Recently, I found interesting clam shell fossils on the shore of Lake Michigan, in Southwestern Michigan USA. The first sample below clearly reveals the hardened muddy sediment that has completely encrusted its shell.
The clam fossil below has been completely replaced by minerals and is petrified to stone. It’s likely the mold of the original clam when sediment filled in the space where the soft body parts once lived. Its smooth surface is a telltale demonstration of Lake Michigan’s sand and wave action.
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 abductor 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 i.e. squid, snails, clams, chitons, octopus, nautilus)
Class: Bivalve or Pelycopod (Animals 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)