a catastrophe dumped massive amounts of sediment on the organisms while they were still alive. One side affect of the rapid burial of the organisms is that they are preserved very well because there was not enough time for the decomposers to decompose the organism. Sometimes the impression is still lined with the skeltous chitin present in arthropods.
The Mazon Creek flora is not only characterized by the excellent preservation of specimens, but also by the large range in sizes of fossils and the large numbers of species represented, many of which are not found anywhere else in the world. Common groups found in the flora of Mazon Creek are extinct and extant species of pteridophytes. They often grew to be the size of large trees though the species of today rarely acquire that size. Some examples of pteridophytes present during the Carboniferous are the lycopod scale trees, which were extremely abundant worldwide and were much larger than lycophytes are today. An undistorted lycopod megaspore with its parenchymatous gametophyte (the gametophyte contained tissue that was capable of dividing) was found by Darrah as mineralized tissue, organic tissue that was replaced with minerals from the environment.
The lycopods represent a major element in coal along with the sphenophytes, which were also one of the most widely distributed flora in Mazon Creek, lining the waterways like they do today. The Pteridopsida (ferns) were present, bearing spores on their leaves as they do today, but there were also fernlike seed-bearing ferns, the pteridosperms. The pteridosperms became extinct after the Carboniferous period along with the majority of scale trees and sphenophytes. Primitive ancestors of conifers, cone-bearing plants, were also present at this time. The seeds of both the pteridosperms and the early conifers are well preserved in the fossil record.
The fauna of Mazon Creek is also diverse and well preserved, but they are not as well represented as the flora. The major groups present in the fossil record are the Crustacea and Uniramia, mainly insects. Aquatic life was very diverse with animals suited to fresh, brackish and marine environments, including: shrimp; bivalves such as mussels and clams; cephalopods; fish; sharks (chrondrichthyes); coelenterates such as jellyfish; crustaceans such as crabs; and the Tullimonster (Tullimonstrum), shown below.
The Tullimonster is placed in its own phylum and is unique to the period. Johnson and Richardson (1956) describe the Tullimonster as being a marine, carnivorous, worm-like creature closely associated with the jellyfish and having a soft body with no internal skeleton. It is hypothesized that storms blew seawater and sea organisms further inland than they would naturally occur, thus the Tullimonster and other marine organisms were introduced inland where they were buried and fossilized. This allowed researchers today to study these organisms that would normally not be preserved. Evidence of marine forms of life is also suggested by the fossilized clam trails and burrow mottles. Other marine animals that can only be found in the fossil record from Mazon Creek are hagfish and lampreys.
Terrestrial forms of animal life are represented by the arachnids (spiders & scorpions), amphibians, and mainly by the insects which make up approximately half of the fauna that is present in the fossil record. The insects are grouped mainly by wing characteristics since this is the only part of the insect that is clearly preserved sometimes. The features of the head, thorax and legs also help to characterize an insect if they are available.
Statistical analysis has shown two different assemblages of fossils from the Mazon Creek area divided into the Braidwood Biota and the Essex Biota; each of these is a different ecosystem with a unique assortment of flora and fauna. This distinction was first characterized by R.G. Johnson and E.S. Richardson. The Essex collection consists of 3214 specimens whereas the Braidwood collection consists of only 1213 specimens. Coelenterates, echinoderms, and the Tullimonster are exclusively found in the Essex Biota. Arthropods represent the largest percentage of animals present in the fossil record, but the relative percentage in the two different biotas differ: arthropods consist of only approximately 45% of the fauna from the Essex biota, but represent 79.5% of the fauna from Braidwood. The ratio between flora and fauna also differ for the two different collections. In the Braidwood collection, the number of plant specimens outnumbers the number of animal specimens available by 100 to 1; whereas in the Essex collection, the relative numbers of both types of specimens are about equal.
From the flora and fauna present, one can deduce the different environments associated with the two different collections. Such paleoenvironmental studies were first performed by C.W. Shabica. The Braidwood biota is thought to come from a coal swamp forest that is a mixture of a terrestrial and freshwater environments, located above sea level and close to the shore. The Essex environment is thought to be a sub-aqueous, interchannel flood plain inhabited mainly by freshwater biota but also by marine biota for brief periods because of storms which blew the sea inland. After such storms, an increase in sediment brought down to the delta like environment would bring freshwater, terrestrial remains and sediment with which to bury and fossilize the marine animals temporarily inhabiting the area. Because of the constantly changing environments in the interdistributary bay, an area where many different sources of water meet, the environment was unable to support large animal populations though small local populations were possible for brief periods in time.
These constantly shifting marine, freshwater, and brackish environments did not occur at the latitude that Illinois is at today. Rather, Illinois and most of North America were equatorial. The flora present in the fossil record compares with that in the tropics today; there are no annual rings suggesting uniform growing conditions without seasonal fluctuations.
Mazon Creek is well known for its diverse and excellently preserved specimens of organisms from the Carboniferous period. One factor leading to the large numbers of specimens available for study is the presence of amateur collectors who, along with the professional collector, collected nodules from the shale heaps since the 1850's, building up large collections. Also, the strip mines expanded the number of fossil sites by exposing shale that would normally remain buried. Included in the specimens found in Mazon Creek are the first fossils from major groups, such as the only known fossilized hagfish, and unique species such as the Tullimonster. The excellent preservation of fossils allows studies of soft-bodied organisms that are usually destroyed and not present in the fossil record. Mazon Creek provides a diverse and well preserved insight into life during the Carboniferous Period.
Learn more about Mazon Creek fossils at the Illinois State Museum.
Johnson, Ralph G.; Eugene S. Richardson, Jr. 1970. Fauna of the Francis Creek Shale in the Wilmington Area. Pg. 53-59 in Depositional Environments in Parts of the Carbondale Formation-Western and Northern Illinois. Illinois State Geological Survey Guidebook Series No. 8.
Nitecki, Matthew H. 1979. Mazon Creek Fossils Symposium. New York Academic Press.
Richardson, Eugene S. Jr. 1956. Fieldiana: Geology, vol. 12. Field Museum of Natural History, Chicago.
Shabica, Charles W. 1970. Depositional Environments in the Francis Creek Shale. Pg. 48 in Depositional Environments in parts of the Carbondale Formation-western and Northern Illinois. Illinois State Geological Survey Guidebook Series No. 8.