Mazon Creek Coprolite
Coprolite (undetermined fish or amphibian origin) • Trace fossil (Ichnofossil); Coprolite (fossilized excrement)
Geological Period
Carboniferous (Pennsylvanian subperiod)
Estimated Age
307 to 309 million years
Preservation Type
Authigenic mineralization (Replacement/Carbonization) within a siderite concretion
Condition Assessment
Fair to Good; the specimen is a partial nodule showing internal structure, but the outer shape is weather-worn and lacks high-definition spiral markings
Taxonomic Classification
Phylum: Chordata (likely source); Ichnogenus: Likely Malformis or similar ichnotaxon
Morphological Description
Irregular, somewhat cylindrical to flattened mass with a lumpy, heterogeneous texture. The specimen shows small circular pitted structures and inclusions that may represent undigested organic matter or bone fragments.
Rock Matrix
Ironstone concretion (Siderite / Iron Carbonate) with a characteristic reddish-brown oxidation layer
Formation & Location
Francis Creek Shale Member of the Carbondale Formation, Mazon Creek area, Illinois, USA
Size & Dimensions
Approximately 4-6 cm in length; typical for small aquatic vertebrate waste from this locality
Value & Rarity
Estimated Market Value
$15 - $40 USD
Auction Estimate
$20 - $50 USD
Rarity Assessment
Common; coprolites are among the most frequently found fossil types in the Mazon Creek nodules
Scientific Significance
Provides critical data on ancient trophic levels and diets of Pennsylvanian fauna. Coprolites help reconstruct the food web of the Mazon Creek estuarine environment.
Preparation Recommendations
Keep in a dry environment to prevent further oxidation. No aggressive chemicals should be used; light brush cleaning only. If the nodule is unstable, a consolidation with Paraloid B-72 may be applied.
Notable Features
Features a distinct contrast between the dark interior inclusions and the oxidized red-orange siderite exterior. The pitted surface suggests a rapid mineralization process before complete decomposition.