Nature On the left is the Maelestes gobiensis; on the right is a living hairy-taled mole Parascalops breweri from Allegheny County Click photo for larger image.

The discovery of a fossil of a new early mammal has sparked an analysis that suggests all placental mammals, including humans, in existence today had a common ancestor that lived at most 65 million years ago in the Northern Hemisphere.

Maelestes gobiensis was found in 1997 in Mongolia's Gobi Desert, and was stored in the American Museum of Natural History until 2003. Then, John Wible, mammal curator at the Carnegie Museum of Natural History, took a closer look and realized it was a new species.

Dr. Wible brought it to the Pittsburgh museum to study it and its relationships to other animals.

In a paper published yesterday in Nature, he and colleagues from the American Museum, the University of Louisville and the University of Cambridge concluded that Maelestes is a Eutherian mammal, a category that umbrellas placentals and their extinct relatives. Maelestes was not placental and its line became extinct.

At about 71 to 75 million years old, it lived around the time of dinosaurs velociraptor, oviraptor and protoceratops. The fossil is fairly complete.

"Most of the Cretaceous record for mammals is just made up of teeth and maybe bits and pieces of jaws," Dr. Wible noted. "So when you find something that's three-dimensionally well-preserved and preserves a certain proportion of the skeleton, you're very excited."

Still, the fossil is "not really all that interesting," he said with a chuckle. "It's really the analysis we did -- that's really the more interesting part of this."

Paleontologists and molecular systematists have long debated, both with one another and within their fields, when the mammalian version of Eve arose.

The findings support a traditional paleontologic view that there was an explosion of mammal diversity after dinosaurs died off around 65 million years ago, during what is known as the Cretaceous-Tertiary, or K-T, boundary.

One of those animals likely gave rise to modern-day placentals, which comprise 5,080 species of the 5,416 species of living mammals. With the exception of a few monotremes, which are egg-layers like the platypus, the remainder are marsupials or pouched mammals, like the kangaroo.

Molecular studies, in which DNA information from living species is plugged into mutation-based clocks, have suggested the ultimate placental mammal ancestor might have lived anywhere from 80 million to 140 million years ago. If so, it would have lived alongside dinosaurs.

"What we were able to show -- I think very conclusively -- with the data we have today is that none of those Cretaceous forms has anything to do with any modern placental group," Dr. Wible said.

Molecular analyses may arrive at different estimates because scientists cannot sequence the DNA of everything. As he put it, "more than 50 percent of the diversity of life is [already] dead" and the genetic material no longer available for collection and comparison.

Also, scientists don't sequence the same genes, examine the same animals or use the same models for mutational clocks. In addition, the clocks are calibrated to fossil finds.

Yet "the fossil record is clearly imperfect," said Dr. Wible, adding "the chances of something turning into a fossil are very slim." The new study's estimates aren't out of line with some of the younger molecular dates, he noted.

So "I would not be surprised if I went out to the field tomorrow and I found a Cretaceous rabbit that was 80 million years old," Dr. Wible said. Finding one that was 120 million years old would shock him, however.

In a Nature commentary, Richard L. Cifelli, of the Sam Noble Oklahoma Museum of Natural History, and Cynthia L. Gordon, of the University of Oklahoma, called the findings "eye-popping."

The study "is ground-breaking because it brings a wealth of new data into play; it includes every informative Cretaceous fossil and is based on comparison of more than 400 anatomical features," they said.

Integrating paleontological and molecular approaches could yield more information, they added.

Dr. Wible and colleagues at the Carnegie Museum are now working with paleontologists and molecular experts at other institutions, including the American Museum, Stony Brook University, University of California Riverside and Texas A&M, on the mammalian component of a National Science Foundation-funded Tree of Life program.

"It's a study like this that's ultimately going to be able to address the discrepancies between the fossil evidence and the molecular evidence," Dr. Wible said.

Anita Srikameswaran can be reached at anitas@post-gazette.com or 412-263-3858.