Monday, December 28, 2015

Rastellum, another unusual fossil oyster

Last November, I had a post concerning a cork-screw shaped fossil. I return to the subject that continues to intrigue me; namely, unusual shaped oysters (clams). This post is about a species of genus Rastellum, an extinct genus whose geologic time range is approximately 170 to 66 million years ago, during the Middle Jurassic to the end of the Cretaceous. This genus is characterized by small to large size; elongate-narrow to comma-shape; and the valves are almost equal in size and shape. The two valves have very distinctive radial ribs called plicae. Although the valves (left and right) look similar, only the left valve (the one that is attached to some foreign object) has a noticeable muscle scar. In the three pictures immediately below, the muscle scar is visible as a small depression.

The surface where the two valves of a clam join together is called the commissure. On many clams it is a simple, straight line. Rastellum has a "zig-zag" commissure, which enabled the feeding process. This "zig-zag" line is readily observable in the third picture below.






These images are  three views (left valve, right valve, and side view; in that order) of a closed-valved specimen Rastellum carinatum (Lamarck, 1819) of  Early Cretaceous (Albian Stage) age from Texas. The longest dimension is 2.25 inches (55 mm).

The last image is of another specimen of R. carinatum, but this one shows the interior of a left valve. The ligament area of oysters is very distinctive. The ligament is a groove where originally there was elastic material (now decomposed) which helped the oyster hold together and open its valves. The muscle scar of oysters is also very distinctive. It is commonly large and almond shaped. Its function was to firmly hold the valves together.

Interior of a left valve of Ratellum carinatum. The longest
dimension is 1.6 inches (40 mm).

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