Mammals can use their forelimbs to swim, fly, jump, climb, dig, and nearly everything in between, yet the question of how all that diversity evolved has remained a vexing one for scientists.

To help answer that, Harvard researchers are turning to one of the most unusual mammals around: echidnas. These sprawling, egg-laying mammals have many anatomical features in common with mammal ancestors, and so can help bridge the gap between extinct and modern-day species.

Using a detailed, musculoskeletal model of an echidna forelimb, Sophie Regnault, a postdoctoral fellow, and Stephanie Pierce, associate professor of organismic and evolutionary biology, were able not only to shed light on how the little-studied echidna’s forelimbs work, but also to open a window into understanding how now-extinct mammals might have used their limbs. The model is described in a paper published in the Royal Society Open Science.

“Echidnas are not very well-studied, and little is known about their biomechanics.” Regnault said. “There are few related species, and echidnas themselves can be difficult to study because they have very large spines hiding underlying movements. We made this virtual model using CT scans that allow us to look in closer detail at how the skeleton and muscles interact with one another.”

The researchers discovered that the bony anatomy and muscles work together to optimize limb leverage and mobility for some movements. In particular, the configuration of muscles supports limb rotation important for the echidna’s sprawling gait.