Invertebrate Fossils
Invertebrate fossils are abundant in NJ from the Cretaceous and Tertiary periods. The most common are the oysters Exogyra and Pycnodonte, which are found in vast numbers at many localities. A layer several feet thick of these oysters are found at the base of the Navesink formation, and can be seen at Poricy Brook, Big Brook, and other well known collecting spots. Belemnites are another fossill also commonly found in this layer. They are the internal skeleton of a mollusk related to the squid. The translucent belmnite guards are composed of the mineral calcite, though they are sometimes replaced by vivianite.
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Belemnitella americana |
Although the original shell material of the thick oyster shells usually survives, most thin shelled clams and snails are usually destroyed, leaving only a mold or cast of the shell. The Woodbury formation, the layer from which Hadrosaurus foulkii was found, is one of the few formations to have well preserved shell material of gastropods, ammonites, and delicate clams. The thick, heavy clay protected the shell from leaching, a process where water dissolves calcium carbonate and other minerals from fossils. The outcrops of fossil bearing sediment in NJ are unconsolidated, meaning they haven't fully hardened into rock. Thus, using a shovel or trowel one can easily dig through the Cretaceous and Tertiary sands, clays, and marls. Unfortunately, these layers are quickly weathered, slump, and become overgrown by vegetation. Sites where fossils were once abundant often become completely uncollectable after a few years. Because of this, the best places to find fossils are either construction sites with fresh excavations, or quickly flowing streams eroding the stream bank.
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Cretaceous pelecypods
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Cretaceous gastropods |
Ammonites are usually found as fragmentary casts, though complete specimens of Placenticeras placenta over 18 inches have been found. At Atlantic Highlands, a unique fauna of well-preserved heteramorphic ammonites and baculites are present.
Crustacean fossils include crabs and lobsters, but only the claws are common. They are often found in phosphate or siderite nodules, which need to be broken open carefully to prevent destroying them.
Brachiopods are rare in the Cretaceous, though at the contact between the Paleocene Hornerstown and Vincentown formations there is a layer several feet thick made almost entirely of the articulate brachiopod Oleneothyrus harlani.
Coelenerates are not common in the Cretaceous formations. Trococyathus woolmani is one of the few corals found in these layers. Flabellum mortoni, another solitary coral, is found in Paleocene and Eocene formations. Bryozoans likewise become much more common in the Tertiary. Much of the Paleocene Vincentown formation is made almost entirely of bryozoans.
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