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New Fossils Show 160-Million-Year-Old Mammals Were Just as Cool as Dinosaurs

The discovery of two new “mammaliaform” fossils are a testament to the badassery of our earliest forebears.
​Illustration of Agilodocodon scansorius. Image: April Neander, University of Chicago.

​The Mesozoic era is so heavily associated with dinosaurs that it can be easy to forget our own mammalian lineage got its start there too. But mammals didn't just eek out a living in this dino-dominated period. According to two papers published today in Science, they absolutely thrived in it.

The papers describe newly discovered fossils that represent the earliest arboreal and subterranean ancestral mammals ever found. These creatures both belonged to a group known as the mammaliaforms, which thrived about 160 million years ago, during the Jurassic period.

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"Mammaliaforms are extinct, distant relatives of modern mammals," biologist Zhe-Xi Luo, an author on both Science papers, told me over email. "[T]hey can provide the ancestral condition from which the more derived features of modern mammals evolved."

Image reconstruction from CT scanning the jaws of Agilodocodon and Docofosor fossils. Credit: April Neander, University of Chicago.

For example, the new tree-dwelling species, named Agilodocodon scansorius, displayed obvious adaptations for its canopied environment, including sharp claws and teeth honed for harvesting tree sap. Likewise, the burrowing animal, named Docofossor brachydactylus, was mole-like in its morphology. It's likely they even developed the ability to nurse their young.

"Several mammaliaforms, including the docodonts described in our Science papers, show evidence that they already evolved lactation," confirmed Luo. The key tip-off is that docodont specimens indicate that these animals went through two generations of teeth. "[T]his tooth replacement pattern is correlated with lactation during their early growth."

These animals are often seen as a kind of sideshow to the dinosaurs

"The extinct docodonts and several other mammaliaforms show exactly the same diphyodont tooth replacement, thus they must have had lactation as well," he added.

Not only do these new fossils push the timeline of mammalian evolution a little farther back, they also demonstrate the ecological diversity and evolutionary success of ancestral mammals. These animals are often seen as a kind of sideshow to the dinosaurs, but the fossils tell a different story.​

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The Docofossor fossil (left) and the Agilodocodon fossil (right). Image: Zhe-Xi Luo, the University of Chicago.

Indeed, when I asked Luo in what ways the dinosaurs shaped early mammalian evolution, he countered that instead, we should be asking in what ways did dinosaurs not affect mammalian evolution.

"[P]aleontologists have accumulated many Mesozoic mammals to show that, despite the dominance of dinosaurs, early mammals had done some great evolutionary experiments and exploited many niches that are just as diverse and interesting as those occupied by modern small mammals," he said.

Trees and burrows, for example, weren't the only habitats mammaliaforms carved out for themselves in the Pangean world. "Besides the newly discovered Agilodocodon as an arborealist, and Docofossor as a subterranean mammal, we also know the docodont Castorocauda is a swimmer," Luo told me.

"Some other mammaliaforms, such as Morganucodon, are terrestrial mammals," he continued. "Stem mammals were never as common as dinosaurs in the Mesozoic beds. But they are no less diverse than dinosaurs, and exploited many ecological niches."

It's natural to be captivated by the sheer spectacle of dinosaurs, and to marvel over their largesse and variety. But as Luo and his colleagues have amply shown with their studies, the animals that kicked off our own evolutionary line were no small shakes either.