Mazon Creek Jellyfish or Soft-Bodied Organism in Ironstone Nodule
cf. Tullimonstrum gregarium or problematic soft-bodied organism • Body fossil / Mold and carbonaceous compression within a concretion
Geological Period
Carboniferous
Estimated Age
309 to 307 million years ago
Preservation Type
Carbonaceous compression and mold preserved inside a siderite (ironstone) nodule
Condition Assessment
Fair to Good, showing typical weathering of the outer nodule and moderate definition of the internal organic impression
Taxonomic Classification
Phylum Incertae Sedis, Class Incertae Sedis, Order Incertae Sedis
Morphological Description
Oval to teardrop-shaped concretion containing an elongated, segmented or longitudinally structured central body with possible radiating structures at one extremity. Surface shows iron oxide staining and weathering along cleavage planes.
Rock Matrix
Sideritic ironstone concretion (clay-ironstone nodule)
Formation & Location
Carbondale Formation, Mazon Creek region, Francis Creek Shale Member, Illinois, USA
Size & Dimensions
Approximately 5 cm to 8 cm in length, typical for small-to-medium Mazon Creek concretions
Value & Rarity
Estimated Market Value
$50 - $150
Auction Estimate
$40 - $120
Rarity Assessment
Uncommon to Rare depending on the exact taxonomic identity and clarity of the internal organism
Scientific Significance
Important for understanding Konservat-Lagerstätten preservation and the soft-bodied fauna of Late Carboniferous tropical estuarine environments
Preparation Recommendations
Mechanical preparation using fine pin vises or gentle freeze-thaw splitting if the nodule has not yet fully opened; store in a stable, dry environment to prevent pyrite disease or further iron oxidation
Notable Features
Contained entirely within a classic oval ironstone concretion typical of deltaic Mazon Creek-style deposition