The planets earliest mammals are an important part of our evolutionary history, yet remain shrouded in mystery. One of them, Mixodectes pungens lived in western North America during the early Paleoceneabout about 66 to 56 million years ago. It was first discovered over 140 years ago by paleontologistand 1881 Popular Science profile subjectEdward Drinker Cope. But most of what we know about them came from tiny jawbone fragments and fossilized teethNow, we may have some answers, thanks to a well-preserved 62 million-year-old skeleton. This particular Mixodectes pungens specimen represents a mature adult that weighed about three pounds, lived in trees, feasted on leaves, and may be among our closer evolutionary relatives. The findings are detailed in a study published March 11 in the journal Scientific Reports. Get the Popular Science newsletter Breakthroughs, discoveries, and DIY tips sent every weekday. By signing up you agree to our Terms of Service and Privacy Policy.A 62-million-year-old skeleton of this quality and completeness offers novel insights into mixodectids, including a much clearer picture of their evolutionary relationships, study co-author and Yale University anthropologist Eric Sargis said in a statement. Our findings show that they are close relatives of primates and colugosflying lemurs native to Southeast Asia making them fairly close relatives of humans.Mixodectes belonged to an extinct family known as mixodectids andCretaceous-Paleogene extinction event that killed off non-avian dinosaurs about 66 million years ago. Many scientists believe that this mass extinction paved the way for the rise of mammals.The skeleton in this study was collected in New Mexicos San Juan Basin by study co-author and New Mexico Museum of Natural History & Science paleontology curator Thomas Williamson. The specimen includes a partial skull with teeth, spinal column, rib cage, forelimbs, and hind limbs.By the anatomy of its limbs and claws, the team believes that it lived in trees and could vertically cling to tree trunks and branches. Its molar teeth had crests that are used to break down abrasive material, which suggests an omnivorous diet heavy on the leaves.The skeleton, discovered in New Mexicos San Juan Basin, is the most complete of its kind known to exist. CREDIT: Chester et al. 2025 This fossil skeleton provides new evidence concerning how placental mammals diversified ecologically following the extinction of the dinosaurs, study co-author and Brooklyn College paleontologist Stephen Chester, said in a statement. Characteristics such as a larger body mass and an increased reliance on leaves allowed Mixodectes to thrive in the same trees likely shared with other early primate relatives.According to the team, Mixodectes was rather large for a tree-dwelling mammal in North America during the early Paleocene. For example, the Mixodectes skeleton in this study is significantly larger than a partial skeleton of Torrejonia wilsoni. This small arboreal mammal from an extinct group of primates called plesiadapiforms was discovered alongside it. While Mixodectes primarily ate leaves, Torrejonia mostly ate fruit. The authors believe that these distinctions in both size and diet suggest that mixodectids likely occupied a unique ecological niche in the early Paleocene that set them apart from their tree-dwelling contemporaries.Mixodectids were also euarchontans, a group of mammals that consists of treeshrews, primates, and colugos. To clarify just where Mixodectes fits on the evolutionary tree, the team conducted two phylogenetic analyses. One analysis supported that they were archaic primates, while the other did not. However, the latter analysis did verify that mixodectids are primatomorphans. This group Euarchonta was made up of primates and colugos, but not treeshrews.While the study doesnt entirely resolve the debate over where mixodectids belong on the evolutionary tree, it significantly narrows it, Sargis said.