A small mixodectid mammal called Mixodectes pungens had skeletal features adapted to living in trees, largely dined on leaves, and weighed about 1.3 kg, according to an analysis a remarkably complete skeleton discovered in New Mexico.
Mixodectes pungens (foreground) inhabited the same forests as early primates like Torrejonia wilsoni (background). Image credit: Andrey Atuchin.
Mixodectes pungens lived in western North America during the Early Paleocene epoch, around 62 million years ago.
First described in 1883 by the famed American paleontologist Edward Drinker Cope, this species was previously known from fossilized teeth and jawbone fragments.
In new research, Yale University’s Professor Eric Sargis and his colleagues examined a new skeleton of Mixodectes pungens — the most complete dentally associated skeleton yet recovered for a mixodectid mammal — from the Nacimiento Formation of the San Juan Basin, New Mexico, the United States.
“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,” Professor Sargis said.
“Our findings show that they are close relatives of primates and colugos — flying lemurs native to Southeast Asia — making them fairly close relatives of humans.”
The researchers determined that the new specimen belonged to a mature adult that weighed about 1.3 kg.
The anatomy of the animal’s limbs and claws indicate that it was arboreal and capable of vertically clinging to tree trunks and branches.
Its molar teeth had crests to break down abrasive material, suggesting it was omnivorous and primarily ate leaves.
“This fossil skeleton provides new evidence concerning how placental mammals diversified ecologically following the extinction of the dinosaurs,” said Dr. Stephen Chester, a researcher at Brooklyn College, City University of New York and the Yale Peabody Museum.
“Characteristics such as a larger body mass and an increased reliance on leaves allowed Mixodectes pungens to thrive in the same trees likely shared with other early primate relatives.”
Mixodectes pungens was quite large for a tree-dwelling mammal in North America during the Early Paleocene.
For example, the new skeleton is significantly larger than a partial skeleton of Torrejonia wilsoni, a small arboreal mammal from an extinct group of primates called plesiadapiforms, that was discovered alongside it.
While Mixodectes pungens subsisted on leaves, Torrejonia wilsoni’s diet mostly consisted of fruit.
These distinctions in size and diet suggest that mixodectids occupied a unique ecological niche in the Early Paleocene that distinguished them from their tree-dwelling contemporaries.
Two phylogenetic analyses performed to clarify the species’ evolutionary relationships confirmed that mixodectids were euarchontans, a group of mammals that consists of treeshrews, primates, and colugos.
“While one analysis supported that they were archaic primates, the other did not,” Professor Sargis said.
“However, the latter analysis verified that mixodectids are primatomorphans, a group within Euarchonta composed of primates and colugos, but not treeshrews.”
“While the study doesn’t entirely resolve the debate over where mixodectids belong on the evolutionary tree, it significantly narrows it.”
The findings appear today in the journal Scientific Reports.
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S.G.B. Chester et al. 2025. New remarkably complete skeleton of Mixodectes reveals arboreality in a large Paleocene primatomorphan mammal following the Cretaceous-Paleogene mass extinction. Sci Rep 15, 8041; doi: 10.1038/s41598-025-90203-z
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