In rocks of late Precambrian age, "carbon film" fossils have been found that are basically just blackened compressions in the rock. Some are featureless and probably represent bacterial mats, but a few have a stalk and seem to represent some sort of seaweed-like algae, possibly brown algae (phaeophytes). Phaeophytes may go back to the Ordovician but are not common as fossils before the Miocene.
Chromists that secrete skeletons of silica or calcium carbonate are much more common as fossils; some, like diatoms, coccolithophorids, and silicoflagellates, may form extensive rock formations out of their accumulated fossil skeletons. A few problematic fossils that may be silica-secreting chromists have been described from the late Precambrian, but the oldest definite diatoms are late Jurassic in age; coccolithophorids also extend back to at least the Jurassic, while chrysophytes and silicoflagellates date back to the Cretaceous, and most other chromists have very poor fossil records that do not predate the Cretaceous. In the Cenozoic fossil record, fossils of siliceous chromists such as diatoms and chrysophytes may be abundant in freshwater deposits and may be crucial for reconstructing paleoenvironments and for paleolimnology.
More on paleolimnology, including jobs, meeting announcements, and links to related servers may be found at the Paleolimnology Home Page at Indiana University. The US National Geophysical Data Center maintians a data set of planktonic counts and core mineralogy for the last 18,000 years of the Earth's history, with related information on climate reconstructions.
