The Pygmy Marmoset (Callithrix (Cebuella) pygmaea) is an interesting little primate. While it is often said on documentaries or zoo enclosure descriptions that they are among the most "primitive" of primates, marmosets actually have a number of derived features. On their hands, for instance, they have claws called tegulae instead of flattened nails. At first this would seem to be a characteristic that was retained from their ancestors, but tegulae are actually modified nails, meaning that the marmosets evolved claws in something of an evolutionary reversal. Likewise, marmosets often give birth to twins, which suggests that at one time they were larger but as they dwarfed the birth canal became too small to allow for the passage of one large baby, the development of twins (each of which would be smaller) being selected for as size decreased.
Looks cute, but also looks like he could remove a digit or two in the blink of an eye.
Why wouldn't smaller monkeys just have one smaller baby?
I'd think twins would be a primitive trait, since primates have 2 teats. Any other indications of former larger size? (eg. long pregnancy as seen in gibbons IIRC)