Steinkern of a Bivalve (Fossil Shell Cast)
cf. Trigonia sp. or aff. Cucullaea sp. (internal mold, exact genus difficult to confirm without surface shell ornamentation) • Internal mold (steinkern) body fossil; Invertebrate Mollusk
Geological Period
Cretaceous / Jurassic (likely Paleozoic to Mesozoic)
Estimated Age
66 to 200 million years old
Preservation Type
Internal mold (steinkern) formed by fine sediment filling the interior cavity of a bivalve shell prior to the dissolution of the original aragonitic shell material.
Condition Assessment
Good. Complete steinkern with well-preserved outline, hinge ridge, and three-dimensional profile, though lacking outer shell details due to mold preservation.
Taxonomic Classification
Phylum Mollusca, Class Bivalvia, Order Trigoniida / Venerida / Pectinida
Morphological Description
Elongated, sub-trigonal to almond-shaped internal mold showing distinct bivalve bilateral symmetry along the hinge line. A prominent umbonal ridge runs longitudinally, with faint concentric growth or muscular attachment impressions preserved on the surface. Dark, fine dendritic manganese oxide patterns (dendrites) vein across the light tan-grey surface.
Rock Matrix
Fine-grained micritic limestone / calcareous siltstone matrix.
Formation & Location
Commonly found in marine limestone deposits worldwide, such as the Glen Rose Formation (Texas, USA), European Jurassic/Cretaceous chalk and limestone beds, or South American Mesozoic formations.
Size & Dimensions
Approximately 5 to 7 cm in length, consistent with typical mid-sized Mesozoic marine bivalves.
Value & Rarity
Estimated Market Value
$10 - $25 USD
Auction Estimate
$15 - $35 USD
Rarity Assessment
Common; internal molds of marine bivalves are abundant in sedimentary marine formations globally.
Scientific Significance
Provides evidence of ancient benthic marine environments, shallow-water shelf deposits, and local taphonomic conditions leading to selective shell dissolution.
Preparation Recommendations
No heavy mechanical preparation required. Store in a dry, low-humidity environment to prevent degradation of minor mineral inclusions. Clean gently with a soft dry brush.
Notable Features
Features prominent dark manganese dendritic mineral formations branching across the exterior surface, creating an aesthetically appealing tree-like contrast against the pale limestone cast.