62 million years ago this mysterious mammal called Earth home

For over a century, the early Paleocene mammal named Mixodectes pungens has remained an enigma. First identified in 1883, its remains consisted of small fragments of fossilized teeth and bones – not enough for paleontologists to be able to draw many conclusions.

Researchers at that time could only guess about the creature’s habits and its position in evolutionary history.

Recently, however, a far more complete fossilized skeleton of one of these ancient mammals has been analyzed, and this has shed light on aspects of the animal’s anatomy, behavior, diet and evolutionary connections.

The research team found that this early mammal, that lived in western North America about 62 million years ago, was tree-dwelling, weighed around three pounds, and fed primarily on leaves.

Mixodectes related to primates and colugos

The researchers determined that Mixodectes was more closely related to primates and colugos – also known as flying lemurs – than previously thought.

“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,” said Eric Sargis, professor of anthropology in Yale University’s Faculty of Arts and Sciences. 

The skeleton, which includes a partial skull, teeth, spine, rib cage, and limbs, was discovered and excavated in New Mexico’s San Juan Basin, under a federal permit.

The well-preserved remains have helped scientists reconstruct how the species, which roamed the Earth shortly after the extinction of the dinosaurs, moved, ate, and interacted with the environment.

Built for climbing

The study revealed that Mixodectes was built for life in the trees. Its limb structure and claws suggest that it could cling to almost vertical surfaces, much like modern primates.

Its molar teeth were shaped for breaking down tough plant material, indicating an omnivorous diet with a strong preference for leaves.

“This fossil skeleton provides new evidence concerning how placental mammals diversified ecologically, following the extinction of the dinosaurs,” said  Stephen Chester, associate professor of anthropology at Brooklyn College/CUNY and lead author of the study.

“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.”

Unique ecological niche

At almost three pounds (.4 kilograms), Mixodectes was quite large for a tree-dwelling mammal of its era. It was much larger than another small, forest-dwelling mammal, Torrejonia wilsoni, that was discovered with it.

While Mixodectes ate primarily leaves, Torrejonia consumed mostly fruit, suggesting that the two species occupied different ecological niches.

Mixodectes and the evolutionary tree

The researchers conducted two phylogenetic analyses to determine Mixodectes’ evolutionary placement.

One analysis suggested it was an archaic primate, while the other classified it as a primatomorphan – part of a group that includes primates and colugos, but not tree shrews.

“While the study doesn’t entirely resolve the debate over where mixodectids belong on the evolutionary tree, it significantly narrows it,” commented Sargis.

These discoveries not only enhance our understanding of Mixodectes but also provide insight into how early mammals evolved and diversified in changing forest environments.

Looking back at the post-dinosaur world

The finding of this Mixodectes fossil offers a glimpse into the ecosystems that arose in the aftermath of the mass extinction that eradicated the dinosaurs.

The initial Paleocene was an era of accelerated evolutionary transformation, as mammals started to diversify and utilize the ecological niches left vacant by the demise of the large reptiles.

During this time, the forests of North America were lush and humid, with plenty of room for arboreal mammals, such as Mixodectes, to flourish.

The fact that they had adapted claws for climbing indicates that they competed and coexisted with other arboreal species, such as early primate ancestors.

Their diet split – with Mixodectes being leaf specialists and other species preferring fruit – could have reduced direct competition and enabled several species to coexist in the same environment.

Apart from its role in ecology, the adaptation of Mixodectes as a specialist tree-dweller is in keeping with the wider pattern of mammals adapting to make use of tree habitats.

These initial experiments in climbing, grasping, and dietary specialization laid the foundation for the eventual emergence of primates, which set the stage for the evolutionary path that would eventually give rise to humans.

The full study was published in the journal Scientific Reports.

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