Foraminifers
Image from University of California Museum of Paleontology (fair use policy)
Foraminifers (fora-MIN-ifers) are the one-celled version of mollusks. They are protists belonging to the order Foraminiferida, in the Alveolate lineage of the eukaryotes (cells with nuclei). Geologists tend to call them "forams" to save time. Forams make skeletons for themselves, either external shells or internal tests, out of various materials (organic material, foreign particles or calcium carbonate). Some live floating in the water (planktonic) and others live on the bottom sediment (benthic). This particular species, Elphidium granti, is a benthic foram.
Forams are a very important group of indicator fossils because they occupy rocks from Cambrian age to the modern environment, covering more than 500 million years of geologic time. And because the various foram species live in very particular environments, fossil forams are strong clues to the environments of ancient times—deep or shallow waters, warm or cold places, and so on.
Oil drilling operations typically have a paleontologist nearby, ready to look at the forams under the microscope. That's how important they are for dating and characterizing rocks.
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