sedimentary
Fossil Brachiopod Cast (Atrypa species)
Phylum Brachiopoda, Class Rhynchonellata, Order Atrypida
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
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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.
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