Hundreds of millions of years ago in the Paleozoic seas of the Ordovician, Silurian, and Devonian periods, brachiopods were the dominate shelled creature of the sea floor. Clams on the other hand just occupied a small area of this habitat. As a result clam fossils are harder to find. In this collector's experience, the ratio is at least 1 to 100 of clam to brachiopod fossils found. In addition, clams tend to just be molds (steinkerns) of the inside of shells or imprints left on limestone.
In the modern world, the fortunes of the brachiopods and clams have reversed with the clams the more dominant animal and brachiopods scarce.
These fossils can be acquired on eBay CLICK LINK HERE (auction ends April 4, 2011)
The Ordovician (468-458 million years ago) clams are just molds of the internal shell they include the Ambonychia and Modiolopsis.
The Silurian period (438-408 million years ago) is represented by Cypricardina arata?. This fossil has some nice contrast with the limestone matrix.
The Devonian period (408-360 million years ago) fossils are Limoptera cancellata?, Paracyclas elliptica, and the Paracyclas lirata?. These fossils show the most promise if cleaned more to reveal further details of the fossil imprint.
|Modiolopsis, Limoptera cancellata?, Paracyclas elliptica|