Rudist Bivalve Internal Mold (Steinkern)
Rudist bivalve cf. Requienia or Caprinula • Internal mold / steinkern, invertebrate body fossil
Geological Period
Cretaceous
Estimated Age
100 to 66 million years ago
Preservation Type
Internal mold (steinkern) formed by sediment filling the shell cavity, followed by complete dissolution or replacement of the original aragonitic shell material.
Condition Assessment
Good condition for a weathered surface find; solid and complete as a steinkern, though showing natural water-worn or weathering smoothing on the outer surfaces.
Taxonomic Classification
Phylum Mollusca, Class Bivalvia, Order Hippuritida, Family Requieniidae or Caprinidae
Morphological Description
The specimen exhibits a deeply indented, asymmetrical, hollowed-out morphology typical of an internal mold (steinkern) of a reef-building rudist bivalve. It shows smooth, weathered contours with a distinct curved indentation where the opposing valve or internal cavity once impressed into the soft sediment, now lithified.
Rock Matrix
Buff to tan colored Cretaceous limestone, typical of central Texas hill country carbonate deposits.
Formation & Location
Fredericksburg Group or Edwards Formation, Kerr County, Texas
Size & Dimensions
Approximately 4 to 5 inches across, scaling against the US nickel placed nearby for reference, which is standard for medium-sized rudist steinkerns from this region.
Value & Rarity
Estimated Market Value
$20 to $50
Auction Estimate
$15 to $40
Rarity Assessment
Common throughout the Cretaceous limestone outcrops of central Texas.
Scientific Significance
Demonstrates the presence of ancient Cretaceous shallow-water carbonate platforms and rudist reef ecosystems that once covered much of Texas, providing insights into Mesozoic tropical marine paleoenvironments.
Preparation Recommendations
No intensive preparation required. Gentle washing with water and a soft brush to remove surface dirt is sufficient. Store in a stable, dry environment.
Notable Features
The pronounced natural negative impression creating a bridge-like or arched structure, formed by the unique asymmetrical valve growth habit of Cretaceous rudist clams.
Notes
Kerr county Texas