Devil's Toenail
Gryphaea arcuata • Body fossil (Invertebrate Bivalve Mollusk)
Geological Period
Jurassic (specifically Lower Jurassic)
Estimated Age
180 - 200 million years
Preservation Type
Recrystallization (calcitic preservation) with internal sediment filling.
Condition Assessment
Fair to Good. The primary left valve is mostly complete, though the distal edges show some erosion and the smaller right valve is missing.
Taxonomic Classification
Phylum: Mollusca, Class: Bivalvia, Order: Ostreida, Family: Gryphaeidae, Genus: Gryphaea
Morphological Description
Characterized by a thick, heavy, strongly curved left valve resembling a hook or claw. The shell shows distinct growth lines (lamellae) and a smaller, lid-like right valve (absent in this specimen). The coiled umbo is highly prominent.
Rock Matrix
Typical of argillaceous limestone or grey shale; the interior appears to be filled with a fine-grained micritic mudstone.
Formation & Location
Blue Lias Formation of the United Kingdom (Dorset/Somerset coast) or similar Jurassic marine deposits in Europe.
Size & Dimensions
Approximately 5-8 cm in length; typical for mature individuals of this species.
Value & Rarity
Estimated Market Value
$10.00 - $25.00 USD
Auction Estimate
$15.00 - $40.00 USD (typically sold in small lots)
Rarity Assessment
Common; widely distributed and abundant in Jurassic marine strata.
Scientific Significance
Gryphaea is a classic example of iterative evolution and adaptation to soft, muddy seafloors. Its shape allowed it to remain stable in silt without sinking or being overturned by currents.
Preparation Recommendations
Mechanical cleaning with a soft brush and water. If matrix is stubborn, a mild acetic acid bath could be used, but with caution to avoid damaging the calcitic shell. Store in a dry environment.
Notable Features
The specimen clearly demonstrates the 'coiling' of the shell that led to the folklore name 'Devil's Toenail.' The interior cavity shows the attachment area for the adductor muscle.