“The amount of organized bone in the outermost layer, or cortex, of the femur strongly correlates with the length of the lactation period,” Weaver said. “Marsupials have long lactation periods and a lot of organized bone in the outermost cortex. The opposite is true for placentals: a short lactation period and much less organized bone in the outermost cortex.”

The outermost layer of organized bone was laid down after birth as the femur’s diameter increased. For tiny marsupial newborns, bones must grow much more to reach adult size, so they deposit a greater amount of outer organized bone compared to placentals, according to Weaver.

“This is compelling evidence that multituberculates had a long gestation and a short lactation period similar to placental mammals, but very different from marsupials,” he said.

Based on this correlation, the researchers estimate that multituberculates had a lactation period of approximately 30 days—similar to today’s rodents.

These findings cast further doubt on an old view that marsupials have a “more primitive” and placentals “more advanced” reproductive strategy. The common ancestor of multituberculates, placentals and marsupials may have had a placental-like mode of reproduction that was retained by placentals and multituberculates. Alternatively, multituberculates and placentals could have evolved their long-gestation and short-lactation reproductive methods independently.

Future studies of multituberculate life history may clarify which explanation is true, as well as other outstanding questions of this, and other, ancient branches of our mammalian family tree.

“The real revelation here is that we can cut open fossil bones and examine their microscopic structures to reconstruct the intimate life history details of long-extinct mammals,” Wilson Mantilla said. “That’s really incredible to me.”

Additional co-authors are former University of Washington undergraduate researcher Henry Fulghum, now a graduate student at Indiana University; University of Washington postdoctoral researcher David Grossnickle; University of Washington graduate students William Brightly and Zoe Kulik; and Megan Whitney, a University of Washington doctoral alum and current postdoctoral researcher at Harvard University. The research was funded by the National Science Foundation, University of Washington, Burke Museum, Society of Vertebrate Paleontology, Paleontological Society and American Society of Mammalogists.