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The origins of mammals are crucial to understanding human evolution, yet many details remain elusive. One notable early mammal, Mixodectes pungens, inhabited western North America during the early Paleocene era, approximately 66 to 56 million years ago. The species was initially identified more than a century ago by paleontologist Edward Drinker Cope, who was profiled in the May 1881 issue of Popular Science. However, much of what has been learned about these creatures is derived from small fossilized remains such as jawbone fragments and teeth collected over the years.
Recent developments may shed light on these ancient mammals, thanks to the discovery of a remarkably preserved skeleton estimated to be 62 million years old. This individual of Mixodectes pungens represents a mature specimen weighing around three pounds that lived in trees, primarily consuming leaves, and is considered a close evolutionary relative to some primates. The details of this significant find are discussed in a study published on March 11 in the journal Scientific Reports.
Eric Sargis, a co-author from Yale University, remarked on the discovery, stating, “A skeleton of this quality and completeness provides new insights into mixodectids and clarifies their evolutionary relationships.” The research suggests that these creatures are closely related to primates and colugos, the latter being flying lemurs found in Southeast Asia, potentially connecting them to our own lineage.
Mixodectes is part of an extinct family known as mixodectids, living during a period that followed the Cretaceous-Paleogene extinction event that eliminated non-avian dinosaurs. This extinction is widely believed to have facilitated the rise of mammals by creating ecological niches that earlier species could fill.
The skeleton was excavated in the San Juan Basin of New Mexico by Thomas Williamson, a curator from the New Mexico Museum of Natural History & Science, and includes a range of skeletal features such as a partial skull, spinal column, rib cage, and limbs.
Analysis of the fossil’s limb structure and claws indicates that Mixodectes was adept at climbing, suggesting a lifestyle spent in trees. Its molar teeth displayed distinct features that would have suited an omnivorous diet focused mainly on leaves.
Mixodectes, known to be the most complete fossil of its kind, offers insights into the diversification of placental mammals in the wake of the dinosaur extinction. Co-author Stephen Chester emphasized how these traits, including an increased body size and leaf-based diet, allowed Mixodectes to coexist with early primate relatives in shared arboreal habitats.
The study points out that Mixodectes was relatively large compared to other early Paleocene mammals. For context, it surpasses a contemporary species—Torrejonia wilsoni, a smaller arboreal mammal known from partial remains. While Mixodectes primarily consumed foliage, Torrejonia was more fruit-oriented. This variation in size and dietary preference suggests Mixodectes occupied a distinctive ecological role during its time.
Moreover, mixodectids belong to the group known as euarchontans, which encompasses treeshrews, primates, and colugos. The research team used phylogenetic analyses to clarify the position of Mixodectes on the evolutionary tree. Although one analysis indicated they might be archaic primates, others affirmed that mixodectids belong to the primatomorphan subgroup within Euarchonta, which consists of primates and colugos but excludes treeshrews.
“While this study does not conclusively resolve the ongoing debate regarding the evolutionary position of mixodectids, it significantly narrows down potential classifications,” Sargis remarked.
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