What critical physical difference related to tail function separates New World monkeys from Old World monkeys?

Many New World monkeys possess fully or partially prehensile tails capable of gripping.

A significant evolutionary divergence between the primate infraorders leads to distinct differences in caudal structure and function, particularly concerning the tail. Many New World monkeys, a group that includes tamarins, have evolved tails that are prehensile, meaning they have the physical capability to grasp or hold onto branches. This gripping ability provides an extra limb for stability and locomotion within the canopy. Conversely, Old World monkeys fundamentally lack this feature; their tails are primarily used for balance and do not possess the muscular structure or dexterity required for gripping objects or branches, marking a clear anatomical distinction resulting from their divergent evolutionary paths.