Unicellular rhodophytes, of which there are a few living today, may go back well into the Precambrian, but since none of the Precambrian fossils in question contain pigments, they cannot be identified confidently as red algae. Multicellular rhodophytes were present in the late Precambrian; the oldest may be as old as 1.25 billion years.
Because of their ability to secrete calcium carbonate, calcareous red algae have a better Phanerozoic fossil record than many other groups of algal protists. Most limestone deposits of reef origin consist largely of the skeletons of coralline algae, and because these are often associated with petroleum deposits, there has been a great deal of attention focussed on these fossils. Despite this attention, we still do not fully understand how the rhodophytes precipitate calcium carbonate; the mechanism is not as well studied as those in bone and shell deposition.
Visit Michael Rasser's pages on FOSSIL CORALLINE ALGAE (Corallinaceae: Rhodophyta).