Some dinosaurs could rise like giants until they grew too big

About 66 million years ago, two South American sauropods could rise onto their hind legs and remain there for relatively long periods, especially when they were young. This ability may have helped them reach leaves high in trees, appear more intimidating to predators, attract mates, or reproduce.

The dinosaurs, Uberabatitan from Brazil and Neuquensaurus from Argentina, were modest in size compared with the largest sauropods. Even so, they were roughly comparable to modern elephants. Adult Uberabatitans may have grown as long as 26 meters, making them the largest dinosaurs known from Brazil.

New research suggests that their ability to stand upright declined as they grew. Younger animals were better able to support themselves on two legs, while adults probably experienced much greater strain because of their increasing weight.

The findings come from a study supported by FAPESP and published in the journal Palaeontology. The international research team included scientists from Brazil, Germany, and Argentina.

Testing Dinosaur Bones With Engineering Tools

To investigate how sauropods handled the forces involved in standing upright, the researchers turned to a computational method commonly used in engineering.

Their goal was to estimate how much stress gravity and body weight placed on the femur, or thigh bone, when each dinosaur shifted its weight onto its hind legs.

"Smaller sauropods like these had a bone and muscle structure that allowed them to stand more easily and for longer on their two hind legs. Larger ones were probably also able to stand, but for a shorter time and with less comfort, since the position caused a lot of stress on the femur," summarizes Julian Silva Júnior, a postdoctoral researcher at the School of Engineering of São Paulo State University (FEIS-UNESP) in Ilha Solteira, Brazil.

Silva Júnior is the study's first author. He carried out the research during an internship at the University of Tübingen in Germany with a scholarship from FAPESP.

The team created digital reconstructions of the femurs of seven sauropod species. The selected dinosaurs represented different evolutionary branches, body sizes, and anatomical features. Their models were built from fossils preserved in natural history museums around the world.

Simulating the Forces of Standing Upright

The scientists used finite element analysis (FEA), a method that breaks a structure into many small sections and calculates how each part responds to pressure, weight, heat, or other forces. Engineers often use the same approach to test whether bridges, buildings, and machines can withstand stress.

"Using this technique, we performed two simulations. One dealt with the extrinsic scenario, simulating the force coming from outside to inside. In this case, gravity and the animal's own weight on the femur when the dinosaur was standing on its hind legs. In the other, we analyzed the intrinsic scenario, the force that the muscles would exert on the femur," Silva Júnior explains.

By combining the two simulations, the researchers estimated the total stress experienced by the femur of each species.

The lowest stress levels appeared in the two South American sauropods. One was a juvenile Uberabatitan ribeiroi (named after the Brazilian municipality of Uberaba, where it was found, and coincidentally, Silva Júnior's hometown). The other was Neuquensaurus australis (found near the Neuquén River in Argentina).

Both lived during the Late Cretaceous period, about 66 million years ago.

Stronger Bones Gave Smaller Sauropods an Edge

The researchers found that the two species had particularly robust femurs. Their thicker, sturdier bones were better able to spread out the forces created when the animals stood upright.

"They had more robust femurs and could dissipate stress better. The bigger ones had very large muscles and even giant femurs, but not enough to support their weight. That doesn't mean they couldn't stand up, but they probably chose the best time to do so, because it must have been an uncomfortable position," says the paleontologist.

Larger sauropods may still have been capable of rising onto their hind legs. However, the simulations suggest that they could not hold the pose as comfortably or for as long.

Adult Uberabatitan individuals probably faced the same problem. Although the juvenile examined in the study was well suited to standing upright, fully grown animals would have carried far more weight. That additional mass likely placed them under levels of stress similar to those experienced by other giant sauropods.

Why Sauropods May Have Stood on Two Legs

Standing upright could have provided several important advantages.

Sauropods were plant eaters, so rising onto their hind legs may have allowed them to reach vegetation high in trees that was unavailable to shorter animals. The posture could also have played a role in reproduction by allowing males to mount females or perform visual displays to attract potential mates.

The pose may also have served as a defensive strategy. By lifting the front of the body into the air, a sauropod would have appeared even larger and more threatening to approaching predators.

When supported by both hind legs and the tail, the animal would have formed a tripodal stance, meaning that three points of contact helped stabilize its body.

Important Limits of the Study

The researchers note that their models did not include every structure that would have affected how the dinosaurs stood.

For example, the simulations did not account for cartilage, the flexible tissue that cushions joints and can help absorb and distribute stress. They also did not model the support provided by the tail while the dinosaur was in a tripodal position.

Because cartilage was not analyzed in any of the seven specimens, the researchers assumed that it played a similar role across the species. This means the method is most useful for comparing the dinosaurs with one another rather than producing an exact measurement for each individual animal.

"The tool we use is very efficient for comparisons, even if the answer isn't exact for each one. By comparing representatives from different lineages, we can get a fairly accurate picture of how these animals behaved millions of years ago," says the researcher.