Skeletons of radiolarians
The skeletons of radiolarians and foraminiferans form a primary constituent of ocean bottoms where they form 30% or more of the sediment. It is called a radiolarian ooze or foram. However at depths below 4000 m, the shells tend to dissolve due to pressure. The shelled amoeboid protozoa are the only large group of protozoa that have a fossil record.
Figure: Skeletons sturucture of radiolarians
The radiolarians are amongst the oldest known fossils. Extensive accumulation of foram shell In the mesozoic and cenozoic eras contributed to the formation of great limestone and chalk deposits in different parts of the world. The quarries from which the Egyptian pyramids were built are composed mainly of foram shells.