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.