sedimentary (fossilized organic remain)

Brachiopod Fossil (Spiriferid)

Order Spiriferida (Phylum Brachiopoda); mineralized as Calcite (CaCO3) or Chert (SiO2)

Hardness: 3 (if calcite) to 7 (if silicified); Color: Tan, grey, or cream; Luster: Dull to earthy; Structure: Bilateral symmetry with a 'wing-like' shape and distinct radial ribbing (costae).

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

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

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Physical properties

Hardness: 3 (if calcite) to 7 (if silicified); Color: Tan, grey, or cream; Luster: Dull to earthy; Structure: Bilateral symmetry with a 'wing-like' shape and distinct radial ribbing (costae).

Formation & geological history

Formed in marine environments during the Paleozoic era (approx. 450 to 250 million years ago). They were bottom-dwelling filter feeders that became buried in sediment, with their shells mineralizing over millions of years.

Uses & applications

Primarily used as 'index fossils' to date rock layers; also collected for educational purposes, personal collections, and as decorative natural history items.

Geological facts

Spiriferids are often called 'butterfly shells' due to their wing-like shape. While they look like clams (clams are mollusks), brachiopods have a completely different internal anatomy and symmetry axis.

Field identification & locations

Identify by looking for the 'hinge line' and the characteristic fold and sulcus (a dip in the middle). Common in limestone outcrops of the Midwestern USA, the UK, and Russia. Collectors should look in riverbeds or road cuts through sedimentary layers.

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