Subsurface Laboratory Methods 99 



These branches show on the surface of the fossil as rows of small open- 

 ings or clusters of openings in regular rows circling around the fossil. 



Ecologically these algae grow in shallow water, on mud flats, or in 

 sheltered portions of reefs. Fossil forms occur in many marls, calcareous 

 shales, and limestones. 



Geologically they are known in rocks ranging in age from the Early 

 Ordovician to the present, although they appear to have made their greatest 

 development during the Triassic and Jurassic. 



They have been extensively studied in certain areas of Europe,^ 

 where it has been demonstrated that the individual species have short 

 geologic ranges; consequently, they can be used as guide fossils. ^° Very 

 few have been described from North America, although a careful search 

 will undoubtedly show them to be present in many localities. 



Charophyta 



The Charophyta are a distinctive, isolated group of algae (fig. 43) 

 usually classed with the green algae (Chlorophyta) . Calcium carbonate 

 is usually precipitated around the tips of the branches and spore cases. 

 These structures are the only portions of many of the plants that are 

 calcified. The spore cases (oogonia) form a distinctive fossil. With their 

 spiral ornamentation they are readily recognized, as no other microfossil 

 even closely resembles them in structural constitution. 



At the present time the Charophyta inhabit only fresh- and brackish- 

 water environs, but during the Paleozoic and possibly during the early 

 Mesozoic it appears that some forms may have lived in shallow-water, 

 near-shore, marine environments. Their remains occur in great numbers 

 in many of our continental deposits,^^ and for some such formations they 

 are distinctive microfossils. 



According to Peck and Recker,^^ 



Considering the difficulty of establishing fine divisions in the classification 

 of the Charophyta, it is believed that they will never be of value for the 

 discrimination of small stratigraphic units. Yet they are of real value in period 

 differentiation: continental deposits of Jurassic, Lower Cretaceous, Cretaceous, 

 and Lower Cenozoic ages can be readily differentiated on the basis of Charo- 

 phyta oogonia. 



Geologically, their remains have been noted in rocks ranging in age 

 from the Devonian to the present. 



General 



Fossil algae as a group require a great deal of further study. Recent 

 work has shown them to be widespread and abundant. They have useful 



" Morellet, L., and Morellet, J., Les Dasycladacees du Tertiaire Parisien: Soc. geol. France 

 Mem. vol. 21, no. 1, p. 43, 3 pis., 1913; Tertiary Siphoneous Algae in the W. K. Parker Collection: 

 BritJBh Mus. Nat. History, 55 pp., 7 figs., 6 pis., 1939. 



■"' Pia, J., Die Siphoneae verticillatae vom Karbon bis zuir Kreide : Zool.-bot. Gessel. Wien Abh., vol. 

 11, pt. 2, 1920. 



^' Peck, R. E., Fossil Charophyta: Am. Midland Naturalist, vol. 36, no. 2, pp. 275-278, 1946. 



" Peck, R. E., and Recker, C. C, Eocene Charophyta from North America: Jour. Paleontology, 

 vol. 22, no. 1, pp. 85-90, 1948. 



