Eocene oyster Cubitostrea sellaeformis

If you have been a reading my posts for the last three years, you will know that I have a strong interest in fossil oysters. I return to them with this post, which concerns the middle Eocene (about 45 million years old) oyster Cubitostrea sellaeformis (Conrad, 1832), known from Alabama, Georgia, Louisiana, North Carolina, South Carolina, Texas, and Mexico. This oyster has a strongly arched, large-size shell, and, if you look at the digital image below, you can see the U-shaped "line" that separates the two valves shown in side view.

I collected this complete specimen in 1989, when I visited southern Alabama. The specimen is from the upper part of the Lisbon Formation. The lower valve (left valve), which is thick and heavy,  sat on the bottom of the shallow ocean. The upper valve (right valve), which is lid-like is smaller and comparatively lighter.

This is the exterior of the lower valve, which is 14 cm long.

This is the exterior of the upper valve, which is 13 cm long. The "ears" (auricles) are part of the other valve.

These are the interiors of both valves, with the lower valve on the left side of the picture, and the right valve on the right side of the picture. If you look closely, you can see the muscle scar, especially on the upper middle part of the lower valve.

If you are wondering what caused the peculiar shape of this species, "join the crowd." No one has determined the answer with any degree of certainty. Its large size allowed it to live in shallow-marine waters in front of barrier beaches (unlike brackish-water, lagoon-living smaller oysters). It is possible that individuals crowded together and took on unusual shapes so as to withstand agitated-water conditions. The "ears" possibly served as stabilizing "anchors" for the lower valve.

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).

Ilymatogyra, the Late Cretaceous cork-screwed-shaped oyster

Ilymatogyra is an odd-shaped oyster genus that belongs to an extinct group known as the exogyrine oysters, which were common during Mesozoic times. The genus is Late Cretaceous (Cenomanian Stage) in age and has only a few known species. It occurs in some Gulf Coast states (e.g., Texas, Mississippi), Oklahoma, Mexico, and Africa.

Ilymatogyra is characterized by having a left (lower) valve with a cork-screw shape and a right (upper) valve that is much smaller and “trap-door” like. Ilymatogyra lived on soft bottoms in mid-shelf environments about 25 to 50 m in depth, and the high-spiral shape of the valves most likely helped the oyster from becoming buried, thereby preventing its gills from being clogged up during intervals of high sedimentation.

The following three pictures are three views (apertural, abapertural, and side) of a specimen (41 mm height) of Ilymatogyra arietina from Mississippi.

The calcite shells of this genus are commonly replaced by pyrite (“fools gold”), which is made up of iron and sulfur. The following two pictures are of a pyritized I. arietina from the Del Rio Formation near Austin, Texas.