Brachiopod (specifically a Productid or Spiriferid type)
Mucrospirifer cf. grabaui or Atrypa sp. • Body fossil/Cast; Invertebrate
Geological Period
Devonian to Mississippian
Estimated Age
360 to 419 million years
Preservation Type
Replacement or recrystallization (calcitic), commonly found as weather-worn valve shells in shoreline deposits.
Condition Assessment
Fair to Good; the exterior ornamentation is visible, but the edges are eroded and it is a single valve rather than a complete articulated specimen.
Taxonomic Classification
Phylum: Brachiopoda, Class: Strophomenata/Articulata, Order: Productida (cf.)
Morphological Description
A single pedicle valve displaying a sub-triangular humped shape with concentric growth lines and faint radial costae. The umbo is moderately incurved, and the hinge line appears wide.
Rock Matrix
Removed; likely originated from Traverse Group limestone or shale common in the Michigan Basin.
Formation & Location
Traverse Group or Dundee Limestone; found near Sterling State Park, Michigan (Lake Erie/Lake Huron drainage).
Size & Dimensions
Approximately 3-5 cm in width, which is typical for adult specimens of Devonian articulate brachiopods.
Value & Rarity
Estimated Market Value
$2.00 - $10.00 USD
Auction Estimate
$5.00 - $15.00 (generally sold in bulk lots)
Rarity Assessment
Common; these are frequently occurring index fossils in Michigan's Paleozoic strata.
Scientific Significance
Provides evidence of the Devonian 'Age of Fishes' marine environment in the Michigan Basin, indicating a shallow, warm epicontinental sea.
Preparation Recommendations
No invasive preparation needed; clean with soft brush and water. Store in a dry, stable environment to prevent shell degradation.
Notable Features
Features distinct concentric growth rings (lamellae) that record the individual's growth phases in ancient sea floor sediments.
Notes
Found at Sterling State Park in Michigan