Prolecanitid Goniatite (Ammonoid)
Prolecanites cf. henslowi • External mold / Impression of an invertebrate cephalopod
Geological Period
Carboniferous (Mississippian to Pennsylvanian)
Estimated Age
300 to 350 million years
Preservation Type
Impression (external mold) with faint iron oxide (limonite) staining
Condition Assessment
Fair; the fossil is incomplete as it is only a partial external mold and lacks the original shell material or internal suture detail.
Taxonomic Classification
Phylum: Mollusca, Class: Cephalopoda, Order: Goniatitida, Family: Prolecanitidae
Morphological Description
Small planispiral shell showing a tight inner whorl and a wide, shallowly depressed outer whorl. The specimen is an external negative impression. It displays a smooth shell surface lacking complex ornamentation, which is characteristic of primitive goniatites.
Rock Matrix
Fine-grained silty mudstone or shale with significant ferruginous (iron-rich) weathering
Formation & Location
Likely from the Moorefield Formation or similar Carboniferous shales of the American Midwest (Arkansas/Oklahoma) or similar deposits in the UK/Europe.
Size & Dimensions
Approximately 10-12mm in diameter; typical for juvenile or small-species goniatites found in these formations.
Value & Rarity
Estimated Market Value
$5.00 - $15.00 USD
Auction Estimate
$10.00 - $20.00 USD (typically sold in bulk lots)
Rarity Assessment
Common; goniatites of this size are very frequently found in Paleozoic marine shale units.
Scientific Significance
Provides evidence of deep-time marine paleoecology; goniatites are essential index fossils for stratigraphic dating of the Paleozoic era.
Preparation Recommendations
No further mechanical preparation suggested as the rock is brittle. Keep in a dry environment to prevent further oxidation of iron minerals in the matrix.
Notable Features
The specimen is a 'negative' mold, meaning it is the impression left in the mud rather than the shell itself. The brownish-orange color indicates the presence of secondary limonite or siderite mineralization.