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Protists were a dominant form of life on Earth 1.5 billion years ago. While protozoans evolved early and have survived to the present day as unicellular organisms, they have undoubtedly undergone considerable evolutionary change. That many species must have become extinct as others appeared can be deduced from the limited fossil record of protozoans. Extinct fossil foraminiferan species, for example, number around 34,000, while there are only about 4,000 described living species.
Only a small number of protozoans, most of which are testate amoebae, have left fossil remains. The calcareous shells of the foraminiferans and calcium-secreting cocolithopores (a group of algae), for example, produced substantial geologic strata in the chalk formed during the Cretaceous period (145.5 million to 65.5 million years ago) and the well-developed foram-limestones of the Paleozoic Era(542 million to 251 million years ago), Early Cretaceous Epoch (145.5 million to 99.6 million years ago), and Cenozoic Era (65.5 million years ago to the present). The fossil-forming radiolarians date to late Precambrian times, and the testate lobose amoeba Melanocyrillium dates to the late Precambrian geologic record of the Grand Canyon in northwestern Arizona, U.S. The testate amoeba Nebela is found in deposits from the Cretaceous Period.
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