Inoceramid Bivalve Shell Fragment
Inoceramus cf. platys / Inoceramus aff. balticus • Body fossil (Calcitic shell fragment); Invertebrate bivalve mollusks
Geological Period
Upper Cretaceous
Estimated Age
66 to 100 million years
Preservation Type
Recrystallization/Unaltered prismatic calcite structure. This type of preservation is unique to Inoceramids because their shells were originally thick, stable calcite rather than aragonite.
Condition Assessment
Fair; the specimen is an incomplete fragment (shred) lacking the hinge or margins, though the internal prismatic structure remains well-preserved.
Taxonomic Classification
Phylum: Mollusca, Class: Bivalvia, Order: Pectinida, Family: Inoceramidae, Genus: Inoceramus or Cataceramus
Morphological Description
A thick, platy fragment of a large bivalve shell showing distinctive prismatic structure. It exhibits clear concentric growth lines (rugae) on the surface, typical of Inoceramid bivalve clams. The shell cross-section reveals elongated calcite crystals perpendicular to the shell surface.
Rock Matrix
No significant matrix attached, but commonly associated with chalk, marl, or calcareous shales.
Formation & Location
Commonly found in the Niobrara Formation (Great Plains, USA) or the Selma Group (SE USA), and equivalent Upper Cretaceous chalk formations in Europe (UK, Germany).
Size & Dimensions
Approximately 4-6 cm in length; this represents a small fragment of what would have been a shell reaching 30-100 cm in diameter.
Value & Rarity
Estimated Market Value
$5.00 - $15.00 USD
Auction Estimate
$10.00 - $25.00 USD (typically sold in bulk lots for study)
Rarity Assessment
Common; such fragments are ubiquitous in Upper Cretaceous marine deposits globally and are frequently found as 'shreds' in chalk outcrops.
Scientific Significance
Inoceramid fragments are valuable index fossils for Cretaceous marine stratigraphy. Their presence indicates oxygenated seafloors or specialized adaptations to low-oxygen environments in ancient epicontinental seas.
Preparation Recommendations
Requires minimal preparation; keep dry as the prismatic structure can delaminate over time. Display away from direct sunlight to prevent further brittleness.
Notable Features
The most notable feature is the 'honeycomb' or 'prismatic' cross-section of the shell, which allows identification of the family even from small, isolated fragments.