I will consider support for the dwarfing hypothesis in New World monkeys. Since evolution has shown to result in a general increase in body size, the case of reduced body size in the New World primates is quite unusual. To explain the phenomenon, the dwarfing hypothesis has been proposed (Martin, 1992).
The dwarfing hypothesis implies that there must have been selective pressure that favored a reduction in body size. Phyletic dwarfing is then presumed to be fundamental to the further development of the species' characters (Martin, 1992). Callitrichids are not the only primates that are thought to have undergone phyletic dwarfing. For example, there are also dwarf lemurs and dwarf bushbabies (Martin, 1990).
I will consider the dwarfing hypothesis in the context of analyzing whether the morphological features of the callitrichids are primitive or derived. Although the callitrichids have many common features (including twinning, small body size, reduction of the molar tooth row, simplification of the upper molars, and claws) in this paper I will focus on the morphological features: body size, claws and the dental features of the Callitrichidae (twinning is an important common character of the callitrichids, but will not be considered here). One important hindrance to finding support for the dwarfing hypothesis has been gaps in the fossil record. However, using the available fossil data and modern studies of callitrichids I will show why each feature is primitive or derived, and how it applies to the dwarfing hypothesis. The New World Monkeys
The New World monkeys (Platyrrhines) can be divided into two families: the Cebidae and the Callitrichidae (the Callimico is also a Platyrrhine and shares many of the characteristics of the callichitrids, but the position of the Callimico within the Platyrrhines will not be considered here). The Callitrichidae family includes the marmosets and the tamarins, and will be the main focus of this discussion. Specifically, there has been disagreement as to whether the callitrichids are primitive or whether they are really specialized dwarf forms (Martin, 1992). Dental Characters
The callitrichids have triangular upper molars without a prominent hypocone (as is characteristic of the larger-bodied New World monkeys). This makes sense, as smaller-bodied primate species (such as the callitrichids) have less developed upper molars than larger-bodied primate species (Martin, 1992). Although it has been claimed that a missing hypocone on the upper molars is a primitive retention, fossil evidence shows that the ancestral anthropoid had a hypocone. It is therefore evident that the New World monkeys have secondarily lost the hypocone (Ford, 1980). Martin (1992) supports this conclusion, formally using the sister group principle. As Martin writes, "According to this principle, when there is a choice between two alternative character states as the ancestral condition for a particular group, the presence of only one of these characters states in the sister group identifies that as the likely ancestral state" (Martin, 1992). The sister group principle shows that the simple upper molars are a secondary reduction since Old World simian primates and fossil simians from the Fayum have more complex upper molars with a hypocone. Martin suggests that secondary reduction of the upper molars in the callitrichids occurred because of a change in their diet to insectivorous (insectivory is associated with small body size). The more triangular upper molars of the callitrichids are better for insectivory than the rectangular, hypocone-bearing molars of the larger, frugivorous true monkeys (Martin, 1992).
While the ancestral placental mammal dental formula included three upper and lower molars, the Callitrichidae have lost the third molars. The loss of teeth is the general trend for evolution of the primates, and loss of...