Timetable of Clams


Timetable of Bivalve Drawing (Six Clams and One Scallop)

Clams fall under the class of animals called “Bivalia” which are soft bodied, filter-feeding animals with two identical shell halves. Some attach themselves to a substrate on the sea bottom while others simply hang around deep in the depths, and yet a few are capable of free swimming, namely the scallops. Most, are marine salt water animals while a few varieties have evolved into fresh water habitats. I have drawn a few samples of bivalves and or clams. They are amazing animals, one of Earth’s original complex organisms and they are still with us today after 500 million years! The drawings represent seven species from all three of Earth’s timetable eras including the Paleozoic, Mesozoic and Cenozoic. Find out their genus names and brief descriptions from the oldest to most recent, along with fossil samples primarily found on the internet. 

Middle Left Orange Colored Rendering: Ctendonta has smooth surface with fine concentric growth lines and teeth along the hinge plate. Ordovician – Silurian (505 million years ago (mya) to 408 mya)

Top Right Pinkish Colored Rendering: Modiolopsis has asymmetrical thin valves crossed by an oblique depression. Ordovician-Silurian (505 mya to 408 mya)  


MODIOLOPSIS Fossil Clam Shell

Middle Bluegreen Colored Rendering: Byssonychia has sharp steeply inclined beak near end of hinge; usually has strong radial ribs. Mostly Upper Ordovician (400 mya)


BYSSONYCHIA Source: Wendell Rickets (All Rights Reserved) https://invertebrateme.wordpress.com/2012/03/10/byssonychia-sp/

Top Middle Yellow/Brown Colored Rendering: Goniophora has a lopsided shell with a prominent beak and ridge extending to the rear margin. Silurian – Devonian (438 mya – 360 mya)  

Bottom Left Goldish/Brown Colored Rendering: Pteria has thin inequilateral shaped valves with a long straight hinge merging into large unequal wings. Jurassic to recent (245 mya – today)

Bottom Right Tanish/Peach Colored Rendering: Glycimeris has symmetrical circular outline with pointed beak. Cretaceous to Recent (144 mya – today)

Top Left Deep Blue Colored Rendering: Pecten is a genus of many well known bivalves otherwise know as large scallops. Valves have strong radial ribs and are symmetrical except for slightly unequal wings. Mississippian to Recent (360 mya – today)

Bivalves feed the world. Bivalve oysters, scallops and clams are near the bottom of the food chain which many marine and freshwater species depend on for a food source. But don’t forget about land creatures such as otters, for one, and we humans who especially enjoy a treat of clam chowder. Their shells, secreted from calcium carbonate, can be beautiful and are used to adorn our homes in many creative ways.  Scan_Pic0009


Simple as a Clam

Did you know, in 2007, off the coast of Iceland, a clam was discovered to be at least 405 years old. It was declared the world’s oldest living creature by North Wales, Bangor University researchers. Hmm . . . maybe they’re not so simple after all!

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.


Clam Fossil

Reverse Side of Clam Shell Fossil

Reverse side of clam fossil above

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.


Completely Fossilized Clam Shell or Clam Shell Mold


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.

Kingdom:    Animalia

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)  



A Devonian Seascape Draing Showing  Clam, Crinoids (Sea Lilies) Petosky Corals (Hexagonaria) and Bryozoans