sedimentary

Fossil Brachiopod Cast (Atrypa species)

Phylum Brachiopoda, Class Rhynchonellata, Order Atrypida

AI-generated identification · may be incorrect

Hardness: 3 (calcite) to 7 (if silicified); Color: Tan, beige, or grey; Luster: Dull or earthy; Structure: Biconvex with distinctive radial ribbing (costae); Cleavage: None; Specific Gravity: 2.7

Hardness: 3 (calcite) to 7 (if silicified)
Color: Tan, beige, or grey
Luster: Dull or earthy

Identified March 22, 2026More sedimentary →

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Physical properties

Hardness: 3 (calcite) to 7 (if silicified); Color: Tan, beige, or grey; Luster: Dull or earthy; Structure: Biconvex with distinctive radial ribbing (costae); Cleavage: None; Specific Gravity: 2.7

Formation & geological history

Formed through the process of permineralization or replacement in marine environments during the Paleozoic era (approx. 430-360 million years ago). They lived in shallow, warm seas anchored to the seafloor.

Uses & applications

Primarily used as index fossils for geological dating, educational specimens in museums, and as decorative items for amateur fossil collectors.

Geological facts

Brachiopods are often mistaken for clams, but unlike clams (which have left and right shells), brachiopods have upper and lower shells. The Atrypida group became extinct during the Late Devonian mass extinction event.

Field identification & locations

Identify in the field by the symmetrical shell shape along the midline and the presence of radial ribs. Commonly found in limestone or shale outcrops in the Midwest USA and worldwide. Look for 'lamp-shell' profiles.