Illustration by González
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As a hatchling, the dinosaur Mussaurus patagonicus walked on all fours. But as the dinosaur grew, its center of mass changed and it shifted to two-legged locomotion.

Illustration by González

Like humans, this dinosaur crawled before it walked

Dinosaurs like the "mouse lizard" are among the few animals that made this particular shift in age-related locomotion, fossils suggest.

It’s a special moment when a baby learns how to go from crawling on all fours to walking on two legs, especially considering how rare this transition is in the animal kingdom. Only a few other species are thought to make the shift from crawling to upright walking as they age—and they’re all dinosaurs.

Now, researchers have added compelling new evidence for one member on that list: a sauropod named Mussaurus patagonicus. (Also see fossil footprints from sauropods that lived in what’s now Scotland.)

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A baby Mussaurus patagonicus was small enough to fit in human hands.

When Mussaurus hatched, it would have been small enough to fit cozily inside a human hand, earning it a genus name that means “mouse lizard.” But after just eight years of gobbling up ferns and other plants, the same creature would have weighed more than a ton.

This dramatic explosion in body size and shape brought with it a shift in the way the animal moved, scientists argue in a new study published today in the journal Scientific Reports.

Using a remarkable series of nearly complete fossils that depict three key stages of life, the scientists were able to create interactive 3-D models of the early Jurassic dinosaur and work out where its center of body mass would have been as it aged. Early in life, the animal’s large head and neck would have tilted its body forward onto the support of well-developed forearms. But as its tail grew, the center of gravity would have shifted to the pelvic region, drawing the animal upward into a stance that allowed for walking on two legs.

“We don’t know if this pattern applied to all sauropods,” says study leader Alejandro Otero, a paleontologist at the La Plata Museum in Argentina. “But the fact that this [group of animals] shifted their movements in a way so similar to humans is fascinating.”

Dinosaurs 101 Over a thousand dinosaur species once roamed the Earth. Learn which ones were the largest and the smallest, what dinosaurs ate and how they behaved, as well as surprising facts about their extinction.

Snapshots of prehistory

One of the things that makes studying extinct animals so difficult is that scientists can usually only see a tiny snapshot of each creature’s life. Often, that snapshot does not include hatchlings or juveniles, which are smaller and less likely to fossilize. (Find out how fossil eggs are revealing more about dinosaur parenting styles.)

“There are few dinosaurs for which we have a good series from eggs or babies to adults, and Mussaurus has become one of those, which opens up some exciting new questions not feasible 20 years ago,” says study coauthor John Hutchinson, a professor of evolutionary biomechanics at the Royal Veterinary College at the University of London.

It’s taken over half a century to assemble the set of fossils that informed the current study, for instance. Scientists call these ontogenetic series, and they are highly prized.

“We typically think of each species of dinosaur represented by one monolithic entity, but like every other animal, they have a growth series that we usually just don't get a chance to see, since dinosaur fossils are so rare,” says Shaena Montanari, a paleontologist and AAAS Science and Technology Policy Fellow who was not involved in the study.

Dinosaurs did it first

Mussaurus is just one branch on the dinosaur family tree that may have switched locomotion modes as the animals aged.

“Several dinosaurs have been hypothesized to do it, including Maiasaura, Iguanodon, Psittacosaurus, Dryosaurus, and Massospondylus,” says Kimi Chapelle, a Ph.D. candidate studying locomotion shifts in dinosaurs at the University of the Witwatersrand in South Africa.

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Paleontologists have what's known as an ontogenetic series for Mussaurus patagonicus, a set of nearly complete fossils that depict three key stages of life, including this juvenile skull.

What’s more, some species displayed shifts in the opposite direction, going from walking on two legs to walking on four, says Chapelle, who was not part of the new study. A propensity for switching things up is even reflected in dinosaur evolution.

“Ancestral dinosaurs are bipedal, but quadrupedality evolved independently across two major lineages,” Chapelle says. That includes the Ornithischia branch, which produced many famous quadruped dinosaurs, such as Triceratops, Stegosaurus, and Ankylosaurus. (See a stunning ankylosaur fossil found by accident in a Canadian mine.)

Despite all this shifting around among dinosaurs, the scientists had a really difficult time finding an example of such behavior in living animals.

“Humans ended up being the best and probably the only really well-known example,” says study coauthor Andrew Cuff, a postdoctoral researcher at the Royal Veterinary College at the University of London. “It’s a really rare thing to see, which is why it’s so exciting to see it in the fossil record of this species.”