CENOZOIC ERA: AGE OF MAMMALS 



597 



mammals and closely approach those of its most specialized descend- 

 ant, Zeuglodon. It is thus seen that the Eocene whales were not 

 descended from the Mesozoic marine reptiles, but from the land 

 mammals, just as the ichthyosaurs and mosasaurs (p. 555) were 



after Gilmore, and restoration modified after Osborn.) 



descended from land reptiles. The specialized zeuglodonts constitute 

 a side branch and are not true whales. They became extinct before 

 the close of the Eocene. 



Ancestors of Existing Whales. — The earliest known ancestors of modern sperm 

 whales are believed to have been small, Eocene marine mammals (Microzeuglodon), 

 whose modified descendants in the Miocene have been called "shark-toothed" whales 

 because they had teeth somewhat similar in appearance to those of a shark. The 

 Miocene whale differs from its Eocene ancestor (Microzeuglodon) in the number and 

 simplicity of the teeth and in the skull, which resembles that of existing toothed whales. 

 With these Miocene shark-toothed whales (Squalodonta) begins an almost unbroken 

 series which leads to the sperm whale. By one investigator it is stated that the evolu- 

 tion from the shark-toothed whale to the sperm whale is sudden and almost " explosive," 

 the entire evolution being completed in a very small section of the geological time of 

 the Upper Miocene. Dolphins and whalebone whales are known only from the 

 Miocene, but probably date from an earlier epoch. Sea cows (Eosiren) are mingled 

 with the remains of zeuglodonts in the Eocene deposits of Africa. 



REFERENCES FOR MARINE MAMMALS 

 Abel, O., — The Genealogical History of the Marine Mammals: Smithsonian Rept., 



1907, pp. 473-496. 

 Beddard, F. E., — The Book of Whales. 

 Beddard, F. E., — Mammalia. 

 Lucas, F. A., — Animals of the Past, pp. 58-64. 

 Osborn, H. F., — Age of Mammals, p. 171. 



