Pelecypods (Bivalves)
As part of the Mollusc family, Pelecypods (peh-les'-i-pods) include oysters, clams, mussels, scallops, and cockles, and they have been found in some of the oldest marine rocks known (Cambrian) – while are also found today in rivers and seas
Photo by Ken Hiller; from the Cretaceous
I grew up in Illinois where, in the past, pearl buttons were made from clam shells from the Illinois and Mississippi Rivers.
Unfortunately, this industry caused near extinction of some species.
Mother Of Pearl Buttons from http://www.newyorksocialdiary.com/node/2336
Besides buttons, though, other bivalves such as Cockels and Mussels are also well known sources of protein and so are fished for and raised as food stock.
Others are wonderful sources of pearls.
Most bivalves have two shells that are mirror images of each other, so the right and left halves of the shell itself look the same. Where the shells fasten together to make a hinge, they have teeth and sockets that fit into those of the opposite shell. The outside of the shell generally is ornamented with ribs, spines, and growth lines.
When you compare bivalves to brachiopods, you can see the differences in how the shells fit together and are arranged.
Most pelecypods form shell banks in the seas or rivers on sand and mud flats. Many burrow into the mud or sand and even into wood or rock. Some oysters attach themselves to rocks, and others creep about the sea floor by means of a hatchet-shaped foot thrust between the open valves. See below.
A few (scallops) move by jet propulsion, forcing water in a jet stream from openings in the beak end.
Pennsylvanian bivalves in limestone, collected near Bonner Springs, Kansas (note bryozoan fragments on right side of photo). From GeoKansas
Fossils
Bivalves have a long history.
Cambrian of south-east Gondwana (South Australia)
Body fossils of mussels in sandstone.
Copyright © 2000-2009 Groundspeak Inc. All Rights Reserved.
Their fossils first appear in rocks that date to the middle of the Cambrian Period, about 510 million years ago.
Although the group became increasingly abundant about 400 million years ago during the Devonian Period, bivalves really took off following the massive extinction at the close of the Permian Period. |
