World's biggest fossil single-celled organisms

This post concerns Foraminifera (also called "forams"). They are single-celled organisms and are classified as protozoans. Most forams are so small (amoeba-size) that a microscope is necessary to see them. There are some exceptionally large fossil forams, however, that are coin size (see below).

Large forams belong to the orbitoids, which has three families:
1) discocyclinds (Late Cretaceous to Eocene)
2) nummulitids (Paleocene to Eocene)
3) lepidocyclinids (Eocene to Miocene)

Orbitoids are indicative of ancient

 warm, shallow seas.

Pseudophragmina clarki
maximum diameter 6.5 mm

Discocyclinids can be so abundant as to make discocyclinid limestones or sandstones (especially in Baja California Sur, Mexico). Compared to the other two orbitoids, discocyclinids are smaller and resemble lentils. The earliest known discocylinids found along the west coast of North America are of early Eocene age. 

Nummulites gizehensis
maximum diameter 25.5 mm

Nummulitids are larger than discocyclinids. Nummulites (note: nummulus is Latin, meaning small coin) is a common genus, and one of its species, Nummulites gizehensis (Lamarck, 1801) is a major component of the limestones that make up the Eygptian pyramids. This species, which can be as large as 30 mm in diameter) represents the largest known cells of all time. This species

Comparison between a specimen of N. gizehensis and a
 USA quarter (23 mm dimeter)

was a bottom dweller (benthic organism) in seas during the middle Eocene (about 45 million years ago). It most likely had a symbiotic relationship with a green alga, which probably accounted for the large size of the foram. For more details about Nummulites and pictures showing their very complicated interior, the link is HERE

cross-section of the interior of Nummulites from Egypt
maximum diameter of foram 18 mm

Lepidocycylina californica (one corner broken off)
maximum diameter 24 mm

Lepidocyclinids can be found locally in southern California and in Baja California Sur, Mexico. Lepidocyclina californica, of early Miocene age (about 20 million years old) is known from the Santa Ynez River valley, northern Santa Barbara County.