If you’ve ever been embarrassed after face-planting in public, a primitive animal called Ossinodus pueri knows how you feel. A few hundred million years ago, one of these guys had an awkward fall that scientists are still talking about today.
In what’s now Queensland, Australia, an Ossinodus—which kind of looks like a big salamander—was crawling around and apparently stumbled, breaking its right leg. The fracture didn’t fully heal before the creature died, and the injury was preserved in the fossil record. Now, scientists think that this broken bone might shake up what we know about vertebrates’ transition from life in the water to life on land.
Around 380 million years ago, the tetrapods—the group that contains all four-limbed vertebrates, including mammals, birds, reptiles and amphibians—evolved from fishlike animals. A few million years later, the earliest known tetrapods fully adapted to life on land; a group of small lizard-like animals called the Casineria were walking around in what’s now Scotland. The details of what happened in between—how and when early tetrapods first scrambled onto dry land—aren’t entirely clear. This was a “pivotal phase of vertebrate history,” says biologist Peter Bishop, but fossils from the group’s early days out of the water are scarce.
The fossils of that unlucky Ossinodus are some of the few available from that murky period around 333 million years ago, and they’re very important to scientists who are trying better understand the evolution of landlubber tetrapods. Bishop and his team were studying these bones when they decided to take a closer look at the Ossinodus’ fracture, which is the oldest known broken bone in a tetrapod, to learn more about the injury and the behavior of the animal that bore it.
The researchers took a CT scan of the fossil and made a computer model of it that they could bend and push to figure out how it was broken. After subjecting their model to different amounts of force on different parts of the bone from various directions and recreating the fracture, they ruled out a bite from another animal as the cause and also decided that the injury probably didn’t happen in the water. The amount of the force needed to cause the fracture would have been very difficult to achieve in the water because of the drag and the cushioning effect that water would create, and the force wasn’t concentrated in one spot like a bite would be.
Instead, the researchers think the injury likely happened on land due to a fall. The fall wouldn’t have had to have been very far—just shy of 3 feet—to produce the force needed, and the break even looks like the fractures that people experience when they fall onto an outstretched arm.
The ancient injury and a few other details about Ossinodus’ bones suggest that the animals spent a good part of their lives on land and had the bones to support their weight while crawling around. If that’s true—a sample size of one isn’t a whole lot to draw sweeping conclusions from—that would make Ossinodus the oldest tetrapod adapted to life on land and push the group’s emergence from the water back by two million years.