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Paleozoic Partner Highlight

click to see a reconstruction of a Mazon Creek ecosystem...

Map of North America 300 million years ago (mya). During the Pennsylvanian, North America straddled the equator (dashed red line). Mazon Creek (green star) was tropical, covered by coastal swamps and deltas. Paleogeographic basemap by Ron Blakey (Colorado Plateau Geosystems.)

Field Museum of Natural History (Chicago):
Mazon Creek Fossil Collection

In 1673, the first coal found in the New World was reported along the Illinois River near Utica in LaSalle County, Illinois. In the mid-1800s, another important find was made in the same area. Fossils were discovered along Mazon Creek, a tributary of the Illinois River. Amateurs and professionals created many large and important collections. The Field Museum of Natural History houses the most comprehensive collection of Mazon Creek fossils with more than 40,000 specimens in the Invertebrate Collections alone. Perhaps the most famous is the Illinois state fossil, Tullimonstrum gregarium (the Tully Monster). In recognition of their significance, the Mazon Creek fossil beds were designated as a National Historic Landmark in 1997.

The fossils are found in the Middle Pennsylvanian aged (~300 million years old) Francis Creek Shale in northeastern Illinois, from parts of Grundy, Kankakee, LaSalle, Livingston and Will counties. Fossils are usually subdivided into two groups: those from terrestrial and freshwater environments (called the Braidwood biota), and those from marine environments (called the Essex biota). During the Pennsylvanian, Illinois was situated far from its current location. The Mazon Creek region was tropical, located just north of the equator (see map). The region was covered by coastal swamps and deltas with fresh- and brackish-water rivers, ponds, and estuaries flowing to a shallow sea that covered western Illinois.

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A selection of Mazon Creek fossil animals. Left to right: shrimp-like animal, Mamayocaris jaskowskii; Tully Monster, Tullimonstrum gregarium; insect, Eubleptus danielsi. Scale bars are 1 cm. Photos courtesy Ian Glasspool and Paul Mayer (Field Museum of Natural History).

A selection of Mazon Creek fossil plants. Clockwise from left: conifer Cordaites borassifolius, fern Spiropteris sp., seed fern Alethopteris serlii, seed fern Macroneuropteris scheuchzeri, conifer Cordaitanthus sp., and seed fern Stephanospermum konopeonus. Scale bars are 1 cm except Cordaites (10 cm). Photos courtesy Ian Glasspool and Paul Mayer (Field Museum of Natural History).

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Mazon Creek fossils are often exceptionally well-preserved. Most often, the fossils are found in sideritic (ironstone) concretions (rounded to elliptical nodules that must be cracked open to find the fossil) in the Francis Creek Shale. As well as hard parts, these concretions often preserve the soft tissues of organisms not typically found in the fossil record. Indeed, many of the Mazon Creek animals are unique and found nowhere else in the world. The Mazon Creek fauna and flora are also known for their exceptional diversity, a phenomenon partially due to the large number of collected specimens. More than 300 animal species and about 125 species of plants have been discovered. The Mazon Creek plant record is the most diverse North American flora of the Pennsylvanian Period. Scroll down for a partial list of animal and plant fossils.

The Mazon Creek plant fossils are dominated by ferns. Other common fossils include an extinct group of seed plants (pteridosperms), lycopsids, and horsetails. Less common are an extinct order of gymnosperms called cordaites. Lycopsids such as Lepidodendron andrewsii were the dominant coal-forming vegetation during the Pennsylvanian. They are the oldest living vascular plant group, having existed for more than 400 million years.

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.

With 4.5 billion years under one roof, The Field Museum is your passport to travel around the world and back in time, all without checking a single bag. Meet SUE, the largest and most complete T. rex ever found. Discover things you never knew, and things you never even knew you were interested in.

Mazon Creek Animal Phyla

    Cnidaria (corals; e.g., Anthracomedusa turnbulli)
    Nemertea (ribbon worms)
    Nematoda (roundworms)
    Pripulida (priapulid worms)
    Chaetognatha(arrow worms)
    Echiura (spoon worms)
    Annelida (ringed worms)
    Onychophora (velvet worms)
    Arthropoda (arthropods; e.g., horseshoe crab Euproops danae, spider Athrolycosa antiqua, scorpion Eoscorpius carbonarius, and dragonfly Oligotypus mackowskii)
    Mollusca (molluscs)
    Brachiopoda (brachiopods)
    Echinodermata (echinoderms)
    Hemichordata (hemichordates)
    Chordata (chordates; e.g., hagfish Myxinikela siroka, shark Bandringa rayi, amphibian Pseudophlegethontia turnbullorum, and reptile Cephalerpeton ventriarmatum)
    Incertae sedis (e.g., the Tully Monster Tullimonstrum gregarium; unclear taxonomic classification above genus).

Mazon Creek Plant Groups

    Pteridophyta (ferns; e.g., Lobatopteris sp., Acitheca unita, Pecopteris mazoniana, and P. squamosa)
    Pteridospermophyta (seed ferns; e.g., Macroneuropteris scheuchzeri, Neuopteris ovata, Laveienopteris rarinervis, Stephanospermum braidwoodensis, and Odontopteris aequalis)
    Sphenophyta (horsetails; e.g., Annularia sphenophylloides, A. stellata, Calamites suckowii, and Sphenophyllum emarginatum)
    Lycophyta (lycopsids; e.g., Lepidostrobophyllum majus, Cormophyton mazonensis, Polysporia mirabilis, and Lepidophloiois protuberans)
    Coniferophyta (cordaites; e.g., Cordaites borassifolius, and Artisia sp.)

Article by Ian Glasspool (Field Museum of Natural History, Adjunct Curator & Paleobotany Collections Manager)

2013 Paleozoic Partner feature articles:

| January: Fossils of the 2013 National Fossil Day Artwork | February: Paleontological Research Institution, Museum of the Earth | March: Falls of the Ohio State Park | April: Field Museum of Natural History, Mazon Creek Collection | May: Prehistoric Trackways National Monument | June: Cincinnati Museum Center | July: Glacier Bay National Park & Preserve | August: University of Michican Museum of Paleontology, Silica Formation Fossils | September: Yale Peabody Museum of Natural History, Beecher's Trilobite Bed | October: Guadalupe Mountains National Park | November: Utah Geological Survey, Millard County Cambrian Fossils | December: Denver Museum of Nature and Science, High-Altitude Mass Extinction |

Last updated: April 1, 2013