Final answer:
The Plesiadapiforms were not classified as true primates due to their smaller front teeth, lack of bipedalism, and smaller brains. They lacked the advanced dexterity, vision reliance, and sophisticated brain development characteristic of modern primates.
Step-by-step explanation:
The Plesiadapiforms displayed several characteristics that differentiated them from true primates. Although Plesiadapis, the oldest known primate-like mammal, exhibited primate tendencies, it lacked certain primate features that were essential for classification within the Primate order. Specifically, Plesiadapiforms had small front teeth, compared to primates which developed larger canines and incisors for a more varied diet. Plesiadapiforms did not demonstrate the upright posture indicative of bipedalism, which is observed in many primate species. Also, they hadn't developed the brain size increase, particularly in the frontal lobes, which is associated with the higher levels of intelligence seen in primates. Modern primates are characterized by evolutionary advantages such as enhanced dexterity, increased reliance on vision over smell, and sophisticated brain development. Thus, traits such as a fully erect posture, the shortening of arms relative to legs, increased hand dexterity, and significant brain size increment in front lobes, coupled with a decreased bone mass of the skull and face, were absent or not fully evolved in Plesiadapiforms, marking them as not quite a primate.