Brachiopod Death Assemblage / Shell Bed
cf. Plaesiomys or Dalmanella species • Body fossils (Invertebrate); Lagerstätte-style shell accumulation
Geological Period
Middle to Late Ordovician
Estimated Age
450 - 460 million years
Preservation Type
Recrystallization and replacement (calcitic to partially silicified or dolomitized) within a highly weathered matrix.
Condition Assessment
Fair to Good. While the individual shells are weathered and the matrix is crumbly, the high density of fossils and the three-dimensional relief of the cluster provide good visual interest.
Taxonomic Classification
Phylum Brachiopoda; Class Rhynchonellata; Order Orthida; Family Plectorthidae or Dalmanellidae
Morphological Description
The specimen consists of a dense cluster of articulated and disarticulated brachiopod valves. Individual shells show distinct costae (radial ribbing) and hinge lines characteristic of the Orthida order. There is a mixture of pedicle and brachial valves, some exhibiting slight convex-concave profiles and sub-quadrate to semi-oval outlines.
Rock Matrix
Weathered bioclastic limestone (Packstone/Grainstone) with argillaceous (clay-rich) silt and iron-oxide staining typical of the Nashville Group.
Formation & Location
Bigby-Cannon Limestone or Catheys Formation; Nashville Basin, Tennessee (Central Basin).
Size & Dimensions
The composite block is approximately 15-20 cm in length; individual shells range from 1 cm to 3 cm in diameter, which is typical for these Ordovician genera.
Value & Rarity
Estimated Market Value
$40 - $85 USD
Auction Estimate
$30 - $60 USD
Rarity Assessment
Common (Common locally within the Nashville Basin, though attractive clusters are desirable for educational use).
Scientific Significance
Provides evidence of high-energy storm deposits (tempestites) in the ancient tropical seas that covered Tennessee. It illustrates the high biodiversity and population density of filter-feeding brachiopods in the Ordovician benthos.
Preparation Recommendations
Mechanical cleaning with soft brushes; chemical stabilization with a reversible consolidant (like Paraloid B-72) to prevent the matrix from further crumbling. Keep in a dry, low-humidity environment.
Notable Features
Highly dense 'coquina' style accumulation; the specimen preserves a specific moment of environmental deposition where shells were swept into a topographical depression or tidal channel.
Notes
This is a fossilized composit of largely small clam-shaped shells. It was found in the bottom of a deeply-excavated pit pond located in the Inner Nashville Geologic Basin of Tennessee.