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Invertebrate Fossils of NJ

Clams
 

Class: Bivalvia (Linnaeus, 1758)
 

Clams have two shells connected by two adductor muscles and have a powerful burrowing foot. In most New Jersey collecting locations fossil clams are only found as internal molds or casts known as steinkerns. This is the result of acidic ground water dissolving the original shell material (see aragonite dissolution discussion). A few locations in New Jersey do produce original shell material and I've included examples of both types of preservation.
 

(Richards et al, 1962)  lists well over 100 species from New Jersey.


 

Typical Cretaceous find
Most of the clams found in NJ are internal molds or casts which do not hold up well to stream wear.
I find trying to identify steinkerns difficult and normally don't even try.

Monmouth County, NJ


 

 maybe the most common fossil bivalve found in New Jersey.
 This example is from the old Inversand marl pit, workers there referred to  these as "squirrel heads".


 

 


Cyprimeria depressa (compression fracturing)
Woodbury Formation - Cretaceous

A few areas in southern New Jersey do produce original shell material. Most of these shells are
either small or very fragile and require a good deal of patience to collect and prep.

At one site C. depressa was the dominate species but not a single complete specimen was recovered
due to compression fracturing (see explanation at bottom).
 


 



Examples of original aragonite shell material (5 mm)

Lucina sp. with bore hole
Caestocorbula crassiplica
Woodbury Formation
Cretaceous
 


 

 

Hexagonal platelets of aragonite give mother of pearl it's iridescence.

 


 

References and Notes:

compression fracturing - compaction-induced breakage occurs when muddy sediments compress. Shells will rotate towards the horizontal plane as a response to stress, but remained largely intact unless in contact with other shells (M. Zuschin et al,

Richards, H. G., 1958. The Cretaceous fossils of New Jersey. (Part 1). New Jersey Dept. of Conservation and Economic Development. New Jersey Geology Survey Bulletin, vol. 61, 266 p., 46 pls.


 Richards, H. G., 1962. The Cretaceous fossils of New Jersey (Part 2). New Jersey Dept. of Conservation and Economic Development. New Jersey Geology Survey Bulletin, vol. 61, 237 p.


Wade, B., 1926. The fauna of the Ripley Formation on Coon Creek, Tennessee. U.S. Geological Survey, Professional Paper no. 137, 272 p., 72 pl.


Zuschin, M., M. Stachowitsch, and R. J. Stanton Jr., 2003. Patterns and processes of shell fragmentation in modern and ancient marine environments. EarthScience Reviews, vol. 63, pp. 33 82.
 

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