Brachiopod (Lamp Shell)
Mucrospirifer cf. mucronatus • Body fossil; Marine invertebrate
Geological Period
Devonian (specifically likely Middle Devonian)
Estimated Age
380 to 410 million years
Preservation Type
Recrystallization/Replacement; likely calcitic or slightly silicified preservation typical of Michigan basin specimens.
Condition Assessment
Good to Fair. The specimen shows significant weathering and rounding on the wing-like extremities (mucronations) due to water/beach abrasion, though the central rib structure remains clear.
Taxonomic Classification
Phylum: Brachiopoda; Class: Rhynchonellata; Order: Spiriferida; Family: Mucrospiriferidae; Genus: Mucrospirifer
Morphological Description
Specimen exhibits a characteristic 'butterfly' or semi-elliptical shape with a straight, elongated hinge line. Visible features include a prominent medial fold and sulcus, and numerous fine radiating costae (ribs) that originate from the beak and fan outwards toward the commissure.
Rock Matrix
Small traces of grey calcareous shale or limestone; the specimen appears relatively clean indicating it was weathered out of its original matrix by wave action.
Formation & Location
Commonly associated with the Traverse Group or Dundee Limestone. Location (Sterling State Park, MI) suggests material potentially transported by glacial drift or sourced from the nearby Devonian outcrops of the Michigan Basin.
Size & Dimensions
Approximately 2.5 to 3.5 cm in width; this is a standard size for a mature Mucrospirifer specimen.
Value & Rarity
Estimated Market Value
$3.00 - $8.00 USD
Auction Estimate
$5.00 - $15.00 USD (typically sold as part of a lot)
Rarity Assessment
Common. This is one of the most prolific and easily identifiable fossils found in the Devonian strata of the Midwest United States.
Scientific Significance
Provides evidence of the shallow, subtropical epicontinental seas that covered Michigan during the Devonian. These fossils are key index fossils for stratigraphic correlation in the Great Lakes region.
Preparation Recommendations
No heavy preparation needed. Gentle cleaning with a soft toothbrush and water can remove sand. Store in a dry, stable environment; no specialized consolidants are required given the robust mineralization.
Notable Features
The specimen is a classic example of a 'winged' brachiopod. Despite the beach wear, the primary diagnostic hinge line and fold are well-preserved enough for genus-level identification.
Notes
Found at Sterling State Park Beach area. 5-7-2026