GENERAL ZOOLOGY 



Recent 



Pleistocene 



Pliocene 



Miocene 



Oligocene 



Eocene 



Hipparion 

 (and related genera) 



Merychippus 



Anchitherium 

 (and related genera) 



Archaeohippus 



Parahippus 



Miohippus 



Mesohippus 



Epihippus 



Orohippus 



Hyracotherium (Eohippus) 



Fig. 20.6. Summary of the main lines of evolution of horses in relation to the geolos;ic time 

 scale. The general characteristics of some of these genera are described in the text, and 

 reconstructions of some are shown in the preceding figure. (From E. H. Colbert, Evolution 

 of the I'ertebrates, copyright 1955 by John Wiley and Sons, Inc., reprinted b\- permission.) 



and loss of the supporting pad with the development of specialized ligaments 

 connecting the toe bones; (2) increase in length of the distal parts of the 

 limbs, with fusion of the radius and ulna and of the tibia and fibula; (3) in- 

 crease in complexity of the teeth, with differentiation for grazing and for 

 grinding; and (4) increase in the size of the body and in the relative size 

 of the brain. In none of these trends has there been a steady progression 

 toward modern conditions; but these are the chief points of contrast when 

 modern Equus is compared with its remote ancestors. 



The Elephant Family. Another example of a relatively complete series 

 of fossils demonstrates the succession of types apparently ancestral to 

 modern elephants (Fig. 20.7). The oldest recognizable representative of the 

 elephant family is Moenlhenum, known from fossils in late Eocene deposits 

 in what is now the Libyan Desert of Africa. Descendants of this form can be 

 traced to all the other continents except Australia. Moeritherium had no 

 proboscis (trunk) and no well-developed tusks; it apparently used its 

 prominent front teeth for grubbing roots, more or less as swine do. As in 

 the history of horses, the descent of elephants involved many separate lines. 



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