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CENOZOIC ERA

PALEOGENE PERIOD

Paleocene Epoch

65-55 million years ago

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Paleocene animals known as plesiadapiform were primitive relatives of primates.

Plesiadapiformes, which are either primitive primates or a sister group to primates, possessed a postorbital ligament which protected the eye laterally. Among other things, such support may ensure better vision while chewing. The frontal bones of the primitive plesiadapiform Palaecthon formed a process over the lateral portion of the orbit. This process was not present in basal mammals such as Morganucodon. In more advanced primates, this forms postorbital process from the frontal bone fused with the jugal (zygomatic) bone to form a complete postorbital bar. The zygomatic bone joined the temporal to form a postorbital bar and the lacrimal is confined to the orbit.

Later Paleocene forms such as Phenacolemur, Plesiadapis, and Ignacius are placed in a group called the plesiadapiforms. The Plesiadapiforms may not be a monophyletic group (Martin, 1993; Martin 1990). Their diversity in the Paleocene and Eocene was reduced after more modern primates evolved. The only living descendants of the plesiadapiforms are the flying lemurs of the Order Dermoptera (the closest living relatives of the Order Primates). The relationship is most evident in the elongated phalanges of the middle fingers and toes (Beard, 1990; Kay, 1990; Hamrick, 1999).


Palaechthon of the Middle Paleocene had a tooth formula of 2,1,3,3/2,1,3,3-the same as in modern prosimians and New World Monkeys. Although there was no complete postorbital bar to enclose the eye socket from behind (as in modern primates), the posterior portion of the orbit was constricted (Carroll, 1988).