Tully Monster / Soft-bodied Bilatarian
Tullimonstrum gregarium (the Illinois state fossil) • Body fossil (compression/carbonization); soft-bodied invertebrate
Geological Period
Carboniferous (Pennsylvanian subperiod)
Estimated Age
307 to 309 million years
Preservation Type
Carbonaceous compression with siderite concretion encasement (Mazon Creek style)
Condition Assessment
Fair to Good; the fossil is partially weathered and lacks high-contrast detail, though diagnostic diagnostic structures like the proboscis are visible
Taxonomic Classification
Phylum: Chordata (disputed); Class: Unknown; Order: Unknown; Family: Tullimonstridae; Genus: Tullimonstrum
Morphological Description
The specimen shows an elongated, torpedo-shaped body with evidence of a long, slender proboscis and a distinctive transverse bar (eye bar) typical of the species. Visible features include the faint outline of the body trunk and the distal portion of the proboscis with a small terminal pincer or claw.
Rock Matrix
Ironstone/Siderite concretion with characteristic reddish-brown oxidation on the exterior and greyish-blue unweathered interior
Formation & Location
Mazon Creek Fossil Beds, Francis Creek Shale Member of the Carbondale Formation; Will and Grundy Counties, Illinois, USA
Size & Dimensions
Approximately 4–6 cm in length; representative of a juvenile or small adult specimen compared to the 35 cm maximum for the taxon
Value & Rarity
Estimated Market Value
$150 - $450 USD
Auction Estimate
$200 - $500 USD
Rarity Assessment
Uncommon; while common in specific Mazon Creek localities ('Essex' marine fauna), they are globally rare and found exclusively in this formation
Scientific Significance
Highly significant as it represents one of the most enigmatic organisms in the fossil record; it continues to be the subject of intense debate regarding its classification as either a vertebrate (stem-lamprey) or an invertebrate (mollusk/worm)
Preparation Recommendations
Keep in a low-humidity environment to prevent further oxidation of the siderite matrix. No aggressive mechanical preparation should be attempted on soft-bodied impressions; use only soft brushes and localized stabilization with Paraloid B-72 if flaking occurs.
Notable Features
Features the unique transverse 'eye bar' and the elongated proboscis; the specimen is preserved in a split concretion which is the classic diagnostic presentation for this locality.