Short answers, pulled from the story.
New World monkeys first crossed the Atlantic Ocean during the Eocene epoch approximately 40 million years ago. This voyage occurred on a floating island of vegetation when the ocean was significantly narrower than it is today. The crossing separated them from their African ancestors and left them to evolve in isolation across South America and Mexico.
New World monkeys are distinguished by a broad, flat nose with nostrils that face sideways rather than downward. This anatomical feature gave the group its scientific name Platyrrhini, which is derived from the Greek words for broad nosed. This structure serves as the primary phenotypic distinction between them and the Catarrhini, or down-nosed simians that include Old World monkeys and apes.
The Atelidae family, which includes spider monkeys, woolly spider monkeys, howler monkeys, and woolly monkeys, possesses a tail that functions as a true fifth limb. This prehensile tail allows these primates to suspend their entire body weight from branches while their arms and legs remain free for foraging and locomotive activities. This trait has evolved at least two distinct times in platyrrhines and is absent in all other simians.
New World monkeys possess a dental formula of 2.1.3.2 or 2.1.3.3, consisting of 2 incisors, 1 canine, 3 premolars, and 2 or 3 molars. This differs significantly from the 2.1.2.3 formula found in Old World anthropoids, including gorillas, chimpanzees, bonobos, siamangs, gibbons, orangutans, and most humans. The presence of these extra premolars has allowed New World monkeys to develop a more versatile diet including fruits, nuts, insects, flowers, bird eggs, spiders, and small mammals.
The Callitrichidae family, which includes marmosets and tamarins, exhibits monogamous pair bonds and substantial paternal care of young. The pygmy marmoset measures just 14 centimeters in length and weighs only 120 grams, making it the smallest monkey in the world. These primates have developed complex social systems that involve cooperative breeding where other members of the group help to care for the young.
The five families of New World monkeys are Callitrichidae, Cebidae, Aotidae, Pitheciidae, and Atelidae. The Callitrichidae family includes marmosets and tamarins, the Cebidae family includes capuchins and squirrel monkeys, and the Aotidae family consists of night or owl monkeys. The Pitheciidae family includes titis, sakis, and uakaris, while the Atelidae family includes howler, spider, woolly spider, and woolly monkeys.