Atrypa Brachiopod
Atrypa cf. reticularis • Body fossil, Invertebrate
Geological Period
Devonian
Estimated Age
380-410 million years
Preservation Type
Recrystallization/Replacement; likely calcium carbonate or silica replacement of the original shell material.
Condition Assessment
Good; the specimen shows clear growth lines and the overall form is intact, though the edges show some signs of prehistoric mechanical wear or more recent weathering.
Taxonomic Classification
Phylum: Brachiopoda, Class: Rhynchonellata, Order: Atrypida, Family: Atrypidae, Genus: Atrypa
Morphological Description
Biconvex shell with a rounded triangular outline. Visible growth lamellae (concentric rings) on the valve surface. The pedicle valve is slightly more convex than the brachial valve. Lacks the typical radial ribbing of some younger species, suggesting a smoother-shelled variety or environmental wear.
Rock Matrix
Commonly found in Devonian limestone or shale; though this specimen appears 'clean', it likely originated from the Traverse Group or Dundee Limestone.
Formation & Location
Likely the Dundee Limestone or Traverse Group fossils commonly found in the Michigan Basin and around the Great Lakes.
Size & Dimensions
Approximately 2-3 cm in width, which is typical for adult specimens of this genus.
Value & Rarity
Estimated Market Value
$3.00 - $10.00 USD
Auction Estimate
$10.00 - $20.00 USD (typically sold in bulk lots for natural history auctions)
Rarity Assessment
Common; Atrypa is one of the most prolific and widely distributed brachiopod genera in Devonian strata worldwide.
Scientific Significance
Brachiopods are important index fossils for the Paleozoic era. This genus specifically helps define Devonian marine environments, indicating shallow, warm coral reef ecosystems.
Preparation Recommendations
Requires minimal intervention. Dry brushing to remove surface debris. Avoid acidic cleaners which will dissolve the calcium-based shell. Store in a padded specimen box or acid-free tray.
Notable Features
Prominent concentric growth lines that mark seasonal or environmental growth pauses; smooth surface implies a specimen from a higher-energy environment where fine ribbing may have been abraded or was evolutionarily absent.
Notes
Found at Sterling state park in Michigan