A scampering, hypothetical ancestor causes an evolutionary stir

What’s cute and furry and has a name only a paleontologist could love?

A tiny theoretical creature dubbed the hypothetical placental mammal ancestor is stealing the hearts of some evolutionary biologists -- and annoying others -- as it raises new debate over just when our early mammal ancestors began diversifying across the globe.

In a paper published Thursday in the journal Science, an international team of researchers described a project in which they used a vast database of fossil and other biological data, as well as DNA evidence, to reverse-engineer a hypothetical ancestor to the largest group of living mammals.


The scientists also theorized that this hypothetical critter would have begun spinning off new species of mammals after the demise of the dinosaurs -- a concept at odds with some current thinking.

“This is fairly novel to reconstruct an ancestor,” said study co-author and paleontologist Mike Novacek, the provost of science at the American Museum of Natural History. “I don’t know of any other.”

Placental mammals make up the largest branch of the mammalian family tree and include humans. Only marsupials and a small number of egg-laying mammals are excluded.

For the last several decades, scientists have debated just when it was that placental mammals began their explosive proliferation. Estimates based on DNA, or so-called molecular evidence, suggest that this began to occur alongside the dinosaurs, and that they managed to survive the mass extinction that occurred roughly 65 million years ago.

The study in Science, however, argues that this speciation began several hundred thousand years after the demise of non-avian dinosaurs. “An environment that radically changed may have offered more opportunities for a new group to take form and radiate,” Novaceck said.

Using a vast online database called Morphobank, a team of 23 scientists built a composite of the hypothetical ancestor based on the morphology, or appearance, of existing and extinct animals, as well as DNA data.

“The placental ancestor was a scampering species that had a diet of insects, a fleshy nose, a light underbelly in its fur, and a long tail,” wrote study co-author Maureen O’Leary, a paleontologist at Stony Brook University in Stony Brook, N.Y. “It was larger than a mouse, but smaller than a rat.”

Not everyone has fallen in love with the furry, hypothetical bug eater however, and discussion is likely to continue for some time as to when placental mammals began to proliferate.

“The bottom line is that this study is not convincing and will not settle the debate,” said Mark Springer, an evolutionary geneticist at UC Riverside, who was not involved in the study. “In contrast to what is reported by authors, the most fascinating result from this study is the tremendous amount of incongruence between the morphological and molecular data.”

Study co-author Novaceck said he did not expect the paper to end discussion, but he did say he hoped it would add weight to an ongoing argument.

Another debate involves the creatures name, and whether it deserves a less cumbersome moniker.

“People who think this organism is cute have asked me for a name,” Novacek said. “I tell them we can’t give it a Latin name, because it never really existed. It doesn’t have a nickname either. It’s just the hypothetical placental mammal ancestor.”

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