Eyeing echidnas: Study models echidna forelimbs to help shed new light on mammal evolution

November 17, 2018, Harvard University

These days, mammals can use their forelimbs to swim, jump, fly, climb, dig and just about everything in between, but the question of how all that diversity evolved has remained a vexing one for scientists.

To help answer it, 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 earlier mammal ancestors, and so can help bridge the gap between extinct and other modern-day mammals.

Using a highly-detailed musculoskeletal model of an echidna forelimb, Sophie Regnault, a Post-Doctoral Fellow and Stephanie Pierce, Associate Professor of Organismic and Evolutionary Biology, were able to not only shed more light on how the little-studied echidna's forelimb works, but also open a window into understanding how extinct mammals might have used their forelimbs. The model is described in a November 14 paper published in Royal Society Open Science.

"Echidnas are not very well-studied, and little is known about their biomechanics." Regnault says. "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 certain kinds of movements. In particular, the configuration of muscles support limb rotation important for the echidna's sprawling gait.

"This model gives us unique insight into not only the echidna but can also guide reconstructions of extinct mammals," Pierce says. "The similarities between the echidna forelimb skeleton and transitional animals can help us to understand the evolution of forelimb diversity in modern mammals."

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Newly discovered dinosaur had 'T. rex arms' that evolved independently

July 13, 2016

Scientists still aren't sure why T. rex had those absurdly small forelimbs, but apparently the look was all the rage in the Late Cretaceous. A newly-discovered dinosaur from Patagonia has similar short, two-fingered claws, even though it's not closely related to the tyrannosaurs. Like Tyrannosaurus rex, the new Gualicho shinyae is a theropod, one of the two-legged, bird-like dinosaurs, but it's on a different branch of the family tree, meaning that the unusual limbs evolved independently rather than arising from a common short-armed ancestor.

"Gualicho is kind of a mosaic dinosaur, it has features that you normally see in different kinds of theropods," says corresponding author Peter Makovicky, The Field Museum's Curator of Dinosaurs, who helped describe the new species in PLOS ONE. "It's really unusual—it's different from the other carnivorous dinosaurs found in the same rock formation, and it doesn't fit neatly into any category."

Gualicho is an allosaurid, a branch of medium-to-large carnivorous theropod dinosaurs. The skeleton discovered is incomplete, but scientists estimate that it was a medium-sized predator weighing around a thousand pounds, comparable to a polar bear. It's very different from the other dinosaurs that lived near it; if anything, it looks most like Deltadromeus, a leggy, carnivorous dinosaur with slender arms found in Africa, which it appears to be closely related to.

Despite its large size, Gualicho's forelimbs were the size of a human child's, and like T. rex, it had just two digits (thumb and forefinger). While Gualicho doesn't explain why so many theropods had reduced forelimbs, it adds to evidence that the trait evolved independently numerous times. "By learning more about how reduced forelimbs evolved, we may be able to figure out why they evolved," explained Makovicky.

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