The Shabica fossil catalogue numbers begin with SF. See below for lists and descriptions of the Shabica and non-Shabica fossils in this display case. At the end of this webpage there are hyperlinks to sources of further information and a list of books on the Mazon Creek fossils.

Extending across five counties in northeast Illinois (and over 800 square miles), the Mazon Creek Fossil Beds represent a snapshot of life along a tropical coastline during the Upper Carboniferous: a time when land areas that are now part of North America and Europe were located near the Equator. Palaeontologists class the area as a Konservat-Lagerstätte, meaning a place with a globally significant lode of diverse and exceptionally well-preserved fossils. Study of the Mazon Creek area has allowed scientists to ‘reconstruct’ a past environment and ecosystem to an incredibly high level of detail.

Environmentally, the sequences (‘cyclothems’) of coal, shale, and other sedimentary rocks show past oscillations in sea level that occurred over a 100,000 year cycle – the same time cycle over which ice sheets have expanded and retreated (and sea levels of have fallen and risen) over the past million years of our current geologic period, the Quaternary. This shows that the climate during the time of the Upper Carboniferous was influenced by cyclic variations in Earth’s orbit around the Sun, like our recent Pleistocene Ice Age.

The coal forming swamp forests of Mazon Creek, and more generally in the Carboniferous world, also reveal the role of changing levels of atmospheric carbon dioxide and oxygen in affecting Earth’s climate and environment. Great ice sheets in the Southern Hemisphere were able to develop in the Upper Carboniferous due to global cooling brought about by millions of years of forest productivity acting as a ‘carbon sink’, reducing the concentration of CO₂ (a greenhouse gas) in the atmosphere as swamp forests became buried, eventually turning into coal. On the other hand, the concentration of O₂ in the atmosphere was increasing owing to the production of plant matter (by photosynthesis) exceeding decomposition rates. The O₂ content of the atmosphere exceeded 30% (it is 21% today) enabling the existence of super-sized insects and arachnids. A dragonfly-like insect that inhabited Carboniferous forests had a wingspan of up to 70cm!

Ecologically, the Mazon Creek Fossil Beds are a time capsule of a tropical coastal ecosystem, containing fossil assemblages that vary along the transition (‘ecotone’) from land to sea. This enables study of the past associations between different terrestrial, freshwater, estuarine, and marine organisms. During a phase of climate warming, when Southern Hemisphere ice sheets decreased in volume, sea level rose bringing the coastline to what is now northeast Illinois. Large areas of swamp forest were flooded, and shallow bays and estuaries formed. The climate also became more seasonal: intense monsoon rains increased the sediment load of rivers, causing flood events that rapidly buried organisms in both inland and shallow offshore locations.

To the north and east, fossils representing the Braidwood Biota consist of land and freshwater plants and animals. Here, some 80% of the animal fossils are arthropods, with amphibians and fish also represented. Fossils of the shallow brackish and marine areas are grouped in the Essex Biota. Represented here are various fish, crustaceans, molluscs, echinoderms, jellyfish, and the enigmatic Tully Monster.

Fossils in Case 49

The fossils from Mazon Creek donated by Dr Charles Shabica are labelled SF (Shabica fossils). The following fossils are on display in Case 49.

Fossil flora on display (representing the Braidwood Biota):

SF.53:1 (and its pair SF.53:2) Calamites sp.

• A genus of ancient horsetails that could grow to heights of 30 to 50 metres. These fossils are of the leaf whorls that grew around the stem of the plant.

SF.56:1 (and its pair SF.56:2) Lepidodendron sp. bark

• Lepidodendron is an extinct genus of tree-like lycopods that could reach heights of 50 metres, with trunks exceeding a metre diameter. These were the canopy forming plants of the Mazon Creek flora, dominating the ancient swamp forests. They are a relative of modern club mosses.

SF.57 Lobatopteris lamuriana (Heer, Wagner, 1958) (a species of true fern)

• The second most abundant plant fossil found in the Mazon Creek area. It was a ‘tree-fern’. Tree ferns could exceed 10 metres in height.

SF.59:1 (and its pair SF.59:2) Macroneuropteris scheuchzeri (Gastaldo, 1977) (a species of seed fern)

• The taxon Macroneuropteris scheuchzeri forma decipens is the most abundant plant type found in the Mazon Creek fossil record.

SF.61 Mariopteris sphenopteroides (Lesquereux, Zeiller, 1886) (a species of seed fern)

• A rare species of seed fern found in the Mazon Creek flora.

SF.63 Pecopteris sp. (a genus that includes true ferns)

SF.65 Pecopteris subcrenulata (Lesquereux, 1879) (a species of true fern)

• This is the third most common species of true fern found in the Mazon Creek fossil record and is unique to the area.

SF.66 Spiropteris sp. (the fiddlehead of a true fern)

• Fossils of developing fern fronds in the form of a ‘fiddlehead’ are given the generic name of Spiropteris when the plant cannot be identified to species level.

SF.67 Sphenopterid (an unidentified plant of the forest understory, likely in the genus Sphenopteris, a type of seed fern)

Fossil fauna on display (representing the Essex Biota):

SF.9:1 (and its pair SF.9:2)Belotelson magister (Packard, 1886) (a crustacean)

• This lobster-like organism is the most abundant fossilised crustacean within the Essex fauna.

SF.20:1 (and its pair SF.20:2) Esconites zelus (Thompson and Johnson 1977) (eunicid worm)

• This is a polychaete annelid (segmented worm) that lived in the shallow marine area, typically 5 centimetres in length.

SF.21:1 (and its pair SF.21:2) Essexella asherae (Foster, 1979) (a type of jellyfish or sea anemone)

• This now extinct type of cnidarian (aquatic invertebrate) is the most abundant taxon of the Essex fauna of the Mazon Creek area, often referred to by collectors as ‘blobs’. Living Essexella asherae measured between 1 and 15 centimetres in length. A recent study suggests that Essexella may have been ancient sea anemones rather than jellyfish.

SF.32:1 (and its pair SF.32:2) Kallidecthes richardsoni (Schram, 1969) (a crustacean)

• This extinct type of shrimp is the second most abundant crustacean represented in the Essex fauna.

SF.41:2 Pieckonia helenae (Thompson, 1979) (goniadid worm)

• Also known as a ‘fish worm’, this polychaete annelid worm lived in the shallow marine area. It was a burrowing predator, living in sand and mud.

SF.43 Rhabdoderma elegans (Newberry) (coelacanth fish with yolk sac)

• An extinct species of coelacanth (lobe-finned fish) that was widely distributed during the Carboniferous Period, with fossils of this species also found in Europe. The presence of the yolk sac indicates that it was a juvenile coelacanth. Coelecanths are ‘ovoviviparous’ meaning that the eggs develop and hatch inside the female, with live birth of juveniles.

SF.45 Solemyid (a type of saltwater clam)

SF.49:1 (and its pair SF.49:2) Tullimonstrum gregarium (Richardson, 1966) (Tully Monster, front part)

SF.50 Tullimonstrum gregarium (Richardson, 1966) (Tully Monster)

• The Tully Monster is unique to the Mazon Creek area, and palaeontologists have long debated its taxonomic classification. Its front section includes eye stalks and a narrow proboscis with a claw at the end. Fossilised front parts are often identified by the flattened eye stalks (transverse bar) oriented perpendicular to the long axis of the fossil. The Tully Monster’s middle section is a segmented trunk, and its back end has a blunt tail with lateral fins.

Carboniferous fossils in Case 49 that did not come from the Shabica Collection:

54 Sigillaria reniformis bark (Brongniart, 1822) (lycopod)

• Sigillaria is another genus of tree-like lycophytes related to Lepidodendron. The origin of this fossil is unknown, but fossils of this species have been found in the Mazon Creek area.

63 Straparollus pentangulatas (a gastropod mollusc)

• A Carboniferous Period marine gastropod. Fossils of this species have been found in the UK. The genus is present, but rare, in the Mazon Creek faunal record.

64 Lepidodendron aculeatum (Sternberg, 1820) stem (lycopod)

• This species is the most abundant type of Lepidodendron represented in the Mazon Creek floral record.

65 Stigmaria ficoides (Sternberg, 1820) (a lycopod root)

• The genus Stigmaria refers specifically to the root systems of various types of lycopsids. Fossils such as these are found in the Mazon Creek area.

74 Lepidodendron sternbergi leafy branches (lycopod)

75 Calamites suckowi (Brongniart, 1828) (stem of an ancient horsetail)

• This species is present, but uncommon in the Mazon Creek floral record.

78 Spirifer striatus (Martin, 1809) (brachiopod)

The Spirifer genus of brachiopod is not represented in the Mazon Creek faunal record, but other brachiopods are, such as Lingula and Orbiculoidea.

Hyperlinks for further information

Mazon Creek flora – Field Museum

Monsters Storm The Field – Field Museum

Mazon Creek Fossil Flora | Smithsonian National Museum of Natural History

About the Mazon Creek Fossils and Deposits (state.il.us)

Exploring the Fossils of Mazon Creek (YouTube video by the Illinois State Museum, from 2021)

What Was The Tully Monster? (YouTube video from 2020)

The Mazon Creek Lagerstätte: a diverse late Paleozoic ecosystem entombed within siderite concretions | Journal of the Geological Society | GeoScienceWorld

An abundant sea anemone from the Carboniferous Mazon Creek Lagerstӓtte, USA – Plotnick – 2023 – Papers in Palaeontology – Wiley Online Library

Books on the Mazon Creek fossils

J. Wittry, ed., A Comprehensive Guide to the Fossil Flora of Mazon Creek, ESCONI in association with Northeastern Illinois University, Chicago, 2020

J. Wittry, ed., The Mazon Creek Fossil Fauna, ESCONI in association with Northeastern Illinois University, Chicago, 2012

J. Wittry, ed., The Mazon Creek Fossil Flora, ESCONI in association with Northeastern Illinois University, Chicago, 2006

C.W. Shabica and A.A. Hay, eds., Richardson’s Guide to the Fossil Fauna of Mazon Creek, Chicago, 1997

D.E. Anderson April 2025