They are the shells of the fossil and present-day marine protozoan Nummulites, a type of foraminiferan.

Nummulites commonly vary in diameter from 1.3 cm (0.5 inches) to 5 cm (2 inches)[2] and are common in Eocene to Miocene marine rocks, particularly around southwest Asia and the Mediterranean in the area that once constituted the Tethys Ocean, such as Eocene limestones from Egypt[3] or from Pakistan.

[3][6] It is not surprising then that the name Nummulites is a diminutive form of the Latin nummulus 'little coin', a reference to their shape.

[7] In 1913, naturalist Randolph Kirkpatrick published a book, The Nummulosphere: an account of the Organic Origin of so-called Igneous Rocks and Abyssal Red Clays, proposing the unconventional theory that all rocks had been produced through the accumulation of forams such as Nummulites.

It is worth highlighting that thanks to the appearance of Nummulites tavertetensis in the Shallow Bentic Zone 15 (SBZ 15), it was possible to date the oldest fossil remains of Sirenio in Western Europe found in a new paleontological site, in Santa Brígida, Amer (La Selva, Catalunya, Spain) near an old quarry of stone limestone with nummulites.