About 66 million years ago, two long-necked, four-legged dinosaurs stood out from other sauropods. They had the ability to stand up on their hind legs and remain upright for long periods of time. This posture probably helped them reach the higher leaves of trees and may have made them appear larger and more intimidating to predators.
Brazil’s Uberabatitan and Argentina’s Neuquensaurus were about the same size as modern elephants. Although considered small compared to other sauropods, an adult Uberabatitan may have reached 26 meters in length, making it the largest dinosaur known from Brazil. However, the ability to remain upright for long periods of time appears to have been limited to young individuals. The findings come from a study supported by FAPESP and published in the journal . paleontologyled by researchers from Brazil, Germany and Argentina.
How scientists measured bone stress
To understand how these dinosaurs managed this behavior, the researchers used a computational approach commonly applied in engineering. Their goal was to estimate the stress that gravity and body weight exert on the femur when the animal stands on two legs.
“Such small sauropods had a bone and muscle structure that allowed them to stand on their two hind legs more easily and for longer. Larger sauropods were probably able to stand as well, but the posture was not comfortable for short periods of time because it caused a lot of stress on the femur,” summarizes Julian Silva Junior, a postdoctoral researcher at the Faculty of Engineering of the State University of São Paulo (FEIS-UNESP) in Solteira, Brazil. Silva Júnior is the first author of this study, which was carried out during an internship at the University of Tübingen, Germany, with a scholarship from FAPESP.
The research team created digital reconstructions of femurs from seven different sauropods. These species represented different evolutionary branches, body sizes, and skeletal characteristics. The model was built using fossil specimens from museum collections around the world.
South American sauropods show low stress
The researchers performed the simulations using finite element analysis (FEA), a technique that predicts how materials will respond to forces and other physical conditions. This technique is often used in engineering projects such as bridge design.
“Using this technique, we ran two simulations: one dealt with an external scenario, simulating forces coming from the outside in, in this case gravity and the animal’s own weight on the femur when the dinosaur was standing on its hind legs. The other analyzed an internal scenario, the forces exerted by the muscles on the femur,” explains Silva Junior.
By combining both scenarios, the researchers estimated the total stress experienced by each dinosaur. Two South American species, Juvenile Uberabatitan ribeiiroi (named after the city of Uberaba, Brazil, where it was discovered, and coincidentally Silva Junior’s hometown) and Neuquensaurus australis (discovered near the Neuquén River in Argentina) had the lowest stress levels in their femurs. Both lived during the Late Cretaceous period, about 66 million years ago.
“They had stronger femurs, which allowed them to distribute stress better. The larger ones had very large muscles and even huge femurs, but they weren’t strong 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 points out that, unlike the younger specimens analyzed, adults of Uberabatitan likely faced the same difficulties as other large sauropods in trying to stand upright.
Why is it important to stand upright?
There are probably several benefits to standing on two legs. Plant-eating sauropods were able to reach tall plants that were normally inaccessible. This posture may also play a role in mating, helping the male mount the female or make a visual display. Additionally, standing up makes you appear larger, which may deter predators.
Research limitations and insights
The researchers note that their simulations do not include cartilage, which may help absorb stress in joints. They also did not consider the role of the tail, which may have provided additional support when the animal balanced in a tripod-like position.
Because cartilage was not directly studied in any of the specimens, the researchers assumed that cartilage functioned similarly in all specimens. “The tools we use are very efficient at making comparisons, even if each answer is not exact. By comparing representatives of different lineages, we can get a fairly accurate picture of how these animals behaved millions of years ago,” say the researchers.
