Belemite Rostrum (Thunderbolt Stone)
Belemnitella cf. mucronata or Hibolites sp. • Body fossil, Invertebrate Cephalopod (internal skeleton)
Geological Period
Jurassic to Late Cretaceous
Estimated Age
66 to 200 million years
Preservation Type
Replacement and recrystallization; originally composed of aragonite, now replaced by robust calcite.
Condition Assessment
Good; the specimen is largely complete but shows signs of surface weathering and erosion at the tips.
Taxonomic Classification
Phylum: Mollusca, Class: Cephalopoda, Order: Belemnitida, Family: Belemnitidae
Morphological Description
A cylindrical, bullet-shaped calcitic structure known as a rostrum. It is smooth, slightly tapered at one end (proximal) and rounded at the other (distal). The cross-section is circular to slightly oval. No visible phragmocone chambers are present in this specific view.
Rock Matrix
None visible (isolated specimen), though typically found in chalk, marl, or limestone formations.
Formation & Location
Common in the Chalk Group of Western Europe (UK, France, Germany) and the Western Interior Seaway of North America.
Size & Dimensions
Approximately 8-10 cm in length and 1.5 cm in diameter; typical for secondary rostrum growth stages.
Value & Rarity
Estimated Market Value
$10 - $25 USD
Auction Estimate
$15 - $40 USD (usually sold in bulk lots at auction)
Rarity Assessment
Common; found in high concentrations in 'belemnite battlefields' across global Mesozoic marine deposits.
Scientific Significance
Belemnites are important index fossils used for biostratigraphy. Their isotopic composition is frequently used to study paleotemperatures of Mesozoic oceans.
Preparation Recommendations
Minimal preparation required. Clean with soft brush and water. Store in a dry environment; avoid acid-based cleaners which will dissolve the calcite.
Notable Features
The specimen exhibits a classic 'cigar' shape with a subtle longitudinal groove (alveolus) often characteristic of the genus Hibolites.