PRINCIPLES OF VERTEBRATE MORPHOLOGY 



27 



of great advantage to memorize the geologic epochs and ages in their 

 order, for it will be necessary in dealing with the extinct representa- 

 tives of the vertebrate classes to assign them without explanation 

 to their appropriate geologic periods. 



The study of geology teaches us that the earth's outer zones have 

 undergone within the period of vertebrate history numerous pro- 

 found changes which in general we may term climatic changes. 



FIG. 6. Chronological chart of vertebrate succession. Successive geologic 

 appearance and epochs of maximum radiation (expansion) and diminution (con- 

 traction) of the five classes of vertebrates, namely, fishes, amphibians, reptiles, 

 birds, and mammals. (After Osborn's " Origin and Evolution of Life " [Charles 

 Scribner's Sons].) 



There have been periods of continental subsidence, accompanied 

 by ocean floor elevations, during which the great continental plains 

 have been covered by comparatively shallow seas. The marine 

 faunas of the seas have migrated into these shallows, and representa- 

 tives of them have been buried in sediment. When the reverse 

 change has occurred and the continental plains have been again ele- 

 vated, the sedimentation of the shallow-sea period forms a great 

 rocky stratum laden with marine fossils. Between periods of sub- 

 sidence millions of years elapsed, and therefore a break in the conti- 

 nuity of the entombed fossils is to be expected. Discontinuity of 

 fossil-bearing strata is the rule. If it were not for this periodicity 



