Fragmented Atlantic Giant Cockle (fossilized/blackened variant)
Phylum: Mollusca, Class: Bivalvia, Order: Cardiida, Family: Cardiidae, Genus: Dinocardium, Species: D. robustum · Cardiidae (Cockle family) · Bivalve (fragment); fan-shaped section of a single valve.
Species
Dinocardium robustum
Shell Type
Bivalve (fragment); fan-shaped section of a single valve.
Family
Cardiidae (Cockle family)
Size
Approximately 2.5 to 3 inches in width. This represents a partial section of a valve that could originally have reached 4 to 5 inches, which is standard for a mature adult.
Color & Pattern
Striking monochromatic pattern featuring alternating bands of slate black, charcoal grey, and off-white. This coloration is a result of anaerobic burial in peat or mud (fossilization/staining) rather than the living creature's original tan and reddish-brown mottling. The patterns follow the concentric growth lines.
Rarity
Common (Beachcombing find). Fragments of this size and color are frequently found on Myrtle Beach, though large, unbroken blackened valves are more desirable.
Habitat
Found in shallow water sandy bottoms, from the intertidal zone to depths of about 100 feet. This specific specimen likely originated from a buried offshore Pleistocene bed before being washed ashore.
Geographic Range
Western Atlantic, from Virginia south to Florida, the Gulf of Mexico, and down to Brazil. This specimen is highly characteristic of the Carolinas.
Description
A flattened, fan-shaped shell fragment displaying heavy concentric growth ridges. The specimen shows significant surf-tumbled wear on the edges, softening the originally sharp radial ribs. The dramatic black-and-white striping gives it a lithic, stone-like appearance, a result of mineral replacement or staining over centuries of burial in sediment.
Key Features
Concentric growth bands, thick shell wall, relatively flat profile (due to being a side fragment), and the signature 'zebra' striping caused by the staining of different shell layers.
Collector Value
Low monetary value ($1-$5) but high aesthetic value for 'beach art' collectors. The high-contrast black and white stripes make these fragments very popular for jewelry making and mosaic art among Myrtle Beach tourists.
Condition Notes
Fair. This is a weathered fragment rather than a whole shell. The edges are broken and rounded by wave action. There are tiny 'pinholes' visible, likely from Cliona sponges (boring sponges) that inhabited the shell before or during its burial.
Interesting Facts
The black color is not natural to the living animal. It occurs when shells are buried in oxygen-poor mud rich in iron and sulfur; the hydrogen sulfide reacts with iron in the sediment to create iron sulfide, which stains the calcium carbonate black. These are often referred to as 'relic' shells, dating back thousands of years.
Ecological Role
As a live bivalve, it is a suspension feeder that filters plankton from the water. As a dead fragment, it provides calcium back to the environment and can serve as a substrate for small organisms like encrusting bryozoans.
Similar Species
Laevicardium mortoni (Yellow Cockle) fragments are smaller and thinner; Mercenaria mercenaria (Quahog) fragments are usually more purple/white and lacks the distinct ribbing of a cockle.
Beach Finding Tips
Look along the wrack line (the line of debris left by high tide) in Myrtle Beach, especially after a storm or a strong offshore wind which churns up older sediment layers.
Notes
Myrtle Beach SC