When mammals took to the skies

The Mesozoic Era, from 252 million years ago to 66 million years ago, is often called the Age of Dinosaurs. To generations of palaeontologists, early mammals from the period were just tiny nocturnal insect-eaters, trapped in the shadows of leviathans. In recent years, scientists have significantly revised the story.

Mammals already had evolved into a staggering range of forms, fossil evidence shows, foreshadowing the diversity of mammals today. In a study published recently, a team of palaeontologists added some particularly fascinating new creatures to the Mesozoic Menagerie. These mammals did not lurk in the shadows of dinosaurs. Instead, they glided far overhead, avoiding predatory dinosaurs on the ground — essentially flying squirrels of the Jurassic Period, from an extinct branch of mammals that probably still laid eggs. The fossils “are most primitive-known mammal forerunners that took to air,” said Zhe-Xi Luo, a palaeontologist at the University of Chicago, USA who led the research.

New branches

The first Mesozoic mammal fossils came to light in the early 1800s, but for generations, palaeontologists struggled to find more than teeth and bits of bone. In the late 1990s, they hit the jackpot. At a site in Northeastern China, hillside after hillside turned out to contain stunning mammal fossils, most dating back about 160 million years. Researchers were suddenly able to examine entire skeletons, some still bearing impressions of skin and hair. As new fossils get unearthed, scientists are using them to draw in many previously unknown branches on the mammal family tree.

All living mammals are divided into three main branches. Platypuses, which still lay eggs, belong to the oldest; their ancestors split off from those of other living mammals roughly 170 million years ago. Millions of years later, the other branch split. One lineage produced the marsupials, such as kangaroos, which finish development in a pouch. The other lineage, our own, makes up the vast majority of living mammal species. Placental mammals all develop inside a uterus, drawing blood from their mothers. Some of the newly discovered mammal fossils belong to these three groups. Others belong to branches no one knew about before. Of those, some diverged from the common ancestors of living mammals, but more primitive mammals split off even earlier.

When palaeontologists looked at the size and shape of these fossils, they found that many did not fit the simple picture of early mammals as tiny insect-eaters. To the researchers’ surprise, a number of extinct species independently evolved bodies resembling those of living mammals. Some swam like otters, for example. Others scavenged, like raccoons, or dug into insect nests like today’s aardvarks.

Exquisitely preserved fossils

In 2007, Jin Meng, a palaeontologist at the American Museum of Natural History, and his colleagues reported finding the fossil of a 160-million-year-old mammal, called Volaticotherium, that looked as if it could glide. Today, placental mammals like flying squirrels and marsupials like sugar gliders travel through the air from tree to tree. But Volaticotherium belonged to a different lineage and independently evolved the ability to glide. They were not the only mammals to do so, it turns out. Zhe-Xi and his colleagues have now discovered at least two other species of gliding mammals from China, which they described in the journal Nature.

The fossils of the new species, Maiopatagium and Vilevolodon, are exquisitely preserved, revealing many details of their anatomy. Wing-like sheets of skin stretched from their cheeks to their legs and tails. They also had remarkably flexible shoulders needed to climb up trees and then manoeuvre through the air during a glide. Zhe-Xi and his colleagues discovered that the two new species are even more distantly related to living gliders than Volaticotherium. They belong to an extinct lineage called haramiyidans, which diverged from the ancestors of all living mammals over 200 million years ago. As a result, they had only some of the traits that define mammals today. While they had fur and were warm-blooded like living mammals, they were more like reptiles in some respects. They had not yet evolved the tiny chain of bones that allow living mammals to hear, for example.

The newly discovered fossils demonstrate unusual adaptations. To support gliding muscles, the animals’ collarbones joined in a V shape — “like the wishbone in a chicken,” said Guillermo W Rougier, a palaeontologist at the University of Louisville who was not involved in the new studies. Jin said that the growing number of fossil gliders showed that many different kinds of mammals followed the same evolutionary path. “They did their own experiments,” he said. The new fossils demonstrate just how many surprises early mammals have left to deliver, Guillermo said.