XXXI 



ARTIODACTYLS 



1. Characteristics of artiodactyls 



The even-toed ungulates, though they can be traced as a distinct 

 line back to the Eocene, may be considered as the latest mammalian 

 herbivores, having radiated out chiefly in the Miocene and attained 

 then a dominance that has persisted to the present day. Except for 

 man and the horse all the large mammals really well established and 

 successful at the present time are artiodactyls. Any attempt to be 

 dogmatic about the reasons for the success of a group of animals is apt 

 to be superficial, but it is not unreasonable to suggest that in this case 

 the result is due to swiftness of foot, combined with keenness of sense 

 and brain, efficient cropping and grinding mechanisms, and especially 

 a complex stomach, allowing the digestion of cellulose by symbionts. 

 Two families of artiodactyls survive without these special features, 

 the pigs and hippopotamuses, water- and forest-living remnants that 

 show us approximately the condition of the group in the Eocene. 



The characters of artiodactyls show a fascinating 'similarity with a 

 difference' to those of perissodactyls. The common origin of the two 

 groups (p. 694) was little above the insectivore stage and nearly every 

 feature has been evolved independently; the general structural simi- 

 larities and detailed differences therefore show the effect produced by 

 similar ways of life on slightly differing populations. For example, a 

 postorbital bar developed in both groups, for attachment of the large 

 masseter muscle, but whereas in the horses it is formed wholly of a 

 process of the frontal bone, in ruminants there is a union of processes 

 of the jugal and frontal. Many such similarities and differences are 

 seen throughout the body, and especially in the limbs. 



The skull of later artiodactyls shows changes of shape to accom- 

 modate the very deep molars and to support the horns that are com- 

 monly found. It becomes very high (as in horses), and there is a sharp 

 kink between the basisphenoid and presphenoid, so that the face 

 slopes steeply downward. The facial bones become large and the 

 parietals restricted to the vertical posterior face of the skull, to which 

 the powerful neck-muscles are attached. In many ruminants there is 

 a scent-gland, lying in a pre-orbital fossa of the skull and opening on 

 the side of the head. The pre-lacrymal fossa is a gap in the skull, where 

 the nasal cavity is separated from the outside only by the skin. 



