sedimentary (fossilized organic remains)

Brachiopod Fossil (Spiriferid)

Phylum Brachiopoda, Order Spiriferida (CaCO3 - Calcium Carbonate)

Hardness: 3 (calcite composition); Color: Tan, grey, or off-white; Luster: Dull to earthy; Shape: Fan-like wing shape with distinct radiating ribs (costae) and a central fold/sulcus; Structure: Bilateral symmetry across the valves.

Hardness: 3 (calcite composition)
Color: Tan, grey, or off-white
Luster: Dull to earthy

Identified March 7, 2026More sedimentary (fossilized organic remains) →

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Physical properties

Formation & geological history

Formed through permineralization or recrystallization of marine organisms in low-energy seafloor environments. Predominantly from the Paleozoic Era (Devonian to Carboniferous periods), dating back 300 to 400 million years.

Uses & applications

Used primarily as index fossils for dating rock strata, educational scientific specimens, and as items for geological hobbyists and collectors.

Geological facts

Spiriferid brachiopods are often called 'butterfly shells' due to their winged appearance. Unlike modern clams, they are not mollusks; they belong to a separate phylum and filtered water using a unique organ called a lophophore.

Field identification & locations

Identify in the field by looking for the 'winged' hinge line and symmetrical ribbing in limestone or shale outcrops. Often found in the Great Lakes region (USA), Kentucky, and the UK. Collectors should look for complete bilateral symmetry which distinguishes them from bivalves.

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