Infraorder Simiiformes
The infraorder Simiiformes was named in...
Distinguishing Characteristics
Plesiomorphies
Dichromatic (two-color) vision, inherited from basal primata, preserved in platyrhini. Modified to trichromatic (three-color) vision in catarhini.
Apomorphies
The diagnostic features used to distinguish monkeys (simiiformes) from other simians (haplorhines), include:
Absent sensory whiskers
Single pectoral pair of mammae
Unsheathed, pendulous penis
Bony plate in back of orbit
Longer gestation
Absent rhinarium
Fully binocular vision
Increased encephalization (see neoteny, below)
More rounded cranium (see neoteny, below)
Neoteny
In addition, simiiformes are neotenous compared to other primates. This means that we retain a more juvenile appearance.
Extant Simiiformes
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References
Williams, B. Kay, R. Kirk, C. New perspectives on anthrpoid origins. PNAS March 16, 2010 107 (11) 4797-4804; https://doi.org/10.1073/pnas.0908320107