Plesiadapiformes

Plesiadapiformes are an extinct order of mammaliforms that lived from the late Paleocene to the early Eocene, roughly 66 to 50 million years ago. They are widely considered to be closely related to primates, and in some classifications are treated as stem primates or as a sister group to true primates. Fossils have been found in North America, Europe, and Asia, with the richest record in western North America.

Anatomy and variation within the group reflect a diverse but primitive mosaic. Plesiadapiforms were generally small to medium in size and often possessed elongated snouts, a relatively small braincase, and distinctive dental features. Their dentition shows a mix of primitive mammalian traits and some derived characteristics that hint at a primate affinity, such as prominent incisors and specialized premolars. Many lineages exhibit adaptations for grasping, though most lack a fully developed postorbital bar, a feature characteristic of later primates. A notable exception is Carpolestes simpsoni, from the early Eocene, which possessed a divergent hallux with a nail rather than a claw, an early indication of primate-like grasping anatomy.

Taxonomy and significance: The group includes several families, such as Plesiadapidae, Carpolestidae, and Microsyopidae. The exact placement of plesiadapiforms within the primate lineage remains debated, with some researchers viewing them as primitive primates and others as a sister group to true primates. Their fossil record provides important insights into the early evolutionary steps toward primates, including shifts in dentition, limb function, and locomotion.