Rice Grain Bryozoan or Ostracod-rich Limestone
cf. Rhinidictya sp. or aff. Eoleperditia sp. • Body fossil (invertebrate index fossils in a death assemblage / coquina)
Geological Period
Late Ordovician (Katian Stage)
Estimated Age
445 to 455 million years
Preservation Type
Recrystallized replacement and selective weathering leading to moldic porosity.
Condition Assessment
Fair to Good. While the density is high, the surface is heavily weathered by modern exposure, losing fine diagnostic microstructure.
Taxonomic Classification
Phylum: Bryozoa; Order: Cryptostomata; Family: Rhinidictyidae (OR Phylum: Arthropoda; Class: Ostracoda)
Morphological Description
The specimen features dense accumulations of small, sub-parallel, elongated elliptical bodies resembling grains of rice. These appear as small recessed molds or weathered casts. Each individual is roughly 3–5mm long with rounded ends and a semi-cylindrical cross-section.
Rock Matrix
Bioclastic limestone (calcarenite to coquina) with a micritic to crystalline calcite matrix, common in the Central Basin's Nashville Group.
Formation & Location
Bigby-Cannon Limestone, Catheys Formation, or Hermitage Formation; Central Nashville Basin, Tennessee, USA.
Size & Dimensions
Individual fossils are approximately 2-5mm in length; typical for small bryozoan fragments or large Palaeocopid ostracods.
Value & Rarity
Estimated Market Value
$20 - $50 (largely as a decorative garden rock or educational hand sample)
Auction Estimate
$30 - $75 (as part of a multi-specimen bulk lot)
Rarity Assessment
Common. This lithology is characteristic of the Ordovician bedrock across Middle Tennessee.
Scientific Significance
High for local stratigraphy; represents a high-energy, shallow marine environment (carbonate platform) typical of the Ordovician epeiric seas of North America.
Preparation Recommendations
No invasive preparation needed. Gentle scrubbing with water to remove modern biological growth. If displayed indoors, a light sealant may enhance contrast.
Notable Features
The high density of individuals suggests a mass-mortality event or a storm-winnowed deposit where currents aligned the long axes of the specimens.
Notes
Deeply excavated pond in Inner Nashville Basin of Tennessee.