Johnson : Morphological Trends among Fossil Algae 435 



bers of this family are probably numerically the most common fossil algae. 

 They changed in detail, but the general morphology and structure changed but 

 Httle. 



Red algae. Calcareous red algae were relatively rare throughout the Paleo- 

 zoic. From the Cambrian to the Pennsylvanian, all found to date appear to 

 belong to two genera of the family Solenoporaceae. During that time they 

 show little morphological change. Then, during the Pennsylvanian, several 

 quite different types of red algae appear. One of these, the genus Archaeolitho- 

 phyllum, has much higher structural features with the tissue differentiated into 

 a well developed hypothallus and perithallus, and definite conceptacles. Also 

 the Pennsylvanian genera show a much greater variety in growth form. 

 Whether this rapid rise of new types represents an evolutionary surge, or for 

 some reason long established groups of plants acquired the calcareous habit 

 and begin to be preserved as fossils, we do not know. 



During the Permian another family of calcified red algae, the Gymnocodia- 

 ceae, appear and in the Late Permian become abundant and widespread, adding 

 additional morphological types. 



The record of Triassic red algae is scanty, but during the upper half of the 

 Jurassic the group undergoes a strong evolutionary push. Within the family 

 Solenoporaceae many developments and new growth forms appear, and repre- 

 sentatives of the family Corallinaceae begin to emerge. The first recorded 

 articulated corallines appear during the Middle Jurassic, and the earliest known 

 crustose corallines during the Late Jurassic. By the end of the Cretaceous, 

 almost all of the common genera of the coralline algae had appeared. They 

 were well established by the middle Eocene and had developed essentially all 

 the morphological features known today. 



Myxophyceae {Cyanophyceae). The only other important type of calcareous 

 algae are the stromatolites. These are calcareous masses of distinctive form 

 and surface markings, commonly showing thin arched laminae, built largely or 

 entirely by the activity of certain types of glue-green algae. They have been 

 reported from rocks as old as the late Archaeozoic, and are faily abundant in 

 the Huronian of few areas. They were the Hmestone building organisms of the 

 Proterozoic and Early Cambrian. With the appearance of limestone building 

 animals in the Cambrian and Ordovician their importance decreases greatly, 

 but they have continued in considerable numbers down to the present day. 

 However, in morphology and structure, they show practically no change after 

 Late Cambrian times, consisting of mats or felts of tiny algal filaments which 

 often trapped some silt or organic debris and was encased in a mold of fine cal- 

 careous dust precipitated by the algae. Commonly they developed colonies of 

 a consistent shape, show growth laminae, but little or no microstructure. 



Bibliography 



Doty, M. S. 1957. Fxology of marine Algae (annotated bibliography). Treatise on marine 

 ecology and paleoecology. Geol. Soc. America Memoir 67. 1: 1041-1050. 



FosLiE, M. & H. Printz. 1929. Contributions to a Monograph of the LUhothamnia. 

 Royal Norwegian Museum of Natural History. Trondheim. 



