55-34 million years ago
Primates adapted to an arboreal life with forward-facing eyes, a postorbital bar to support the eye, grasping hands, and other skeletal features.
One group of mammals, the primates, adapted to life in the trees. Grasping hands are especially useful in slow movements on slender branches (Cartmill, 1974). They developed large, forward-facing eyes and improved vision. Higher primates evolved better color vision which was obviously important in distinguishing between leaves and fruits in the treetops, as the following images demonstrate.
In the adapid Smilodectes and omomyid Teutonius, the frontal bone forms
the superior and lateral borders of the orbit. The interparietal bones
fuse with the occipital. The interparietal forms more of a shelf in non-human
primates and earlier hominids. The two centers of ossification of interparietal
may remain separate bones. After plesiadapiforms, primates possess a groove
in the auditory capsule for the promontory artery. Tree shrews and prosimians
may possess one nasoturbinal, four ethmoturbinals, two ectoturbinals,
and one maxilloturbinal. A vestigial nasoturbinal is present in gibbons.
In primates, the length of the snout shortened.