Horn Coral (Rugose Coral)
cf. Grewingkia canadensis or Streptelasma sp. • Body fossil (Invertebrate, Phylum Cnidaria, Order Rugosa)
Geological Period
Middle to Upper Ordovician
Estimated Age
450 - 460 million years
Preservation Type
Silicification and recrystallization (calcite replaced partially by silica/chert, with iron-oxide staining)
Condition Assessment
Fair to Good. Partial specimen showing internal septal structures exposed by natural weathering and fracture, with minor iron staining and matrix attachment.
Taxonomic Classification
Kingdom: Animalia, Phylum: Cnidaria, Class: Anthozoa, Subclass: Tetracorallia, Order: Rugosa, Family: Streptelasmatidae, Genus: Streptelasma or Grewingkia
Morphological Description
Cross-sectional and longitudinal view of a solitary rugose (horn) coral. Shows prominent radial septa radiating outward from the central calice, crossed by horizontal tabulae and dissepiments that form a ladder-like interior structure. The outer conical wall (epitheca) is weathered away in areas, exposing internal structures.
Rock Matrix
Dense argillaceous limestone / cherty limestone matrix typical of the Ordovician Nashville Group in Middle Tennessee, heavily stained with limonite and iron oxides.
Formation & Location
Bigby-Cannon Limestone or Hermitage Formation, Stones River Group / Nashville Group, Marshall County, Inner Nashville Basin, Tennessee, USA.
Size & Dimensions
Approximately 6 to 8 cm in length and 5 to 6 cm in width, consistent with medium-to-large Ordovician solitary rugosan corals.
Value & Rarity
Estimated Market Value
$15 - $35 USD
Auction Estimate
$20 - $40 USD
Rarity Assessment
Common. Solitary horn corals are among the most abundant invertebrate fossils found within the Ordovician strata of the Nashville Dome region in Middle Tennessee.
Scientific Significance
Illustrates early Paleozoic benthic marine reef communities in the Laurentian shallow epicontinental seas. Rugose corals were major framework builders and key paleoenvironmental indicators for warm, clear, shallow marine environments during the Ordovician.
Preparation Recommendations
Light mechanical cleaning with a soft brush and water to remove loose dirt from pond excavation. Avoid harsh acids (like HCl) as they may damage calcite structures if present alongside silica. Display in a dry environment with standard stable humidity.
Notable Features
Exposed internal longitudinal and transverse septal architecture visible due to weathering; distinctive honey-yellow and iron-brown coloration resulting from limonite mineralization.
Notes
Found in bottom of deeply-excavated pond in Marshall County, Tennessee, Inner Nashville Basin.