Mazon Creek Fossils and Tully Monster

While researching strip coal mining in the northern Illinois, I noticed Mazon River on the maps. I remember when we would take Member's Night tours at the Field Museum that we would see discussions of the fossils they had collected from the Mazon River area. They were especially excited about the Tully Monster (Tullimonstrum gregarium) fossils.

Tully monster fossil and model at Chicago Field Museum; photo by Brian Smith
n Flickr (noncommercial use permitted with attribution / share alike)

A video explains that it was discovered by Francis Tully in 1958, described in 1966 by Dr. Richardson of the Field Museum, and declared the Illinois State Fossil in 1989 because it has been found only in Illinois. (The video says 1958, one of their captions says 1955. The Illinois State Museum agrees with the 1958 date. A YouTube copy of the video in case the Field Museum breaks their link.)

I quote the entire section on the Mazon Creek Fossils in their history page because I'm coming across several links to the Field Museum that no longer work. For example, they changed their name from www.fmnh.org to www.fieldmuseum.org.

The Field Museum and Mazon Creek Fossils

Mazon Creek fossils are exceptionally well-preserved remains of Middle Pennsylvanian (~300 million years old) animals and plants that lived along a subtropical swampy coastline.   The fossils are preserved in sideritic concretions (round to elliptical ironstone nodules) found in the Francis Creek Shale Member of the Carbondale Formation. This type of preservation preserved soft-bodied animals and plants not normally preserved in the fossil record. The Mazon Creek fauna and flora are known for their diversity. This diversity of fossils is partially due to the large number of specimens that have been collected.

The history of study of the Mazon Creek can be divided up into four overlapping phases. The initial discoveries of plants and associated arthropods and amphibians were made in the mid-nineteenth century (Dana, 1864; Cope, 1865; Lesquereux, 1866; and Meek, 1867). Until about 1928 most collections were small and gathered from the classic exposures along Mazon Creek itself. The Field Museum collection grew gradually over this time. However, collecting activities and the number of accessible localities increased dramatically with the era of strip-mining in this part of Illinois from 1928 to 1974.

The work of Langford, Richardson, his colleagues, and many local collectors from 1945 to 1980 broadly constitutes the second phase of study. During this period, large numbers of taxa were discovered and described, and hypotheses were developed to begin to resolve the paleoenvironments represented by different parts of the Francis Creek Shale Member. Intensive collecting by Richardson, Johnson, and his students, and associated local collectors at Pit 11, west of Essex, Illinois, led to the description of a marine -influenced faunal assemblage that sharpened scientific interest in the Mazon Creek Biota as a whole (Johnson and Richardson, 1966; Richardson and Johnson, 1971). The Field Museum consequently became the center of Mazon Creek study (Langford, 1958, 1963; Richardson et al. 1945-1983; Nitecki, 1979; and many others). During this time the growth rate of the collection grew dramatically.

In the late seventies and early eighties these studies expanded into the third phase, through the research of Gordon C. Baird and his colleagues (Baird, 1979; Baird et al., 1985a,b, 1986).  The Field Museum hired Baird in 1976 as a Curator of Fossil Invertebrates and he began a large, detailed Mazon Creek census program.  Baird and his group of collectors were the perfect collecting machine. They collected at 350 strip mine, deep mine, and surface outcrop localities over a 200-square kilometer region in the northeastern Illinois Basin. Collecting trip after collecting trip, the group filled up burlap sacks with Mazon Creek nodules transporting them back to the Museum in Baird's heavily overloaded car. At the Museum Baird put his nodules in plastic buckets filled with water and set them on the Museum roof over winter. The freezing and thawing of the water all winter long split the nodules, revealing any fossils. This method was far superior and more efficient then the traditional splitting of nodules with a rock hammer. Baird's collections were so large that the heavy buckets of rocks and water caused structural damage to the Field Museum's roof! Baird's collecting had to be scaled back a bit. In total Baird and his group collected over 285,000 Mazon Creek nodules.

This project resulted in more-rigorous quantitative analysis of the paleoecological hypothesis first developed by Richardson and Johnson. Baird's results indicate that the Mazon Creek biota contains three major fossil associations representing three different paleoenvironments: (1) a euryhaline estuarine fauna inhabiting river distributary-influenced coastal marine waters (Essex assemblage), (2) a freshwater association comprising a low-diversity autochthonous fauna, and (3) a terrestrial biota including allochthonous plants and animals from swamp, levee, and floodplain environments. The terrestrial and freshwater associations together (2 and 3) constitute the Braidwood assemblage as recognized by Richardson and Johnson. The census sampling conducted by Baird has revealed in detail the distribution and correlation of different biotic and lithic paleoenvironments within the Francis Creek Shale Member. These models have aided the interpretation of comparable nodule biotas in North America and Europe (Baird et al., 1985, 1986).

The 285,000 specimens collected by Baird and his fellow collectors represent the peak in the growth of the FMNH Mazon Creek collection. During this time major collections were also donated to the FMNH by local collectors including: Fagan, George, Greene, Herdina, Klocek, Lietz, Roubik, Sobolik, and Wolff.

The last phase is the gradual decline in collecting due to the end of strip mining and the loss of many collecting localities. The few remaining areas available to collectors today are generally over-collected and overgrown with plants.  After the untimely death of Richardson in 1983, the department needed to deal with the large backlog of specimens that the former intense period of collecting had generated. Scott H. Lidgard was hired as a Curator of Fossil Invertebrates in 1984. He and collection manager Mary Carman, and Mazon Creek Coordinator Bret Beall took on the task of transforming a series of private collections, research collections, and Baird's census collection, plus just large piles of unidentified Mazon Creek nodules into a single, well-curated, systematic collection of Mazon Creek fossils fully available to the research community. This involved sorting, identifying, and organizing tens of thousands of specimens, then cataloging and labeling them, and entering the data into a computer database.  The results are that today the Field Museum's collection of Mazon Creek fossils is the definitive resource for researchers studying the Mazon Creek.

The golden age of Mazon Creek fossil collecting may be over, but there are still new, important finds being made and research about these fossils continues today. There are three fields for which Mazon Creek fossils have special research significance: diagenesis and taphonomy, systematics, and paleoecology.

 The Illinois State Museum better explains the geology of the Mazon Creek Fossils and Deposit.

The Francis CreekShale

Approximately 300 million years ago (during a time geologists call the Pennsylvanian Period) Illinois looked nothing like it does today. Much of it was not even dry land. Much of the area that we now call Illinois was a mixture of swampy lowlands and shallow marine bays.

From the northeast flowed at least one major river system. The river(s) built large deltas through the low swamps and into the shallow bays. The mud that the river(s) carried was deposited in these deltas and bays. This mud turned into a rock called the Francis Creek Shale.

In some ways the area might have been similar to southern Louisiana (USA) and the adjacent Gulf of Mexico. However, the plants and animals would have been very different from today. They were different for at least two reasons. First, many of the plants and animals that are common today had not yet evolved at that time. Second, the climate would have been tropical. The tropical climate was a result of continental drift; 300 million years ago the area was just a few degrees north of the equator.

How the Mazon CreekFossils Formed

Many animals lived in the shallow marine bays. More plants and animals lived in the swampy areas along the rivers. As animals in the bay died they fell to the bottom of the bay. They were joined by plants and animals that died along the river and were washed into the bays.

When the remains of these plants and animals sank to the bottom of the bays, they were rapidly buried by the mud washing in from the river(s). This process protected the remains from being destroyed. Bacteria began to decompose the plant and animal remains in the mud. The action of these bacteria produced carbon dioxide in the sediments around the remains. The carbon dioxide combined with iron from the groundwater around the remains forming siderite (ironstone). The siderite protected the remains from further damage.

The combination of rapid burial and rapid formation of siderite resulted in excellent preservation of the many animals and plants that ended up in the mud.

 
The ISM has a page for many of the plants and animals in the Mazon Creek Deposits. The page for the Tullimonstrum is extensive since it is now the state fossil. Of note is that more recently the fossils have been found in open-pit coal mines in central Illinois. Scientists still have not determined to what other animals they are related.