Polished Ammonite Slice/Cross-section
Desmoceras sp. (cf. Desmoceras latidorsatum) • Body fossil; Invertebrate (Cephalopod); Calcite replacement/Permineralization
Geological Period
Cretaceous (likely Albian stage)
Estimated Age
100 to 113 million years
Preservation Type
Permineralization and replacement. The original aragonite septa and shell have been replaced by translucent honey-colored calcite crystals.
Condition Assessment
Good/Very Good. As a decorative polished slice, it is incomplete (missing half the shell) but provides excellent visibility of internal anatomical structures.
Taxonomic Classification
Phylum: Mollusca, Class: Cephalopoda, Order: Ammonitida, Family: Desmoceratidae, Genus: Desmoceras
Morphological Description
Planispiral coiled shell showing internal phragmocone. The cross-section reveals calcite-filled chambers (camerae) separated by curved septa. The central protoconch is visible, and the outer whorls increase in width towards the body chamber. Siphuncle position is marginal; shell is evolute to involute.
Rock Matrix
Matrix-free; the specimen has been extracted and polished. Likely originated from a fine-grained sedimentary limestone or marl node.
Formation & Location
Likely the Mahajanga Basin of Madagascar, which is world-renowned for mass-produced polished Desmoceras ammonites.
Size & Dimensions
Approximately 3 cm to 5 cm in diameter based on typical commercial specimens of this quality.
Value & Rarity
Estimated Market Value
$10.00 - $25.00 USD
Auction Estimate
$15.00 - $40.00 USD (typically sold in lots)
Rarity Assessment
Common. These are among the most frequently traded fossils globally due to high-volume commercial mining in Madagascar.
Scientific Significance
Provides insights into cephalopod buoyancy through the study of chamber volume and septal spacing. Helps map mid-Cretaceous marine corridors and paleobiogeography.
Preparation Recommendations
No further preparation needed as it is already cut and polished. Store out of direct sunlight to prevent calcite fading; clean with a dry microfibre cloth only.
Notable Features
Features high-clarity calcite mineralization within the chambers, creating a 'stained glass' effect when backlit. Distinctive suture patterns are visible where the septa meet the outer shell wall.