sedimentary

Brachiopod Fossil in Limestone

Phylum Brachiopoda (likely Order Spiriferida), CaCO3 (Calcite)

Hardness: 3 (matrix) to 4 (fossil); Color: Tan, cream, or light grey with dark brown mineral staining; Luster: Dull/Earthy; Crystal Structure: Trigonal (as calcite replacement); Cleavage: Perfect rhombohedral in matrix crystals.

Hardness: 3 (matrix) to 4 (fossil)
Color: Tan, cream, or light grey with dark brown mineral staining
Luster: Dull/Earthy

Identified March 7, 2026More sedimentary →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Formed through the accumulation of marine organic debris and calcium carbonate on the seafloor during the Paleozoic era (approx. 250-500 million years ago); the shell was buried in sediment and lithified into rock over millions of years.

Uses & applications

Educational purposes, personal fossil collecting, and scientific research into paleoecology. The limestone matrix is used industrially for cement and building materials.

Geological facts

Brachiopods are often called 'lamp shells' because they resemble ancient oil lamps. Although they look like clams, they are not mollusks; they belong to their own distinct phylum and possess a unique feeding organ called a lophophore.

Field identification & locations

Identify by looking for the symmetrical 'fan' shape with radial ribbing (costae) embedded in a soft sedimentary matrix. Found worldwide in areas with exposed Paleozoic or Mesozoic sedimentary strata, such as riverbeds and roadcuts in the Midwest USA.

Other sedimentary specimens

See all

Sandstone or Siltstone

Arenite (if sandstone)

Sedimentary

Sandstone

Arenite (primarily SiO2)

sedimentary

Chert Breccia

Brecciated Chert (Microcrystalline Silica)

sedimentary

Sandstone Grain

Clastic Sedimentary Rock Grain (SiO2 dominated)

sedimentary

Shale or Slaty Mudstone

Argillaceous sedimentary rock

sedimentary

Sandstone

Arenite

sedimentary