Operculina sp. is a genus of Foraminifera protozoa of the Nummulitidae family that also contains the well-known, often very common and visible in the nummulitic limestones of the Eocene in Central Catalonia, Nummulites. Operculina is also found in large number as fossil into nummulitic limestones; therefore, some of them should better be known as operculinic limestones.

The evidence from the fossil register proofs that Operculina appeared in the Paleocene, it experienced a great expansion and abundance during the Eocene and still exists today in the benthos of flat coasts, mainly in mangroves.

Operculina is a unicellular organism that secretes a calcareous shell that grows linearly in a spiral by addition of new and larger chambers with a transverse septum as a border. The result is a flat disk-shaped shell, with a maximum diameter of 5 mm. Its inside delicate snail structure looks like a tiny Ammonite. Operculina has a single layer of chambers that result from linear spiral growth. On the other hand, Nummulites has several layers of chambers that result from growth in two perpendicular directions, its shape is rather lenticular, it can be larger than 5 mm and its cells have a constant size. When comparing two adjacent whorls, in Operculina the outer whorl is wider than the inner one, whereas in a cross section of Nummulites the whorls are the same width and the chambers the same size, so the outermost whorl needs more chambers to complete the lap.

The image shows a limestone with various specimens of Operculina that were seen when examining the rock of the slope of Talló (Castellgalí), in the ICHN-Bages’s outing on 26.03.2022.

[photo Jordi Badia]