December 30 , 2016:

by Sid Perkins

Imagine how odd it would be if humans were born with a full set of teeth and gradually went toothless from age 12 onward. A slim, little bipedal dinosaur that lived 160 million years ago, appears to have done something much like that. Limusaurus lost all of its teeth as it aged and swapped them for a smooth beak, a team of paleontologists reports. That never-before-seen transformation in dinosaurs—or indeed in any reptile living or extinct—may have allowed Limusaurus to become increasingly herbivorous as it matured into adulthood, and thus avoid competing with its more omnivorous offspring.

“This is wholly unexpected,” says Stephen Brusatte, a vertebrate paleontologist at the University of Edinburgh, who wasn’t involved in the study. Limusaurus might be the first known member of “a weird group” of dinos, he says. “It’s possible that these dinosaurs are more interesting ecologically than we’ve given them credit for.”

Limusaurus was first described less than a decade ago, based on a handful of relatively complete fossils of adults and near-adults. Standing less than waist high, these lithe, bipedal creatures measured as much as 1.5 meters long from the tip of their snout to the tip of their tail and weighed about 23.5 kilograms (about as much as a medium-sized dog), says Xing Xu, a vertebrate paleontologist at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing. He was a co-author on that first paper as well as the new research. But since 2009, Xu and his teammates have extracted even more fossils of Limusaurus from rocks they’d previously collected at the same site.

Many meat-eating dinosaurs lost teeth as they chomped on prey. And, like Limusaurus, members of some species of tyrannosaurs are known to have lost a handful of teeth as they aged, Xu says. But in losing all its teeth, Limusaurus shows the most radical changes in dinosaurian dental array.

The team’s analyses show that over its life span, Limusaurus experienced at least 77 other anatomical changes. These include shifts in the proportions of its skull and the shape of its upper jawbone, which over time began to curve pronouncedly downward and presumably developed on its exterior a beak. Another change strongly suggests that as it aged Limusaurus gave up omnivory and switched to a plant-based diet: Larger individuals have swallowed stones that ostensibly helped the dinosaurs grind up vegetation and aided digestion, just as gizzard stones do in birds today.

The new findings “show that baby dinos didn’t have to be tiny versions of the adult,” says Thomas Holtz, Jr., a vertebrate paleontologist at the University of Maryland in College Park. Any changes in Limusaurus’ diet over time might have helped different age groups carve out separate niches within their ecosystem and thus reduce competition for food between adults and juveniles, he speculates.

Holtz says there may be more similarly strange dinos to come. A handful of Limusaurus' closest relatives, some of which lived at the same time and some that lived millions of years later, are as of now known only from their skeletons and not their skulls. And those missing heads might show the same sort of anatomical changes that Xu and his colleagues have reported for Limusaurus. Only future finds would be able to tell the tale. “That’s the most annoying thing,” Holtz adds. “There might be a whole lineage of these dinosaurs out there.”