I found these interesting clam shell fossils on the shore of Oval Beach in Saugatuck, Michigan in Southwest Michigan USA. The shell of the darker sample has been completely replaced by minerals and is petrified to stone. Its smooth surface is a telltale demonstration of the lake’s sand and water action. The light colored sample clearly reveals the hardened muddy sediment that has completely encrusted its shell.
Clam can be a term that covers all bivalves. Some clams bury themselves in sand and breathe by extending a tube to the water’s surface. Those varieties usually possess a stronger foot for digging. Bivalves such as oysters and mussels attach themselves to hard objects, and scallops can free swim by flapping their valves together. Clams feed by filtering plankton with their adapted gills, although the digging varieties use their siphoning tubes and more primitive species used special tentacles. Bivalves lack a head and usually have no eyes, although scallops are a notable exception. All bivalves possess a heart, kidneys, a mouth and anus, as well as a circulatory system. With the use of two adductor muscles they can open and close their shells tightly. Very fittingly, the word “clam” gives rise to the metaphor “to clam up”, meaning to stop speaking or listening.
AGE: Bivalves have occupied Earth beginning as early as the Cambrian Period, 510 million years ago, but were especially abundant during the Devonian Period around 400 mya.
Phylum: Mollusk (Invertebrate animals with soft body encased in hard shell)
Class: Bivalve (Possessing two uneven halves called valves which are mirror images of each other joined at one edge by a hinge, i.e. oysters, mussels, scallops, clams)