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Why are tarsiers classified more closely with monkeys and apes than with lemurs and lorises?

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Illustration comparing the body plans of representative anthropoids, showing a male western lowland gorilla, a male human, a male bonobo, a female Bornean orangutan, and male and female northern white-cheeked gibbons, all labeled and depicted to scale. — primate Representative apes (superfamily Hominoidea).

Tarsiers are classified with monkeys, apes, and humans in the suborder Haplorrhini because they share specific anatomical features that distinguish them from lemurs and lorises, which belong to the suborder Strepsirrhini.1

The classification is based on several key biological differences:

Nose Structure: Unlike the moist, hairless muzzle of strepsirrhines, tarsiers possess a dry, hair-covered nose, a trait they share with other haplorrhines.21
Visual System: Tarsiers lack the tapetum lucidum—a reflective layer behind the retina found in lemurs and lorises—and instead possess a fovea, an area of enhanced vision.13
Reproductive Biology: Haplorrhines, including tarsiers, have a more complex, vascularized uterine wall that forms a hemochorial placenta, which allows for intimate contact between maternal and fetal bloodstreams. In contrast, strepsirrhines have a less intimate epitheliochorial placenta and do not undergo menstruation.1

These shared derived characteristics indicate that tarsiers are more closely related to the anthropoid primates than to the lemur-loris lineage.1