Earliest Placental Mammal Described in Breathtaking Detail

BY Jenny Marder  February 8, 2013 at 3:45 PM EST

Some 65 million years ago, an asteroid possibly as big as Mount Everest plowed into the Earth, wiping about 70 percent of all species, including dinosaurs, off the planet. Why this event killed off the dinosaurs but spared other species, including many mammals, birds, turtles, starfish and frogs, is a great mystery.

Also disputed has been whether placental mammals emerged before or after this event. Placental mammals are those that give birth to live young after nourishing them via a placenta rather than laying eggs. That includes 5,100 living species — among them, us. But a team of scientists claim to have resolved this latter question, according to a new study, published in Thursday’s issue of the journal, Science.

A tiny, scrambling, chipmunk-like creature, they found, is the common ancestor to all placental mammals. It ate insects, climbed trees, weighed less than half a pound, and gave birth to blind, hairless babies. And it emerged after the dinosaurs died out, not before. They’ve given it a very unimaginative name: the “hypothetical placental ancestor.”

From this strange creature sprang all the placental mammals we know today: monkeys and mice, dolphins and deer, bats and dogs and humans.

It’s a fascinating finding, but most astonishing is the amount of detail the scientists have managed to gin up about this hypothetical super ancestor.

It would have ranged from the size of a chipmunk to that of a shrew. It had a long tail, sharp teeth and well-developed sense of smell. Its brain had a feature that linked both hemispheres, called the corpus callosum. (We have that too.) Like marsupials, it bore its young live. But unlike marsupials, it probably had a long gestation period, said Michael Novacek, provost and curator at the American Museum of Natural History and one of the study’s authors.

Novacek and other scientists explain more in this video by the American Museum of Natural History:

Researchers pieced together a picture of this animal after painstakingly recording and mapping some 4,500 physical traits for 83 mammalian species. They did this using a matrix-like computer program called the MorphoBank. Over six years, members of the 23-member team entered DNA and morphological data — physical characteristics — on these species.

“The way this worked, you could be working in the cloud, in this massive matrix, entering a character there, and you suddenly see other images pop up on the matrix, because a guy from Argentina was entering data there,” Novacek said.

By the end, they had 390,000 cells, or windows, each describing a particular feature for a particular species. A feature could include the presence or absence of teeth or the shape and texture of those teeth, for example.

“We were looking at really minute evolutionary features on a very small scale, said Michelle Spaulding, also a study author, from the Carnegie Museum of Natural History. “Things like the formation or condition of bones in the skull, the number of toes, the number of vertebrae, or stripes on a tail.”

They spent years nailing down which features to include, and eventually assembled the data and entered it into a high-powered computer. The computer mapped that data, producing a tree that showed the relationships of these mammals. And using all the characteristics and relationships, they were able to trace the species to a common ancestor, and create a fantastically detailed description of that animal.

The next step, Spaulding said: to add more and more mammals.

“The beauty of it being on the web platform MorphoBank is anyone can work on it,” she said. “If you have a fossil and you want to know what it is, you can access this matrix. You don’t have to travel. It’s great for graduate students or full-blown scientists that just don’t have the resources to travel.”

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Tom Kennedy and Patti Parson contributed to this report.

Photo credit: An artist’s rendering of the hypothetical placental ancestor, a small, insect-eating animal. Image courtesy of Carl Buell.