A 333-million year old fossil recently uncovered in Australia is shedding light on the evolution of land-dwelling vertebrate animals, suggesting that they transitioned to land earlier than thought.
Led by paleontologists from Queensland University of Technology (QUT), the team conducted an analysis of a partially fractured bone belonging to Ossinodus pueri - a large, primitive, four-legged, salamander-like animal. Given the nature of the broken bone, it has pushed back scientists' notions of when terrestrial vertebrates moved to land by two million years.
"Its age 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," evolutionary biologist Dr. Matthew Phillips said in a statement.
"The evolution of land-dwelling tetrapods from fish is a pivotal phase in the history of vertebrates because it called for huge physical changes, such as weight bearing skeletons and dependence on air-breathing," he added.
Phillips and his colleagues used high-resolution finite element analysis to better examine the radius bone. The results suggest that the Ossinodus specimen broke its bone due to high-force impact.
"The break was most plausibly caused by a fall on land because such force would be difficult to achieve with the cushioning effect of water," the researcher explained.
Not only that, but two other features confirmed the tetrapod had spent substantial time on land.
"Firstly, the internal bone structure was consistent with re-modelling during life in accordance with forces generated by walking on land," Phillips said. "We also found evidence of blood vessels that enter the bone at low angles, potentially reducing stress concentrations in bones supporting body weight on land."
This combined evidence makes the specimen of Ossinodus the oldest known vertebrate relative to have spent significant time on land. In fact, it is two million years older than the previous record-holder found in Scotland.
The findings, published in the journal PLOS ONE, help to change scientists' perspective of vertebrate animals' evolution from sea to land.
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