For a furry, mouse-like creature that lived 125 million years ago, the ability to climb was no mean feat. It likely helped it scurry up trees to capture insects to eat and to hide in bushes to keep from being eaten itself by hungry theropod dinosaurs.
Climbing ability takes on a special significance in this animal because, as scientists from the Carnegie Museum of Natural History report today in the journal Nature, it is the earliest-known ancestor of today's placental mammals -- mammals, like humans, that nourish their young through a placenta.
Its fossilized skeleton was unearthed in northeastern China's famed Liaoning dinosaur quarry just three years ago, said Zhe-Xi Luo, associate curator of vertebrate paleontology. Not only is the specimen very old, but also it is unusually well-preserved, providing investigators with a full set of bones as well as cartilage, carbonized fur and impressions of fur.
"When you find something like this, it's just incredible," said John Wible, associate curator of mammals, who swears the discovery gave him heart palpitations. He is co-author of the Nature paper with Luo and Qiang Ji of the Chinese Academy of Geological Sciences. Justin Georgi of the museum also contributed.
They dubbed it Eomaia scansoria -- Eomaia is Greek for "ancient mother" and scansoria is Latin for "climber."
Older mammal specimens exist, but placental mammals are of particular interest. Scientists are curious about what might have contributed to the success of placental mammals, in contrast to egg-laying mammals and marsupials, such as kangaroos and opossums, as well as other mammal types that ultimately hit evolutionary dead-ends.
"It seems at least possible that a propensity to climb gave them the upper hand," said Anne Weil, a Duke University paleobiologist who wrote an accompanying commentary. Other examples of early mammals from China seem better adapted to dwelling on the ground than in the trees; none of those lineages have survived to present day. But she cautioned that no conclusions can be drawn from studying a single specimen, even one as good as Eomaia.
If nothing else, the fossil shows that placental mammals were a diverse lot. Younger fossils have revealed features consistent with terrestrial creatures living in sand dunes, or on riverbanks, Luo said. Eomaia didn't necessarily live in trees, but its ability to climb shows that it was able to fill a niche of its own.
Eomaia is just 10 million years older than what had previously been considered the oldest placental fossil. Those remains consisted of little more than a few teeth, however. The next oldest skeletal remains are 80 million years old, leaving a large gap in knowledge, Luo said.
"Now, Eomaia has provided all of that," he added.
In many ways, Eomaia held few surprises. Wible said this fossil appears to be the earliest evidence of fur -- the impressions surrounding the skeleton show both long "guard" hairs and shorter, clumpy underhair. But most scientists assumed early mammals had fur.
The shape of its claws, as well as the shape of its foot, are similar to modern climbing animals. Its teeth suggest it subsisted on insects.
It's not clear whether this was an animal that was active during the day or night. Nocturnal animals normally have enlarged eyes, but Eomaia's skull had been crushed, making it difficult to determine whether it had large eye sockets. Wible guesses that a bush-dwelling Eomaia would not have wanted to go eye to eye with a theropod during daylight hours, however.
The closest thing to a surprise is the presence of epipubic bones, extra bones in the pelvis that strengthen the abdomen wall so that it can carry the weight of a newborn.
Though the young of the Eomaia would be nourished through a placenta while in the womb, the presence of epipubic bones suggests newborns would latch onto a nipple or otherwise hang on the mother for some time following birth, similar to marsupials, which carry their young externally for extended periods.
Eomaia is the earliest known placental, but it's unlikely that it was the first. S. Blair Hedges, a molecular geneticist at Penn State University, has studied the mutation rate of certain genes and developed what's called a molecular clock, providing an estimate of when species branched off from each other. By his calculations, marsupials and placentals diverged on the evolutionary tree about 173 million years ago -- about 50 million years earlier than Eomaia.
"I should be jumping up and down, saying [Eomaia] confirms what I've been saying all along," Hedges said yesterday. But because Eomaia is only 10 million years older than the previous oldest placental specimen, it doesn't do much to narrow the gap between the fossil record and his predictions, he said.
