The first backboned animals to step out of water and walk on dry land were from Australia—not Scotland, as scientists previously thought. Researchers made the discovery when they analyzed a 333-million-year-old broken arm bone.
The bone was from the fossil remains of a very primitive amphibian, called Ossinodus pueri, which looked like a cross between a crocodile and a fish. Significantly older and much larger than the fossils found in Scotland, the Ossinodus was found in 2001 near the central Queensland town of Emerald.
One of the fossils, a 4-centimeter-long arm bone, which had fractured in life and had started to heal when the animal died, was re-examined using CT scans, allowing the team to look inside the fossil.
The bone’s internal microstructure was consistent with what was expected for an animal that spent time walking on land.
Applying the same principles that engineers use to assess whether a building or bridge is adequately supported, the team developed a 3D computer model of the bone and then used engineering computer software to determine what kinds of forces would have caused the fracture.
The results: the animal would have experienced a very large impact force, exceeding five times its own body weight. A fall on land is the most likely explanation for the broken arm bone, says Peter Bishop from the Queensland Museum.
“Forces of that relative magnitude don’t happen easily in water, because water cushions against forces, especially impact forces. In order to fall down, the Ossinodus must have been on land to begin with.”
2 million years older
The age of the fossil raises the possibility that the first animals to emerge from the water to live on land were large tetrapods in Gondwana in the southern hemisphere, rather than smaller species in Europe, says Matthew Phillips of Queensland University of Technology.
“This specimen of Ossinodus is our oldest vertebrate ancestor shown biomechanically to have spent significant time on land. It is two million years older than the previous undoubtedly terrestrial specimens found in Scotland, which were less than 40 centimeters long.”
Other features confirmed the tetrapod had spent substantial time on land. The internal bone structure was consistent with re-modeling during life in accordance with forces generated by walking on land. There was also evidence of blood vessels that enter the bone at low angles, potentially reducing stress concentrations in bones supporting body weight on land.
The discovery highlights the significant role that Australian fossils play in understanding the history of life on the planet, says Catherine Boisvert from the Australian Regenerative Medicine Institute (ARMI) at Monash University.
“Other, older tetrapods are known from elsewhere in the world, but it is unclear if they were capable of supporting themselves on land.”
“With Ossinodus, however, we can say with great confidence that this creature was walking on land 333 million years ago.”
Source: Monash University