stratigraphic classification paleontologic criteria 481 



Paleontologic Criteria 

 general discussion 



The taxonomic value of fossils is universally recognized. Without 

 them the determination of the sequence of geological events would be in- 

 complete, while the correlation of such events in widely separated regions 

 would be practically impossible. The sequence of the fossils was, of 

 course, originally determined by their relative positions in undisturbed 

 strata, and exact advance in stratigraphic paleontology is still largely de- 

 pendent on such basic information. But with the great fund of positive 

 paleontological knowledge now available, and from which a more or less 

 progressive scheme of evolution of plants and animals has been worked 

 out, the expert paleontologist of today ventures an opinion concerning the 

 age of previously unknown fossil genera and species. This opinion is 

 based on principles of evolution and as a rule proves reasonably accurate. 

 To a considerable extent therefore the paleontologist has progressed be- 

 yond proved stratigraphic sequence. 



There are many sorts of fossils, and the taxonomic aspects of the dif- 

 ferent kinds varies considerably. Remains of marine organisms prove 

 submergence of the area in which the strata containing them are found. 

 When well preserved remains of land animals or land plants are asso- 

 ciated in the same layers with the marine fossils then we know that land 

 areas were not far away. Further, when the bed or formation contains 

 only land or fresh water organisms we may fairly assume that this de- 

 posit was laid down in an area above sealevel. Finally, when such land 

 deposits are found to alternate with marine sediments in a section then we 

 may infer that the sea migrated to and fro over the land, such parts of 

 the continents being at times submerged and at other times emerged. 

 Obviously, in a classification based primarily on diastrophic movements, 

 causing emergences and subemergences of continental areas, such alter- 

 nate occurrences of marine and land organisms are of vital importance. 



Naturally land animals and plants flourished chiefly in the emergent 

 stages, and more especially in the late, the intermediate, and the early 

 ages of geologic periods when the emergent phases were dominant. Un- 

 fortunately, land organisms are but rarely found in deposits older than 

 Devonian, but beginning with this period land plants, and later non- 

 marine animals, became very prominent. In the Paleozoics, therefore, 

 land organisms constitute criteria of taxonomically important earth move- 

 ments only in the Devonian and subsequent periods. Certain crustacea, 

 like the Eurypteridas, which are found in pre-Devonian as well as later 



