MORPHOLOGICAL TRENDS AMONG FOSSIL ALGAE 



J. Harlan Johnson 

 Colorado School of Mines, Golden, Colorado 



The algae may be considered as a vast subkingdom of primitive plants that 

 exhibit an enormous range in structure, reproduction, and life history. Struc- 

 turally at the base are unicellular forms, often motile, that are indistinguishable 

 from similar unicellular animals except for the presence in the cell of color spots 

 or chromatophores, which contain photosynthetic pigments. At the other ex- 

 treme are tree-sized multicellular plants in which there is some differentiation 

 of tissue for dilTerent functions. 



For convenience in study and classification the algae are divided into a num- 

 ber of major groups. These groups have been considered as classes by the older 

 authors but the tendency today is to think of them as phyla. They are named 

 on the basis of the pigmentation, for example, the Chlorophyta or green algae; 

 the Rhodophyta or red algae. Nine such groups are recognized in most classi- 

 fications, 11 in others. 



Before considering the morphological trends among fossil algae it will be de- 

 sirable to review two things regarding recent algae. (1) The structural trends, 

 and (2) structural parallelism among the major groups of algae. 



Algal Morphology 



General. The algae show a great range in form, size, and structural develop- 

 ment. At the bottom are the microscopical unicellular forms. These occur 

 in all but two of the major groups and in a number of them no higher structural 

 types have ever developed. A majority of the unicellular forms are motile 

 flagellate types or at least in their life cycle pass through a flagellate stage. 



Structural evolution seems to have followed the steps shown in table 1, with 

 the first three, either 4 or 6, and 5, forming an evolutionary series. 



Parallelism. One of the most striking facts facing a student of algal morphol- 

 ogy is the evidence of parallel evolution and development among the members 

 of the various groups (tables 2 and 3). 



Marked structural complexity of the plant occurs only in two groups, the 

 Rhodophyceae and the Phaeophyceae, with some of the green algae (Chloro- 

 phyceae) reaching a high medium of complexity. It should be emphasized 

 however, that even in these three groups a majority of the known species have 

 simple types of structure. The highest structural features have developed 

 among the brown algae, with some of the reds not far behind. The green algae 

 probably show the greatest diversity of structural types with, however, the 

 highest types missing.* 



Fossil Algae 



General. A review of the structural types and evolutionary trends among 

 Recent algae, as briefly summarized in the previous section, and a study of 



* This has been explained by numerous writers on the basis that the highest types moved 

 ashore and gave rise to the land plants. 



430 



