ATHENS, Ohio Scientists studying the earliest mammals have been stumped for centuries about the function of two pelvic bones found in the fossil record that most mammals don't have today. A study published in this week's issue of the journal Science suggests those bones were involved in locomotion and helped the animals become more mobile, a find that could help researchers pinpoint a key moment in the evolution of mammals.

Biologists at Ohio University and Buffalo State College studied modern-day relations to the earliest mammals opossums, one of the few types of animals alive today that still has the bones in question, called epipubic bones.

In opossums and a few other marsupials, the epipubic bones are attached to the pelvis and jut into muscles of the stomach. "Kind of like you had two pencils in your belly wall coming from your pelvis up to either side of your navel and they can move up and down," explained Steve Reilly, associate professor of biological sciences at Ohio University and lead author of the study.

Epipubic bones have been found in the earliest mammal fossils and remain in some of the marsupials still living today, and scientists had long thought they supported the animals' trademark pouch. If that were the case, the bones and attached muscles would move together on one side of the body when the animals walk. But when researchers placed opossums on a treadmill and observed their bones and muscles in motion with a videoflouroscope, they found that the bones move asymmetrically.

"Instead of moving together, one bone is going up and the other is going down," Reilly said. "The epipubic bones act like fishing poles within the belly wall to pull one at a time diagonally across the body, stiffening the body during each trotting step."

And, he added, the support from the bones that stiffens the body allowed the animals and most likely their ancient ancestors to trot.

"These opossums are marsupials that look almost exactly like the fossils we have of mammals that lived millions of years ago," said Reilly, who has studied the evolution of animal locomotion for seven years. "We believe the earliest mammals probably moved just like the opossums because they're very similar anatomically."

Reilly and his collaborator Thomas White suspect that the development of epipubic bones made the prehistoric creatures more mobile. The increased locomotion made them better predators, helped them to escape predators and allowed them to forage more widely.

"Locomotion contributed heavily to the evolution of mammals," Reilly said, "and these bones had something to do with increasing locomotor efficiency in the very earliest mammals."

As the mammals radiated after the dinosaurs went extinct, the epipubic bones in most mammals, including humans, became fused with the pelvis, which allowed mammals to use many gaits besides the trot.

The findings could have implications for paleontologists, Reilly said.

"If the function of the epipubic bone relates to locomotion, that makes the bones more important as a fossil indicator of increased locomotor efficiency," he said. If scientists study the fossil record and figure out when these bones first appeared, he added, it would shed light on a crucial step in the evolution of mammals.

The research is part of a larger study by Reilly and Ohio University colleague Audrone Biknevicius focusing on the evolution of locomotion, which is funded by a three-year, $295,000 National Science Foundation grant.