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.