The biggest land animals of all time probably needed fluffy “shock absorbers” on their paws so their skeleton could support the tens of tons of weight they could reach. According to a new study, this adaptation was crucial for sauropods (the name given to the group of four-legged, herbivorous, long-necked dinosaurs) to reach their outsized size over tens of millions of years.
The pads on the paws of prehistoric monsters have not yet been directly identified — because they are made up of soft tissue, which breaks down much more easily than bones, it is expected that they will not be preserved in sauropod fossils. The high probability that they were present in the animals is a conclusion that comes from computer simulations of the way they walked and the forces that acted on the bones of their paws.
The conclusions of the analysis are in the most recent edition of the specialized journal Science Advances, signed by a trio of researchers linked to Australian institutions. Andreas Jannel and Steven Salisbury, from the University of Queensland, and Olga Panagiotopoulou, from Monash University, applied the computational methods to sauropod species of different sizes and that lived at different times, which also helped them to trace an outline of how the pads evolved along with the group.
Between 200 million and 65 million years ago, sauropods such as the famous brontosaurus and titanosaurs, which had a great diversity of species in Brazil, were among the largest terrestrial vertebrates on the planet. From the tip of the snout to the tip of the tail, they could measure close to 30 meters. In size, the only animals that surpass them are the larger whales still alive today, such as the blue whale.
As many sauropods far surpassed the size of the largest modern land animals, paleontologists have always tried to understand how they managed to achieve these extraordinary dimensions. In the last century, it was common for them to be thought of as semiaquatic dinosaurs — it would be easier to sustain their body weight if most of it were underwater.
Since then, it has become clear that sauropods had a series of adaptations that allowed them to do very well on dry land despite their enormous size. Compared to large land mammals, for example, they had proportionately much lighter-than-expected skeletons, with pneumatic vertebrae (roughly, with “hollow” areas). The column-shaped paws were also important for the task of supporting the weight of the animals.
All of that helped, but it wouldn’t be enough on its own, say the authors of the new study. The simulations carried out by him indicate that the forces produced by the weight of sauropods on the bones of the legs would be between 5 times and 50 times more intense than those generated by the weight of today’s largest land mammals, such as elephants. This means that without additional help, taking into account only the strength of the bones, it would be inevitable that they would end up suffering fractures simply because of the weight.
When the pads on the paws are added, however, the forces that the bones of the animals’ paws have to withstand are back to what is expected for large mammals, for example. The researchers further estimate that, although the sauropod footprints found today look like plantigrade animals (that is, whose soles of the feet completely touched the ground, like humans), the most likely foot skeleton was digitigrade (with fingertips touching the ground, as with cats and dogs).
The pads would cover, in this case, the middle and back of the paws, forming a kind of “orthopedic insole” that touched the ground on its entire surface. It is also possible that, like a spring, they stored energy from strides and gave the animals an extra boost, reducing the effort needed to walk.
