Though there are several Late Proterozoic fossils (both unicellular and multicellular) which superficially resemble green algae, fossils which can be assigned to the group with some confidence are not known earlier than the Cambrian. This is, perhaps in large part, because the group has traditionally been defined by pigment composition, and not with more easily identified morphological characters. Fossils were thus assigned to this group because of overall resemblance to modern taxa, Recent morphological work has added a number of characters which should aid in recognition of members of this group.
The Dasycladales has perhaps the best-known fossil record of any group of green algae. More than 120 fossil genera have been described, some dating back to Cambrian and Precambrian strata. This fossil diversity contrasts remarkably with the fact that today there are only eight extant genera, including the familiar "mermaid's cup" Acetabularia. The Dasycladales began to rapidly diversify in Middle Ordovician, and are common in all strata until the lower Cretaceous. Both living and extinct species are known primarily from warm marine waters,
Many members of the Dasycladales secrete lime (calcium carbonate) which increases their chances for preservation and later discovery as fossils. The group is easily recognized by their radial symmetry, with a central nonseptate axis to which are attached whorls of lateral appendages which may or may not be branched. Fossil forms are cylindrical, club-shaped, or have a spherical appearance from the density of their branches.
Botryococcus is another "green alga" known from the Carboniferous onward. It is a colonial member of the Tetrasporales, and may be responsible for certain coal and petroleum deposits.
Fossil members of the Charales and Coleochaetales are discussed under their respective groups.
Visit the Gallery of Prehistoric Seaweeds for an overview of marine plants that lived between 700 and 400 million years ago .