Brachiopod (Lamp Shell)
Spiriferid brachiopod (cf. Mucrospirifer or Mediospirifer species) • Body fossil; Marine Invertebrate
Geological Period
Devonian (likely Middle to Late Devonian)
Estimated Age
360 to 419 million years
Preservation Type
Recrystallized calcitic replacement with some surface erosion; likely shell material or a very detailed cast of the original shell.
Condition Assessment
Fair to Good; the hinge line is mostly intact, but the 'wings' show some terminal erosion and the surface detail is slightly weathered due to exposure in a beach/state park environment.
Taxonomic Classification
Phylum: Brachiopoda, Class: Rhynchonellata, Order: Spiriferida, Family: Spiriferidae
Morphological Description
Semicircular to sub-triangular shape with a characteristic long, straight hinge line (alate or 'winged' form). Visible radial costae (ribbing) extending from the umbo to the commissure. Features a central fold and sulcus typical of spiriferid brachiopods.
Rock Matrix
Limestone/Dolomite-rich environment; common in the Michigan Basin formations. The specimen appears largely freed from its matrix but shows calcitic weathering.
Formation & Location
Likely from the Traverse Group or Dundee Limestone. Commonly found in coastal deposits and gravels near Lake Erie/Sterling State Park in Michigan.
Size & Dimensions
Approximately 2.5 to 4 cm in width; this is a typical size range for adult spiriferids of this genus.
Value & Rarity
Estimated Market Value
$5.00 - $15.00
Auction Estimate
$10.00 - $25.00 (usually sold as part of a lot)
Rarity Assessment
Common; spiriferid brachiopods are among the most abundant marine fossils in Devonian strata across North America.
Scientific Significance
Provides evidence of the Devonian 'Age of Fishes' marine environment in the Michigan Basin, a time when the region was covered by a warm, shallow epicontinental sea.
Preparation Recommendations
Keep dry and store in a padded box to prevent further chipping. Can be cleaned gently with a soft brush and water if needed, but avoid harsh acids which will dissolve the calcite.
Notable Features
Distinctively wide hinge line and well-defined radial ornamentation; the symmetry of the valve suggests it is likely a ventral valve.
Notes
Found at Sterling State Park in Michigan.