Brachiopod (Petrified/Silicified Replacement)
Mucrospirifer cf. mucronatus • Body fossil, Invertebrate
Geological Period
Devonian Period
Estimated Age
360 - 410 million years
Preservation Type
Silicified replacement (chalcedony/agate) resulting in a translucent appearance
Condition Assessment
Fair to Good; while chemically replaced and beautiful, it is heavily water-worn and lacks fine surface detail or Sharp 'wings' due to erosion.
Taxonomic Classification
Phylum: Brachiopoda, Class: Rhynchonellata, Order: Spiriferida, Family: Mucrospiriferidae
Morphological Description
A winged or 'butterfly' shaped shell showing high biconvexity. Visible features include a prominent fold and sulcus and distinct costae (ribbing) radiating from the umbo. The specimen shows significant water-wear, smoothing the typical sharp edges of the wings.
Rock Matrix
Missing; the specimen is a weathered 'beach pebble' fossil, likely originally from a limestone matrix such as the Traverse Group.
Formation & Location
Likely from the Traverse Group or Dundee Limestone of Michigan, found as secondary deposit at Sterling State Park on Lake Erie.
Size & Dimensions
Approximately 3-4 cm in width; typical size for a mature Mucrospirifer specimen.
Value & Rarity
Estimated Market Value
$5.00 - $15.00 USD
Auction Estimate
$10.00 - $20.00 USD (typically sold in bulk lots)
Rarity Assessment
Common; brachiopods are the most abundant fossils in Michigan's Devonian strata, though silicified translucent examples are aesthetically pleasing.
Scientific Significance
Provides evidence of the Middle Devonian tropical seas that once covered Michigan; representative of the diverse brachiopod fauna of the Michigan Basin.
Preparation Recommendations
No mechanical preparation needed. Occasional light mineral oil rub can enhance translucency; store in a dry, padded container to prevent chipping of the silica shell.
Notable Features
The most striking feature is the chalcedony replacement, which gives the fossil a gemstone-like quality, allowing light to pass through the edges of what was once an organic shell.
Notes
Found at Sterling State Park Michigan