Polished Sliced Ammonite
Cleoniceras cf. besairiei • Body fossil (Internal Mold/Replacement) - Invertebrate Cephalopod
Geological Period
Cretaceous (likely Albian stage)
Estimated Age
Approx. 100 to 112 million years
Preservation Type
Permineralization and Replacement; internal cavities are often filled with brownish calcite or druzy quartz crystals, with the original aragonite shell typically replaced or removed through polishing.
Condition Assessment
Very Good; the polishing clearly highlights the internal anatomy. The specimen is a 'half' or 'sliced' ammonite, representing a clean cross-section of the organism.
Taxonomic Classification
Phylum: Mollusca, Class: Cephalopoda, Subclass: Ammonoidea, Order: Ammonitida, Family: Desmoceratidae, Genus: Cleoniceras
Morphological Description
Planispiral, involute shell showing internal chambers (septa). The specimen has been cut and polished to reveal the internal phragmocone filled with calcite crystals. Siphuncle position and simple, curved suture lines are visible where the septa meet the outer wall.
Rock Matrix
Originally found in a hard marly limestone or ironstone nodule, but the specimen is now free of matrix and has been polished to a high gloss.
Formation & Location
Tulear (Toliara) region, Mahajanga Basin, Madagascar
Size & Dimensions
Approximately 2.5 to 4 cm in diameter; small for the genus but a typical size for commercially prepared 'pocket' specimens.
Value & Rarity
Estimated Market Value
$5.00 - $15.00 USD
Auction Estimate
$10.00 - $25.00 USD (typically sold in larger wholesale lots or specialized décor auctions)
Rarity Assessment
Common; specifically within the Madagascan export market where these are produced in massive quantities for the global gemstone and fossil trade.
Scientific Significance
Provides diagnostic evidence of marine cephalopod evolution and paleoecology. This genus is an index fossil used for biostratigraphy in the Lower Cretaceous. It demonstrates mineral replacement processes in ancient marine environments.
Preparation Recommendations
No additional preparation needed as it is already cut and polished. Keep away from acidic cleaners which will dissolve the calcite/limestone components. Display out of direct sunlight to prevent potential color fading of deep brown minerals.
Notable Features
Features a high-contrast 'honey' calcite mineralization within the chambers. The distinct spiral growth perfectly illustrates the Fibonacci sequence often found in nature.