Pyritized Gomphoid Sponge (Desmas)
cf. Jereica punctata or aff. Coeloptychium sp. (individual desmas cluster) • Body fossil (Invertebrate); specifically a silicious sponge replaced by pyrite
Geological Period
Cretaceous (Upper/Late Cretaceous, specifically Campanian stage)
Estimated Age
72 to 83 million years
Preservation Type
Permineralization and total replacement; the original organic and silica structure has been replaced by iron sulfide (pyrite/marcasite).
Condition Assessment
Good to Very Good; shows high structural integrity with minimal oxidation ('pyrite rot'), though some fine surface detail is obscured by the heavy mineralization.
Taxonomic Classification
Phylum: Porifera; Class: Demospongiae; Order: Lithistida; Family: Tetracladinidae
Morphological Description
Globular to sub-spherical nodular form composed of interlocking, radiating skeletal structures known as desmas. The surface exhibits a rugged, botryoidal texture characterized by clustered, pointed mineralized protrusions following the original sponge canal system.
Rock Matrix
Likely originated from a Chalk or Marl matrix (e.g., White Chalk Subgroup), though the matrix has been entirely removed by erosion or preparation.
Formation & Location
Commonly found in the Gault Clay or White Chalk formations of England (e.g., Kent or Sussex) and Northern Germany (e.g., Misburg/Höver quarries).
Size & Dimensions
Approximately 2.5 to 4 cm in diameter; consistent with average size for pyritic sponge nodules from European chalk deposits.
Value & Rarity
Estimated Market Value
$15 - $45 USD
Auction Estimate
$20 - $60 USD (usually sold in small lots or as part of a teaching collection)
Rarity Assessment
Uncommon; while pyrite nodules are frequent, well-defined pseudomorphs after specific sponge structures are less common than ammonites or echinoids in similar preservation.
Scientific Significance
Illustrates the process of authigenic mineral replacement in low-oxygen seafloor environments; provides data on Cretaceous sponge diversity and taphonomic processes in deep-water carbonate settings.
Preparation Recommendations
Keep in a low-humidity environment (below 45% RH) to prevent pyrite oxidation. Do not clean with water; use a soft brush and consider a coating of Paraloid B-72 if signs of 'pyrite disease' (white powder) appear.
Notable Features
Total replacement by pyrite/marcasite resulting in a 'metallic' appearance while retaining the 'brain-like' or 'pinecone' texture of the original sponge desmas.