Name: Ilymatogyra arietina
Age: 94-100 million years ago, Cretaceous Period
Location: Texas, USA, Del Rio Formation
Snail-like, coiled shells appear over and over in nature in groups that have little to do with each other, including certain species of oyster such as Ilymatogyra arietina. The repeated geometry is especially surprising in oysters, whose shells usually look more like lumpy cups or plates.
Oyster shells are irregular and asymmetric. Ilymatogyra is no exception. The shell in the photograph is an example. The right half of the shell is the small cap nestled into the rest of the shell on the upper right. The rest of the shell, and all of the spiral, is the left half.
Although living oysters don't form as obvious a spiral as the extinct relative in the photograph, remnants of spiral growth remain in the the early growth of today's oysters. Early in the development of baby oysters, a notch forms at a particular point in the tiny shell. As the oyster grows bigger, the notch deforms the edge of the shell and turns into a spiral growth track.
It's not clear why individuals of Ilymatogrya grew more strongly spiraling shells than other oysters. One proposal is that the kind of continuously growing spiral like the one in the photograph might prevent an oyster from sinking into soft mud on the seafloor, but the idea remains untested.
Specimen number: BEG 21161
References:
Waller, Thomas R. “Functional Morphology and Development of Veliger Larvae of the European Oyster, Ostrea Edulis Linne.” Smithsonian Contributions to Zoology 328 (1981): 1–70.
