Gryphaea (Devil's Toenail)
Gryphaea cf. arcuata • Body fossil; Invertebrate (Bivalve Mollusk)
Geological Period
Jurassic to Cretaceous
Estimated Age
Approx. 201 to 66 million years
Preservation Type
Recrystallization and mineralization. The original aragonite/calcite has been replaced or reinforced by secondary minerals, likely calcite or silica.
Condition Assessment
Fair to Good. The shell shows significant weathering and surface erosion. The primary diagnostic coiled shape is intact, though the fine surface detail is worn.
Taxonomic Classification
Kingdom: Animalia, Phylum: Mollusca, Class: Bivalvia, Order: Ostreida, Family: Gryphaeidae, Genus: Gryphaea
Morphological Description
Characterized by a heavy, thick, calcified shell. The larger lower valve is strongly curved or coiled (planispiral-like), resembling a thick claw or toenail. The smaller upper valve (missing or obscured here) would have acted as a lid. Growth lines are visible as concentric ridges on the outer surface.
Rock Matrix
The specimen is partially freed from a matrix likely consisting of grey limestone or calcareous mudstone. Traces of iron staining (limonite) are visible on the surface.
Formation & Location
Commonly found in the Blue Lias of the UK (Dorset/Somerset) and various marine Jurassic/Cretaceous formations across Europe and North America.
Size & Dimensions
Approximately 4-6 cm in length; typical for mature individuals of this genus.
Value & Rarity
Estimated Market Value
$5.00 - $15.00 USD
Auction Estimate
$10.00 - $25.00 USD (grouped with other specimens)
Rarity Assessment
Common. These are among the most abundant and well-known fossils in Jurassic marine deposits.
Scientific Significance
Gryphaea is a classic example of rapid evolution and adaptation to muddy seafloors. Their curved shells allowed them to sit on soft substrates without sinking, providing insights into Mesozoic benthic marine environments.
Preparation Recommendations
Requires minimal preparation; mild cleaning with a soft brush and water. Avoid acids as they will dissolve the calcareous shell. Seal with a light archival consolidant if dusting occurs.
Notable Features
The specimen displays the classic 'Devil's Toenail' morphology with a high degree of inflation in the left valve, typical of oysters that adapted to life in soft sediment.