434 Annals New York Academy of Sciences 



importance as it controls the amount of information, especially with regard to 

 structure (table 4). 



The most common and the most useful algal fossils are those of calcareous 

 algae. If not recrystallized these commonly show not only the external form 

 and surface features but at least some details of the microstructure. In the 

 case of the coralline algae they actually show the size, shape, and arrangement 

 of the cells in the tissue and details of the reproductive organs permitting def- 

 inite, accurate classification. 



Identificalion and classification of the fossils. This is the most important and 

 frequently the most dithcult part of the study of fossil algae. The remarkable 

 parallelism in structural development and growth form in several of the major 

 groups and numerous orders and families gives a perplexing choice of possible 

 assignments for the fossils, which can only definitely be decided on the basis of 

 internal structure and reproductive organs. As just pointed out, very few of 

 the fossils can give this information except the calcareous algae. 



This means that the calcareous algae are the only groups of megascopic fossil 

 algae for which we have enough solidly based information to be able to discuss 

 the evolutionary morphological trends. 



Morphological trends. Among the green algae two families, the Dasyclada- 

 ceae and the Codiaceae have a long fossil record. Both appear in the record 

 during the Cambrian and continue down to the present. 



Dasycladaceae. The general form for most members of this family suggests 

 a test tube brush, consisting of a central stem from which develop more or less 

 regularly spaced whorls of primary branches. From the tips of the latter may 

 arise tufts of secondary branches, which in some genera may produce tertiary 

 branches. In the earlier, primitive forms the primary branches are not col- 

 lected in regular whorls, but may be irregularly spaced, or develop in more or 

 less regular rows which spiral upward around the central stem. However, 

 genera with regular whorls of primary branches are definitely present during the 

 Silurian period and characterize most of the genera thereafter. From Silurian 

 times on the general trend is toward greater structural complexity, involving 

 greater numbers of whorls, the development of secondary, and tertiary, rarely 

 even quaternary branches, and the differentiation of the branches into whorls 

 of purely vegetative branches, and whorls of fertile sporangia bearing branches, 

 with, in some cases, the modification of certain branches into elaborate holders 

 of sporangia or spores. This trend toward greater elaboration of structure 

 reaches its climax during the Jurassic period, after which a tendency toward 

 simplification begins. This has continued to the present. 



Codiaceae. The early Paleozoic record of this family is meager but sufficient 

 to show that by Ordovician times some members had reached a high structural 

 level quite close to that of present day types, like Halimeda which thev closely 

 resemble. Since then "increased structural complexity suggesting evolutionary 

 changes, such as are seen in the Dasycladaceae, can scarcely be recognized 

 among the Codiaceae. This fact suggests that, as a consequence of vegetative 

 differentiation and evolution from primitive plants sometime during the Pre- 

 cambrian, the family was already well established," (Konishi, 1961, p. 233). 

 Actually, from the Mississippian up into the Lower Cretaceous various mem- 



