Ammonite (Steinkern)
cf. Cleoniceras sp. or Douvilleiceras sp. (genus level uncertain due to erosion) • Body fossil - Internal Mold (Invertebrate Cephalopod)
Geological Period
Cretaceous (likely Lower Cretaceous)
Estimated Age
100 - 145 million years
Preservation Type
Replacement/Internal Cast (Steinkern). The internal cavity of the shell was filled with sediment and mineralized, after which the original outer shell was lost or dissolved.
Condition Assessment
Fair to Good. While the shape is well-preserved, the lack of fine surface detail, significant weathering, and absence of the original aperture reduce the grade.
Taxonomic Classification
Phylum: Mollusca, Class: Cephalopoda, Order: Ammonitida
Morphological Description
A planispiral, coiled shell showing an involute to semi-evolute whorl pattern. The specimen lacks its original aragonitic shell layer, exposing a smooth internal mold. Vague indications of suture lines (the junctions between septa and the outer shell) may be present but are largely obscured by surface weathering.
Rock Matrix
The specimen is free of matrix, but remnants suggest a silty limestone or calcareous sandstone origin.
Formation & Location
Commonly found in the Albian stage deposits of Madagascar (Mahajanga Basin) or the Gault Clay of England.
Size & Dimensions
Approximately 2.5 - 4.0 cm in diameter; a small to medium-sized representative of typical juvenile or small-species ammonites.
Value & Rarity
Estimated Market Value
$5 - $15 USD
Auction Estimate
$10 - $25 USD (standard retail/gift shop category)
Rarity Assessment
Common. This morphological type and preservation state are found in massive quantities across global Cretaceous deposits.
Scientific Significance
Provides data on the distribution of cephalopods in Mesozoic marine environments and serves as an index fossil for stratigraphic dating of sedimentary layers.
Preparation Recommendations
No further chemical preparation needed. Store in a dry environment and handle with care to avoid chipping the calcified surface. Avoid acidic cleaning agents.
Notable Features
The 'Steinkern' preservation style makes it an excellent tactile specimen for educational purposes, demonstrating how ancient shells were filled with sediment before the shell itself vanished.