Which protozoans are especially likely to leave fossils behind?

Protozoans that are most likely to leave fossils behind are those that possess hard, resistant shells or skeletons, known as tests. Because most protozoans lack such structures, their overall fossil record is limited.1234

The two primary groups of protozoans that contribute significantly to the fossil record are:

• Foraminiferans: These marine protozoans produce calcareous (calcium-based) tests. Their remains are so abundant that they form extensive deposits of limestone and chalk, and they are highly valued as index fossils for geological dating and petroleum exploration.245
• Radiolarians: These marine protozoans possess complex, beautifully sculptured skeletons usually composed of silica. Their skeletal remains settle on the ocean floor to form radiolarian ooze, which can eventually transform into sedimentary rocks such as chert and flint.15

Additionally, some testate amoebae have left fossil remains, with records dating back to the Precambrian.4