XXX 



PERISSODACTYLS 



1. Perissodactyl characteristics 



The protungulate and paenungulate herbivorous types achieved their 

 chief radiation and greatest numbers early in the Tertiary period. 

 Their organization was not profoundly different from that of the 

 original eutherians and although a few of them, such as the elephants, 

 have persisted to the present day, most have been supplanted by 

 ungulates that appeared by later modification of the original type. 

 Very roughly we may say that the protungulate is the chief Palaeo- 

 cene mammalian herbivorous type and the paenungulate that of the 

 Eocene. The Perissodactyla, including horses, rhinoceroses, tapirs, 

 and certain early extinct types, then represent a third or Oligocene- 

 Miocene development, supplanting the paenungulates and itself then 

 largely replaced in Pliocene, Pleistocene, and Recent times by the 

 Artiodactyla. This analysis must of course be taken only as a very 

 rough approximation, especially as it is given unsupported by the 

 quantitative data that it evidently requires. It is subject to many ex- 

 ceptions, for example the large development of the elephants in post- 

 Miocene times. 



The early perissodactyls were much like all other early ungulates 

 and it is not easy to characterize the group as a whole. The limb 

 structure developed the mesaxonic condition, with the digits arranged 

 around the third as the main weight-bearing member, the others 

 being reduced. With the power of fast movement the lower part of 

 the limbs became elongated and the upper segments shortened, with 

 reduction of the ulna and fibula, but these are characters found also 

 in artiodactyls. A distinctive feature of the perissodactyls was the 

 plan of the carpus and tarsus (Fig. 466). One distal carpal, the capitate 

 (magnum), became enlarged and interlocked with the proximal car- 

 pals, while in the foot the ectocuneiform developed into a large flat 

 bone, transmitting the thrust through a flat navicular to the talus, 

 which has a flat undersurface, not a pulley-like one as in Artio- 

 dactyla. Modifications of the backbone for carrying great weight or 

 for running were similar to those of other orders (elephants, Dino- 

 cerata), including increase in the number of ribs and the vertical 

 position of the ilium (p. 697). 



