Model of Globigerina bulloides, a species of planktonic foraminifer indigenous to shallower zones of oceanic waters, known as the photic or euphotic zone, where there is enough light for its procreation. Foraminifera are single-celled protozoa sized from under 1 to max 5 mm, having calcareous shells often penetrated by pores, floating in limitless numbers while alive and settling on the seafloor to form vast deposits more or less mixed with mud, later to become sedimentary limestones and marls. They are valuable guide fossils for paleontologists to establish the age of deposits from the Jurassic period to this day.