Evolution and systematics

Although bearing many derived modern features, the extant tarsiers are the most ancestral haplorhine (tarsiers, monkeys, apes, and humans) living primates. The undisputed oldest fossil representative of tarsiers, Xanthorhysis tabrumi, is not less than 50 million years old, and was unearthed by Beard and colleagues in Eocene sediments in China. Once, the tar-siiform primates were widely distributed, fossil remains being found, for example, in Egypt, Germany, France, Thailand, and North America (Wyoming, New Mexico). The extinct tarsiiform family of Omomyidae shares quite a number of traits with the extant Tarsius, such as the olfactory bulb lying above the interorbital septum. But at present, the mosaic of shared and less similar characters does not allow a definitive decision about the probable direct ancestors of the present day tarsiers.

Like all other haplorhine primates, the tarsiers are very probably derived from diurnal ancestors, as they have lost the tapetum lucidum in their eyes, a reflecting layer that maximizes light-gathering capacity. By contrast, this reflecting tapetum is characteristic of all nocturnal strepsirhine (prosimian) primates. Sharing a central fovea in the retina of their eyes, a fused frontal bone, and a posteriorly closed orbit with anthropoid primates, as well as many other features, the tarsiers are recognized, today, as a sister group of the anthropoid primates. Their closest living relatives are South American platyrrhine monkeys. This is also supported by recent findings in molecular genetics.

Therefore, anthropoid primates and tarsiers, together, have to be united in the suborder Haplorhini. As Groves found in 1998, the term prosimian is no longer appropriate in a formal taxonomic sense, but belongs in the realm of folk taxonomy. To include tarsiers within the prosimians, i.e. together with the lemurs, galagos, etc., however, is somewhat misleading, as it groups these higher, haplorhine primates falsely with the less closely related Strepsirhini.

At present, the single extant family, Tarsiidae, includes only one genus, Tarsius, although the validity of a second genus Ra-bienus was seriously discussed by Groves in 1998. Until 1984 only three tarsier species were recognized, but currently six

A spectral tarsier (Tarsius spectrum) in Sulawesi, Indonesia. (Photo by Anup Shah/Naturepl.com. Reproduced by permission.)
A western tarsier (Tarsius bancanus) can turn its head almost 360°. (Photo by Frans Lanting/Minden Pictures. Reproduced by permission.)
The Philippine tarsier (Tarsius syrichta) has a head and body only about 6 in (15.2 cm) long. (Photo by Ron Austing/Photo Researchers, Inc. Reproduced by permission.)

species are recognized. Two or three more species may be added within the coming years.

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