Because the cranial fragments show the absence of a post-orbital plate, they must come from a prosimian. Looking at the primate taxonomy, you can see that the loss of the premolar occurred in a lineage that had already split off from the prosimians, and that already had a post-orbital plate. This is shown in the text on pg. Such independent events ARE unlikely, but they do sometimes occur. He points out that the mandible is about the same size as that of a langur or a spider monkey.
Langurs are also catarrhines, while spider monkeys are platyrhines.
Even though we have no direct knowledge of the face at all, we can be pretty sure that it had a catarrhine nose. Put another way, the fact that both langurs and our fossil share the recent, derived trait of just two premolars, while the spider monkey does not, suggests that the langur and the fossil have a more recent common ancestor than either does with the spider monkey. The closer relative would naturally be more likely to look similar. The next week, you find two tiny molar teeth, which the project director identifies as belonging to a primate that must have been no larger than a medium-sized rat. The teeth have tall, pointy cusps.