176 U.S. NATIONAL MUSEUM BULLETIN 2 73 



where they moved by using- the slender vines as supports. This 

 group of animals included the ancestors of present day Ateles, Brach- 

 yteles, Lagothrix, and Alouatta. The locomotor system developed 

 structural adaptations in accord with arboreal habits. A common 

 genetic background and the same selective forces were responsible 

 for the acquisition of the same muscular pattern, very significantly 

 in the shoulder region, as a response to the needs for brachiation. 

 Presumably when the howler line branched off, the increase in size 

 of the hyolaryngeal organs introduced new mechanical problems for 

 the locomotor system. The changes in the neck severely restricted 

 the ability of these early howlers to progress through swdnging by 

 their arms. They had to revert to quadrupedal habits and either 

 reversed some of their muscular arrangements or retained those 

 which had not yet become fully adapted for brachiation. The latter 

 seems the simpler explanation. The paleontological evidence (Stirton, 

 1951) is suggestive of an early separation of the howlers from the 

 Atelines, but, unfortunately, it is not conclusive. 



