What characterizes Plesiadapiforms compared to true primates?

Plesiadapiforms are an important group in the study of primate evolution, and they are generally considered to be proto-primates or pre-primates, rather than true primates. One of their main characteristics is that they lack defining features associated with more derived primates, such as the postorbital bar, forward-facing eyes, and a grasping hand with a fully opposable thumb. Instead, Plesiadapiforms exhibit primitive traits that are more similar to those of small mammals.

By not possessing characteristics that define true primates, such as those adaptations that facilitate arboreal living and refined sensory abilities, Plesiadapiforms highlight an evolutionary stage before the emergence of the primate order as we recognize it today. This distinction underscores their placement in the primate lineage, showing that while they are mammalian ancestors to primates, they do not quite fit the criteria that would classify them as true primates.