Around 66 million years ago, two types of long-necked, four-legged dinosaurs stood out from other sauropods. They had the ability to rise up on their hind legs and stay upright for extended periods. This posture likely helped them reach leaves high in trees and may have made them look larger and more intimidating to predators.
The Brazilian Uberabatitan and the Argentine Neuquensaurus were roughly the size of modern elephants. Even though they were considered small compared to other sauropods, adult Uberabatitans may have reached lengths of up to 26 meters, making them the largest dinosaurs known from Brazil. However, their ability to remain upright for long periods appears to have been limited to younger individuals. This finding comes from a study supported by FAPESP and published in the journal Palaeontology, led by researchers from Brazil, Germany, and Argentina.
How Scientists Measured Bone Stress
To understand how these dinosaurs managed this behavior, researchers used a computational approach commonly applied in engineering. Their goal was to estimate how much stress gravity and body weight placed on the femur when the animals stood on two 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 first author of the study, which was conducted during an internship at the University of Tübingen in Germany with a scholarship from FAPESP.
The team created digital reconstructions of femurs from seven different sauropods. These species represented a range of evolutionary branches, body sizes, and skeletal features. The models were built using fossil specimens from museum collections around the world.
South American Sauropods Show Lower Stress
The researchers ran simulations using finite element analysis (FEA), a method that predicts how materials respond to forces and other physical conditions. This technique is often used in engineering projects such as bridge design.
“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 both scenarios, the researchers estimated the total stress each dinosaur experienced. The two South American species, a juvenile Uberabatitan ribeiroi (named after the Brazilian municipality of Uberaba, where it was found, and coincidentally, Silva Júnior’s hometown) and Neuquensaurus australis (found near the Neuquén River in Argentina), showed the lowest stress levels in their femurs. Both lived during the Late Cretaceous period, about 66 million years ago.
“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. He notes that adult Uberabatitan individuals, unlike the younger specimen analyzed, likely faced the same challenges as other large sauropods when trying to stand upright.
Why Standing Upright Mattered
Standing on two legs likely offered several advantages. As plant-eaters, sauropods could reach higher vegetation that was otherwise out of range. This posture may also have played a role in mating, helping males mount females or perform visual displays. In addition, rising up could have made them appear larger, potentially deterring predators.
Study Limitations and Insights
The researchers point out that their simulations did not include cartilage, which may have helped absorb stress in the joints. They also did not factor in the role of the tail, which could have provided additional support when the animals balanced in a tripod-like stance.
Because cartilage was not directly studied in any of the specimens, the team assumed it functioned similarly across all of them. “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.
