Spiriferid Brachiopod
Mucrospirifer cf. mucronatus • Invertebrate Body Fossil (Shell)
Geological Period
Devonian
Estimated Age
360 - 419 million years ago
Preservation Type
Silicification/Replacement. The original calcium carbonate shell has likely been replaced by silica (chert or quartz) or minerals from the surrounding matrix, typical of Michigan Devonian finds.
Condition Assessment
Good. The specimen shows significant weathering and rounding on the right wing due to wave action, but the diagnostic hinge and ribbing on the left side remain distinct.
Taxonomic Classification
Phylum: Brachiopoda, Class: Rhynchonellata, Order: Spiriferida, Family: Spiriferidae
Morphological Description
A winged or 'butterfly-shaped' biconvex shell featuring a long, straight hinge line. Visible features include a prominent medial fold and sulcus and distinct costae (ribbing) radiating from the beak to the anterior margin. The width is significantly greater than the length.
Rock Matrix
Dense, gray bioclast-rich limestone or calcareous shale, common to Michigan coastal drift deposits.
Formation & Location
Likely derived from the Traverse Group or Dundee Limestone of the Michigan Basin; commonly found as glacial drift at Sterling State Park along Lake Erie.
Size & Dimensions
Approximately 2.5 - 4 cm in width; this is a typical size for an adult specimen of this genus.
Value & Rarity
Estimated Market Value
$5.00 - $15.00 USD
Auction Estimate
$10.00 - $25.00 USD (typically sold in bulk lots)
Rarity Assessment
Common. Mucrospirifer is one of the most abundant fossils in the Devonian strata of the Great Lakes region.
Scientific Significance
Provides evidence of the warm, shallow epicontinental seas that covered the Michigan Basin during the Middle Devonian. These organisms were benthic filter feeders and are key index fossils for stratigraphic correlation.
Preparation Recommendations
Requires minimal preparation. Cleaning with a soft brush and water is sufficient. Should be stored in a dry environment; avoid acid cleaning as it may damage the mineralized surface.
Notable Features
The elongated 'wing' tips (mucronations) are characteristic of the 'butterfly' brachiopods, though they are often broken or worn away by erosion in beach-found specimens.
Notes
Found at Sterling State Park beach area in Michigan. 5-7-2026