Final answer:
Plesiadapiforms are primeval primate-like mammals that are crucial to understanding early primate evolution, exhibiting beginnings of arboreal adaptations.
Step-by-step explanation:
Plesiadapiforms are often considered the oldest known primate-like mammals, dating back to just after the extinction event that marked the end of the dinosaurs and the beginning of the Cenozoic era. Plesiadapiforms possessed features that suggested a beginning of adaptation to an arboreal lifestyle, although they lacked some of the key characteristics found in modern primates, such as grasping hands and feet. They provide valuable insight into the early stages of primate evolution and help to bridge the gap between more primitive mammals and the first true primates.