158 



AMERICAN MESOZOIC MAMMALIA 



10 





170^ 

 160, 

 190^ 

 200^ 

 210^ 

 220^ 



PERIOD 



CflETACEOUS 



20. 



30 ^TERTIARY 



50^ 



70| 



100. 

 110^ 

 120^ 

 130^ 



m^ 



150^ 



JURA551C 



TRIASSIC 



ERA 



CEN020IC 



nE50Z0IC 



PALEOZOIC 



HAnnALIAN LIFE 



HAN 



"AGE OF HAnnALS" 



j-pueRco 



GREAT SPREAD OF PLACENTAL TAUN AS 



-BEiLir Riven 



■DJ^DOKHT/^ 

 ORDINAL DIFFERENTIATION OF PLACENTALS 



ORIGIN OF PLACENTALS4 MARSUPIALS 



■^nonniSON, PUnBEiCK, TENO/IG^VfH/ 



^STONESFIELO 



KNOWN MAMriALS 



3. A Cretaceous radia- 

 tion, from some center reached 

 by at least the Multitubercu- 

 lata and Pantotheria of the 

 previous radiation, involving 

 more advanced members of the 

 Multituberculata, and, for the 

 first time, marsupials and pla- 

 cental, probably derived from 

 the Pantotheria. 



4. A Paleocene radiation, 

 from some center reached by 

 the Insectivora, involving the 

 archaic placentals, Meseuthe- 

 ria of Osborn. 



5. An Eocene radiation, 

 possibly from the same center 

 as that of the Paleocene radia- 

 tion, involving the modernized 

 mammals, Ceneutheria of Os- 

 born, of ultimate common deri- 

 vation with the archaic mam- 

 mals. 



Each of these five events 

 probably had a definite focus or 

 center, and the five centers 

 probably did not all coincide. 

 Nothing is yet certainly know^n 

 as to the location of any one of 

 them. 



Secondary radiations, in- 

 termigrations, local or vi^ide- 

 spread extinctions, and so forth, 

 w^ere of constant occurrence. 

 Australia was probably reached 

 by the Cretaceous radiation, or part of it, but by no subsequent radiation, except acci- 

 dentally, until recent time. South America apparently lost connection with the rest of 

 the world after the Paleocene radiation had reached it and was not reunited until late 

 Miocene or Pliocene time. 



K FIRST 

 ^f\H/ETIC 



ORIGIN OF MAHMALS 



HI5T0RY o' MAnriALS 



1928. 



Fig. 62. Diagram showing the relative positions in time of the 

 known Mesozoic mammalian faunas and the approximate rela- 

 tive dates of some of the major events in mammalian history. 

 The figures on the left represent millions of years and are 

 essentially those advanced by Professor Arthur Holmes (The 

 Age of the Earth, London, 1927) on a correlation of radio- 

 active and other data. The relative duration of the periods is 

 fairly well established, the absolute chronology still open to 

 question. 



