Internal mold of a Heart Urchin (Spatangoid)
Micraster cf. coranguinum • Body fossil (steinkern/internal mold); Invertebrate (Echinoderm)
Geological Period
Cretaceous (likely Late Cretaceous)
Estimated Age
66 to 100 million years
Preservation Type
Internal mold (steinkern) formed by the lithification of sediment inside the hollow test, which subsequently dissolved away.
Condition Assessment
Fair; the specimen is mostly complete and retains the diagnostic heart shape, but has significant surface weathering, loss of detail, and some modern organic staining.
Taxonomic Classification
Phylum: Echinodermata; Class: Echinoidea; Order: Spatangoida; Family: Micrasteridae (aff.); Genus: Micraster (cf.)
Morphological Description
Heart-shaped (cordiform) morphology with a distinct anterior groove. The specimen lacks the original calcite shell (test), instead showing the internal sediment fill. Some remnants of the ambulacral pathways (petaloids) and the central suture line are visible as faint impressions on the surface.
Rock Matrix
Fine-grained chalky limestone or marl, typical of the Upper Cretaceous 'Chalk' deposits. Some silty residue and lichen growth are present.
Formation & Location
Likely from the Chalk Group of Western Europe (England/France) or the Glen Rose/Edwards Formations of Texas, USA.
Size & Dimensions
Approximately 5cm to 7cm in width; consistent with the typical adult size for mid-sized Spatangoid echinoids.
Value & Rarity
Estimated Market Value
$10.00 - $25.00 USD
Auction Estimate
$15.00 - $35.00 USD (higher if part of a bulk lot)
Rarity Assessment
Common; Heart urchins are found in high abundances in Cretaceous chalk and limestone deposits globally.
Scientific Significance
Provides evidence of benthic infaunal activity in prehistoric seafloor environments. Spatangoids are excellent index fossils for stratigraphy in Chalk deposits and help reconstruct ancient marine depth and substrate conditions.
Preparation Recommendations
Mechanical cleaning with a soft brush and water to remove loose dirt and organic growth. Acid cleaning is discouraged as the mold is likely calcareous and would dissolve. Keep in a dry environment to prevent further weathering.
Notable Features
The specimen clearly demonstrates the 'heart' bilateral symmetry characteristic of irregular echinoids, which evolved to burrow into soft sediment, unlike the radial symmetry of regular sea urchins.