Researchers analyzing the world’s oldest broken (and healed) bone have discovered that a 333-million-year-old amphibian-looking critter spent most of its life on land, not water. The findings on this remarkable paleopathology, published in PLoS ONE this month, push back the origin of our early terrestrial ancestors by at least two million years.

Tetrapods include all of the land-living vertebrates we have today, including some who returned to life at sea (like whales) and some who lost their legs (like snakes). But exactly when and how terrestrial tetrapods originated has been a mystery. Researchers think the advent of vertebrate terrestrialization took place during the Early Carboniferous, but fossils from the first 30 million years of this period are very rare.

To better understand this pivotal behavioral transition, Queensland Museum’s Peter Bishop and colleagues studied the oldest known broken tetrapod bone: the forearm of Ossinodus pueri from the mid-Viséan of Queensland, Australia. This two-meter-long primitive tetrapod lived around 333 million years ago, and it fractured its radius under a high-force, impact-type scenario. The presence of a thickened, hardened callus (see image above) indicates that healing had begun; by the time this animal died, its radius was already partially healed.

When the team reconstructed the forces required to cause the fracture, New Scientist explains, they found that the magnitude of the force was so large (relative to the animal’s size) that the fall must have happened on land. Assuming that Ossinodus weighed 25 kilograms (55 pounds), it would only need to fall 85 centimeters (33 inches) to sustain an impact force sufficient to fracture its radius.

"Those kinds of impact forces are very difficult to achieve in water, because water acts like a cushion," Bishop said, speaking to New Scientist. Additionally, the team also found anatomical features in both its spongy bone tissue and the opening to the nutrient canal in its bones that appear to be important adaptations for terrestrial weight support.

Taken together, these findings suggest that Ossinodus spent a significant portion of its time on land. This pushes back the date for the origin of terrestrial tetrapods into the Carboniferous by at least two million years. And that means that it’s possible that terrestriality in vertebrates first evolved in large tetrapods of Gondwana -- rather than in small, 40-centimeter-long European tetrapods that were widely regarded as the oldest known terrestriality adapted vertebrates.