Illustration of a herd of Apatasaurus dinosaurs

Sauropod dinosaurs — from classic as Apatosaurus to the recently married Patagotitan — were giants of the Jurassic and Cretaceous periods, with the largest species reaching over 100 feet in length and over 80 tons in weight. And while there are multiple reasons why these reptiles grew to such impressive sizes, paleontologists are still unraveling the biological features that allowed these extraordinary dinosaurs to exist. Researchers have learned that there are cushioned feet among them.

Paleontologists have long wondered about sauropod feet. The footprints and tracks left by these dinosaurs show how the bottoms of their feet had soft pads on the underside of the toes. Modern elephants have a similar anatomical arrangement. But it is only now that experts have realized how important these pedal cushions were to the evolution of truly huge dinosaurs; stress tests on virtual dinosaur bones described earlier this year in Scientific progresshighlight how soft tissues helped keep sauropod bones strong.

Read more: Why were prehistoric marine reptiles so huge?

Although experts have extensively studied various parts of sauropod skeletons, paleontologist Andreas Janel of the Natural History Museum in Berlin and his colleagues write that fossils of complete sauropod legs are relatively rare. Using 3D models of dinosaur legs such as Diplodocus, Camarasaurus and Giraffatitanthe researchers used an engineering technique called finite element analysis to study how the presence or absence of a foot pad – as well as different foot positions – affected the mechanical stresses and strains a standing sauropod was dealing with.

Broke your leg?

Understanding how an animal holds its feet is a complex task. Some animals, such as bears and humans, are plantigrade. This means that the entire foot, from the toes to the heel, touches the ground. But many other animals, such as cats and birds, are digitigrade. They effectively stand on your toes like you would when trying to reach the cookie jar on top of the fridge.

For a long time, paleontologists considered sauropods to be flat-footed, plantigrade animals. But Janelle notes that dinosaurs have a bit of an odd arrangement: the earliest sauropod ancestors were digitigrade. Instead of changing foot positions, however, the pads on the bottom of their feet formed cushions that allowed for more contact with the ground. “Sauropods retain an annular foot skeleton, but the soft tissue pad makes their foot functionally plantigrade,” Janelle says.

Paleontologists have devised new tests to determine whether these foot pads are really necessary or just anatomical extras, like the human coccyx. A major consideration of the tests was what experts know as “von Mises stress.” This is a formula that engineers and other researchers use to determine whether the material under study will withstand mechanical stress—such as the forces involved when a large dinosaur stands on an ancient plain — or broke instead.

In any case, Janel and his co-authors found that adding soft-tissue pads to the models reduced the stress placed on the bones of the sauropods’ feet—whether the bones were closer to flat on the ground or on top of the toes. On the other hand, tests without the soft tissue pads were much more prone to fracture. In fact, dinosaurs were so massive that they would break their bones with a single step if they didn’t have a cushion of soft tissue underneath.

“What was amazing to me was that the model results clearly show that sauropod foot bones break in every realistically possible foot posture without the foot pad,” says Stockton University paleontologist Matthew Bonan, who is not participated in the new study. Although paleontologists have yet to find a fossilized sauropod foot pad (a tall order, given that they likely decay quickly and even complete sets of sauropod foot bones are difficult to find), the largest dinosaurs should they had them.

Titan Trailblazers

The fact that this feature was so important to the sauropod’s size led Janelle’s team to reconsider when pads evolved in sauropod history. These giants belonged to a wider group called sauropodomorphs, which date back to the earliest days of the dinosaurs; then, the ancestors of Apatosaurus and Diplodocus they were small, lightly built animals that ran on two legs. However, the fossil record shows that sauropods had enlarged heel pads very early in their history.

The trait will only be refined over time as the creatures get bigger. About 200 million years ago, the line of bipedal sauropodormorphs diverged to move on all fours. Surely these first sauropods could grow to be so large because they inherited the soft tissue pads from their ancestors.

The new research even helps paleontologists understand the small details of how sauropods walked, even though we’re 66 million years too late to witness them walking. Dinosaurs walked heel-to-toe like elephants, Bonan says, but put more pressure on the inner surface of the foot, toward the midline of the body, and had very mobile toes. This flexibility likely helped redistribute weight as these dinosaurs moved, their anatomy based on what sauropods inherited from their earliest ancestors.

The research resolves what paleontologists have only been able to speculate about for 120 years: the pads were part of a constellation of different features that opened up evolutionary opportunities for sauropod dinosaurs to become extremely huge. Laying eggsrefusal to chew and light bones filled with air sacs also played their part in allowing Diplodocus and family to surpass what has been possible for any land-dwelling mammal since.

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