CENOZOIC ERA 195 



Actually there were four successive glaciations during this period. Four 

 times glaciers centering around the Hudson Bay region swept down over 

 the northern tiers of states, extending into Pennsylvania, southern Ohio, 

 and Illinois. During such times reindeer lived as far south as southern 

 New England, and such an arctic animal as the musk ox ranged through 

 Kentucky, Arkansas, and Texas. Each glaciation was followed by an 

 interglacial period during which the climate in a given region was as mild 

 as, if not milder than, it is today. The interglacial periods lasted for many 

 thousands of years; the shortest is estimated to have been of 135,000 years' 

 duration. Some 10,000 years are estimated to have elapsed since the last 

 glaciation (Libby, 1956). Thus it may well be that we are at present living 

 in an interglacial period, that the Recent period of our time chart (Table 

 7.1, p. 137) really forms part of the Pleistocene. The extensive ocean 

 ice of the arctic regions and the glaciers covering Greenland and the 

 antarctic continent remind us that glaciation is not far away. Indeed it has 

 been estimated that a lowering of average annual temperature by only 

 5 C. would bring the ice sheets down upon us again. Will the glaciers 

 return? Only our remote descendants will be able to answer that question 

 with certainty. 



MAMMALIAN EVOLUTION IN THE 

 CENOZOIC ERA 



Disappearance of the dinosaurs at the end of the 

 Mesozoic left a clear field for mammalian expansion. We have already 

 noted the occurrence in Cretaceous times of both marsupial and placental 

 mammals (p. 191). A few of the Cretaceous mammals persisted into the 

 Paleocene, notably opossumlike marsupials, and insectivores. We recall 

 that the latter are the group of placental mammals from which the other 

 orders of placental mammals are believed to have arisen. Among these 

 other orders, representatives of carnivores, primates, and ungulates lived 

 in Paleocene times. The carnivores (flesh eaters) and ungulates (hoofed 

 animals) living at this time were quite unlike their modern relatives, how- 

 ever. Most of them belonged to groups which underwent a relatively 

 rapid evolution during the Paleocene and Eocene and then disappeared. 

 Thus they are sometimes called "archaic mammals" to distinguish them 

 from the "progressive mammals'' whose development was slower but led 

 to modern types of mammals. Most of the archaic carnivores belonged to a 

 group called Creodonta, most of the archaic ungulates to a group called 

 Condylarthra. 



