Fossilized Giant Atlantic Cockle (also known as the Fossil Dinocardium)
Phylum: Mollusca, Class: Bivalvia, Order: Cardiida, Family: Cardiidae, Genus: Dinocardium, Species: Dinocardium robustum · Cardiidae (Cockle family) · Bivalve fragment. This is a section of a single valve (half-shell) showing the characteristic ribbed fan shape.
Species
Dinocardium robustum
Shell Type
Bivalve fragment. This is a section of a single valve (half-shell) showing the characteristic ribbed fan shape.
Family
Cardiidae (Cockle family)
Size
Approximately 2.5 to 3 inches in length. Based on the rib spacing, this fragment came from a large adult shell that would have originally measured 4 to 5 inches in diameter.
Color & Pattern
Charcoal gray to dark slate with hints of tan and off-white. This 'blackened' appearance is a result of fossilization in an anaerobic (oxygen-poor) environment, likely buried in peat or mud for thousands of years. The original species' reddish-brown spots are no longer visible.
Rarity
Common (Beachcombing). While whole fossilized valves are more desirable, fragments like this are frequently washed up after storms or high tides in Myrtle Beach.
Habitat
The living species occupies shallow sandy bottoms in offshore and intertidal zones. This specific specimen is a fossil found in the surf, likely eroded from Pleistocene epoch deposits (approx. 11,000 to 2 million years old).
Geographic Range
Living: Virginia to the Gulf of Mexico and Brazil. Fossilized: Common along the coastline of the Carolinas, specifically Myrtle Beach and the 'Grand Strand'.
Description
This is a thick, robust fragment of a fossilized cockle shell. It features deep, prominent radial ribs that are square-shaped in cross-section. The surface shows fine concentric growth lines crossing the ribs. The dark mineralization gives it a stony, heavy feel compared to modern 'white' shells.
Key Features
Heavy shell thickness, wide and flat-topped radial ribs (~12-14 visible on this fragment), and the distinct dark gray/black 'fossil' mineralization common to South Carolina beach finds.
Collector Value
Minimal ($1 - $5). These are 'pleasure finds' for beachcombers. A fully intact, perfectly blackened fossilized valve would have higher value ($15 - $30) to collectors of local paleontology.
Condition Notes
Fragmentary/Fair. The specimen is a partial piece with weathered edges and several small holes likely made by boring sponges (Cliona) or predatory gastropods before or during the fossilization process. Collectibility Grade: Poor to Fair (suitable for educational sets or beach decor).
Interesting Facts
The dark color is often mistaken for 'pollution' by tourists, but it actually indicates the shell is a fossil from the Ice Age. The living animal has a large, powerful red foot that allows it to jump across the sandy seafloor to escape predators like starfish.
Ecological Role
As a living organism, it was a filter feeder, cleaning the water column. As a fossil fragment, it now provides a substrate for algae and a source of calcium carbonate as it slowly breaks down back into the sand.
Similar Species
Atlantic Giant Cockle (modern white/brown versions) and the Ravenel's Giant Cockle. It differs from the modern version primarily by its dark mineralized color and heavier weight.
Beach Finding Tips
In Myrtle Beach, look for these 'black shells' in the shell line after a strong Northeast wind. They are much heavier than modern shells and will often be found mixed with fossilized shark teeth.
Notes
Myrtle Beach SC