An artistic rendering of multituberculates from the genus Mesodma- a mother and her litter of offspring-- who lived in western North America 60 to 70 million years ago. Fossil evidence indicates that these creatures were the most abundant mammals in western North America just before and directly after the mass extinction event 66 million years ago that killed off the dinosaurs. Image credit: Andrey Atuchin

It’s hard to imagine life on Earth without mammals. They swim in the depths of the ocean, hop across deserts in Australia and travel to the moon.

This diversity can be deceiving, at least when it comes to how mammals create the next generation. Based on how they reproduce, nearly all mammals alive today fall into one of two categories: placental mammals and marsupials. Placentals, including humans, whales, and rodents, have long gestation periods. They give birth to well-developed young—with all major organs and structures in place—and have relatively short lactation periods during which young are nursed on milk from their mothers.

Marsupials, like kangaroos and opossums, are the opposite: They have short gestation periods—giving birth to young that are little more than fetuses—and long lactation periods during which offspring spend weeks or months nursing and growing within the mother’s pouch, or marsupium.

For decades, biologists saw the marsupial way of reproduction as the more “primitive” state and assumed that placentals had evolved their more “advanced” method after these two groups diverged from one another. But new research is testing that view.

In a study published online July 18 in The American Naturalist, a team led by researchers at the University of Washington and its Burke Museum of Natural History and Culture present evidence that another group of mammals—the extinct multituberculates—likely reproduced in a placental-like manner. Since multituberculates split off from the rest of the mammalian lineage before placentals and marsupials evolved, these findings question the view that marsupials were “less advanced” than their placental cousins.

“This study challenges the prevalent idea that the placental reproductive strategy is ‘advanced’ relative to a more ‘primitive’ marsupial strategy,” said lead author Lucas Weaver, a postdoctoral researcher at the University of Michigan who conducted this study as a University of Washington doctoral student.

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