Gryphaea (Devil's Toenail)
Gryphaea cf. arcuata • Body fossil; Invertebrate (Bivalve Mollusk)
Geological Period
Jurassic (frequently Lower Jurassic/Lias)
Estimated Age
145 to 201 million years
Preservation Type
Recrystallization and replacement (calcite replacement of the original aragonitic/calcitic shell)
Condition Assessment
Fair to Good; the coiled left valve is mostly complete, though the shell surface is somewhat eroded and the smaller right valve appears missing or obscured
Taxonomic Classification
Phylum: Mollusca, Class: Bivalvia, Order: Ostreida, Family: Gryphaeidae, Genus: Gryphaea
Morphological Description
Characterized by a heavy, thick, planispirally coiled left valve (the 'toenail') and a smaller, flat right valve (operculum). The specimen shows distinct growth lines (rugae) and a prominent incurved umbo.
Rock Matrix
Fine-grained grey limestone or calcareous mudstone/shale remnants within the crevices
Formation & Location
Commonly found in the Blue Lias of the UK (Dorset/Gloucestershire) and similar Jurassic marine deposits across Europe and North America
Size & Dimensions
Approximately 4-6 cm in length; typical for a mature adult of the species Gryphaea arcuata
Value & Rarity
Estimated Market Value
$5 - $15 USD
Auction Estimate
$10 - $20 USD (often sold in bulk or study lots)
Rarity Assessment
Common; Gryphaea are among the most abundant and well-known Jurassic invertebrate fossils
Scientific Significance
Significant as an index fossil for the Jurassic period; provides evidence of rapid evolution in shell morphology and adaptation to soft, muddy seafloor environments
Preparation Recommendations
Mechanical cleaning with a soft brush and water; avoid strong acids which can damage the calcitic shell; store in a dry environment to prevent matrix degradation
Notable Features
Distinctive 'toenail' curvature and visible overlapping growth lamellae; one section shows possible internal chambering or suture-like patterns due to surface wear