Nose Today, biologists and anthropologists classify monkeys into two groups: New World monkeys, situated in Central and South America, and Old World monkeys, found in Africa and Asia (Stanford, Allen and Anti?? n 2009, 180). The defining feature that separates New World monkeys from Old World monkeys, into platyrrhines and catarrhines respectively, is the fact that platyrrhines generally have broader and flatter noses, with nostrils far apart and facing sideways, and catarrhines have relatively narrower noses that project forward, and nostrils that face downward (Fleagle 1999, 137).
Skull When examining the cranium, it is also distinctive of a platyrrhine if contact between the zygomatic and the parietal bones of the skull can be spotted, whereas, in a catarrhine, a frontal-sphenoid junction can be found instead. Furthermore, New World monkeys have the eardrum connected to the outer ear by a loop made of bone, while alternatively, in Old World monkeys, the tympanic membrane is attached to the external ear by an ectotympanic tube.
The Old World monkeys also tend to have relatively longer and narrower skulls (Ibid.). Teeth In the jaws, there are also recognizable dental differences, in that the platyrrhines have three premolars, giving a dental formula of 2:1:3:3 (Swindler 2002, 40), and the catarrhines have two premolars, giving a dental formula of 2:1:2:3 (Ibid. p. 44). Catarrhines also have sharp-edged, sectorial premolars on either side of the mandible, specialized for honing the upper canines, sharpening them in preparation for tearing apart food. Catarrhine molars are bilophodant as well-teeth that have two transverse ridges.
These molars feature shearing, dagger-like cusps (Fleagle 1999, 186) for grinding the tough cell walls of leaves (Strier 2007, 61). These molars are either comparatively smaller, or missing altogether, in the platyrrhines situated in the Western hemisphere. Diet In conjunction with their dentition, catarrhines and platyrrhines also differ in their diets. With the exception of several species of howler monkey, catarrhines are more folivorous than platyrrhines, and in addition to the teeth they have adapted, they also have specialized digestive tracts to digest leafy material (Stanford, Allen and Anti??n 2009, 183).
Colobines, a subfamily of the catarrhines, have developed sacculated stomachs, in which leaves, and sometimes, even highly toxic seeds can be digested by bacteria in the gut (Strier 2007, 59). Although both Old and New World monkeys feed on fruit, no Old World monkey feeds on insects exclusively (Fleagle 1999, 220). The New World monkeys, on the other hand, are generally more frugivorous as opposed to folivorous, and some species such as the capuchin monkey, the Common Squirrel Monkey and the Red-Handed Tamarin also have insects make up a large percentage of their diet (Ibid. p.143-4).