Adorable Squirrel-Like Creatures Shed Light on the Origin of Mammals

Mammals are incredibly diverse and successful, with member species scattered across land, ocean, skies, and even in space (ISS, represent). But the origins of our awesome clade are murky, and paleontologists have long debated when the earliest mammals first appeared on the Mesozoic scene.

Now, new research on a group of these early forebears, the haramiyids, has presented compelling evidence that mammals split off as a separate group at least as early as the late Triassic. It also reveals that early mammals became surprisingly diverse despite their rather terrifying neighbors—the dinosaurs.

The study was led by paleontologists based out of the American Museum of Natural History and the Chinese Academy of Sciences, and was published today in Nature.

"It has been portrayed that mammals lived in the shadow of dinosaurs were small, nocturnal creatures and their diversity was pretty much known," co-author and AMNH paleontologist Jin Meng told me. "It turns out that mammals' coeval with dinosaurs were much more diverse than previously known."

These findings are based on six well-preserved Chinese haramiyid fossils, dating back 160 million years to the middle Jurassic. Not only do the fossils represent three new species—called Shenshou lui, Xianshou linglong, and Xianshou songae—but they also required the creation of an entirely new clade, which the team named Euharamiyida.

Perhaps most importantly, the specimens put an end to a longstanding controversy over where haramiyids belong in the tree of life. Prior to this study, paleontologists only had a few jaw and teeth bones to go on when it came to categorizing the group.

"For many decades, paleontologists have debated whether haramiyids are mammals or non-mammalian animals that are somehow related to mammals," explained Meng. But the discovery of much more complete specimens changed all that.

"The new fossils we reported show the morphologies from the tip of the incisor to the tip of the toe and these structures are primarily mammalian," he said. "Because the oldest members of the group came from the late Triassic period, roughly 220-200 million years ago, we can infer that the origin time of mammals was at least in the Late Triassic, earlier than many previous research predicted."

These three new species resembled miniature squirrels, and judging by artistic depictions of them, they could have been painfully cute. They would have weighed anywhere between one to 10 ounces; ate insects, nuts, and fruit; and were uniquely adapted for tree-top living.

"They lived in the forestry of the Jurassic Park," said Meng. "The evidence is that they all have a gracile body built and long prehensile tail. But most importantly, their hands and feet have elongated fingers, which is a typical adaptation for grasping tree branches."

According to Meng, mammals of the period occupied many other niches, from land-dwellers, to underground burrowers, and some pioneering swimmers and gliders as well. Clearly, the euharamiyids were just one of many mammalian branches that were flowering in the Jurassic.

Far from Mesozoic shrinking violets, our mammalian progenitors in the age of the dinosaurs were enterprising foragers, and their remains contextualize the long evolutionary history of our clade.