This early sauropod went from walking on four legs to two as it grew

Most long-necked sauropods lumbered on four legs all their lives to support their titanic bulk. But an early relative of such behemoths as Brachiosaurus made the unusual transition from walking on four legs to two as it grew, a new study shows.

Diminutive at hatching, Mussaurus patagonicus (which means “mouse lizard”) began life walking on all fours. But by the time the 200-million-year-old plant eater reached its 6-meter-long adult size, it roamed what’s now Argentina on two legs.  

The changing length of M. patagonicus’s arm bones relative to its body and its inward facing-palms as an adult had hinted at the transition. But for the first time, computer simulations based on a rich fossil record show how a shift in the creature’s center of gravity as it grew enabled a change to bipedal walking, researchers report May 20 in Scientific Reports.   

Researchers took CT scans of fossil bones from six individual M. patagonicus — covering different stages of the species’ development, from 60-gram hatchlings the size of baby chickens to 1.5 metric ton adults the size of rhinoceroses. The researchers added virtual flesh to digitized bones to create 3-D models that allowed them to estimate both the weight and center of gravity of M. patagonicus at many different stages of its life.

Reconstructions of the hatchlings showed that the creature’s center of mass was so far forward that the dinosaurs could move around only by walking on all four legs, says Andrew Cuff, a paleontologist of the Structure and Motion Laboratory of the Royal Veterinary College in Hatfield, England.

As the dinos grew, their center of mass moved back toward their hips, allowing them to walk upright on two legs, Cuff and colleagues found. The transition “is incredibly rare,” he says. “We have struggled to find any other animals aside from humans that go through that transition…. Finding it in the fossil record is pretty exceptional.”

The results suggest these adult dinosaurs turned bipedal because their tail muscles became bulkier and heavier as they grew, moving their center of gravity backward, says Stephen Poropat, a paleontologist at Swinburne University of Technology in Melbourne, Australia, who was not involved in the research. “It is not the changing proportions of Mussaurus’s front legs that is necessitating this change from walking on four legs to walking on two legs as an adult,” he says.

As later long-necked dinos bulked up in size (SN Online: 9/4/14), going to two legs may no longer have been an option. Massive sauropods instead probably started on four legs like M. patagonicus and stayed that way, developing trunklike front legs to bear their weight. “What we gain from this [study] is that there may be a size limit of how big you can get being a biped in this group,” Cuff says.