Prosimians

Often used as models for early primate anatomy and behavior, prosimians include a rich diversity of species exhibiting a complex range of social behavior, dietary specializations, and locomotor habits. Although more geographically widespread in the past, today these generally small primates are restricted to the Old World. Five of the eight major prosimian groups are known exclusively from Madagascar, while other species are found in Africa and Asia. While prosimians are only distantly related to humans, this group has allowed scientists to identify common anatomical and behavioral traits that link all primates.

Traditionally, the order Primates is divided into two major suborders,Prosimii and Anthropoidea. The latter group includes monkeys, apes, and humans, while prosimians are comprised of eight distinct families, including the cheirogaleids, lemurids, lepilemurids (megaladapids),daubentoniids, indriids, galagids, lorisids, and tarsiids. However, this Linnaean taxonomic arrangement has proven controversial, as several recent studies have shown that Prosimii isn't a natural, monophyletic group comprised of a single common ancestor and all of its descendants. Instead, this suborder is a basal grade of primates, linked by primitive characteristics, that includes those species more primitive than Anthropoidea. Modern primatologists have thus advocated a system that divides Primates into two large, monophyletic clades: Haplorhini, which consists of Anthropoidea and tarsiids, and Strepsirhini, which includes all of the traditional prosimian families with the exception of tarsiids. Although based on phylogenetic analyses that have found tarsiids to be closer to anthropoids than to the other traditional prosimians, this nomenclature is still not universally accepted. Thus, this entry will consider all eight of the traditional prosimian families, as well as a handful of fossil groups whose relationships are poorly resolved. [Page 1958]

Source: © iStockphoto/Derek Dammann.

Depending on which classification scheme one follows, there exist between24 and 31 genera of living prosimians, which are further divided into 54 to 68 species. All prosimians share with other primates several basic characteristics, including grasping hands with opposable thumbs, the primitive mammalian claw modified into a flat nail, forward facing eyes that result in binocular vision, large brains, small litter size, and slow growth. The clade Strepsirhini is united by three distinct features: an unusual dental “comb,” a grooming claw on the second digit of the foot, and a laterally flaring talus (ankle bone). The clade Haplorhini, which includes anthropoids and tarsiers, is linked by several features of the soft anatomy, mostly relating to the nose and lips. Overall, the traditional Prosimii (including tarsiers) exhibits a relatively primitive anatomy compared to later primates, including a small brain. As a result, prosimians—especially cheirogaleids and lorisids—have often been used by primatologists as models for the primitive primate condition. However, despite their retention of primitive features, prosimians are a successful group that exhibits a wide range of behavior.